Move implementation utility methods into their own namespace

- Preconditions is renamed to Validation
- Validation is public, allowing simplified validation in
  implementation classes
- CloudEvent.ValidateForConversion is now Validation.CheckCloudEventArgument
- All the MimeUtilities methods are non-extension methods (to avoid
  Intellisense suggesting them)
- BinaryDataUtilities is now public (it's much less of an issue once
  it's in a namespace that other users are somewhat discouraged from
  using)
- Documentation explains the purpose of the namespace, albeit briefly

Signed-off-by: Jon Skeet <jonskeet@google.com>

docs/README.md

@@ -4,3 +4,15 @@ This directory contains documentation on:
 
 - Using the SDK as a consumer
 - Implementing new event formats and protocol bindings
+
+## Implementation utility classes
+
+The `CloudNative.CloudEvents.Core` namespace contains utility
+classes which are generally helpful when implementing [protocol
+bindings](bindings.md) or [event formatters](formatters.md) but are
+not expected to be used by code which only creates or consumes
+CloudEvents.
+
+The classes in this namespace are static classes, but the methods
+are deliberately not extension methods. This avoids the methods from
+being suggested to non-implementation code.

docs/bindings.md

@@ -95,7 +95,7 @@ following pseudo-code as structure:
   - If the message contains content, call the
     `formatter.DecodeBinaryModeEventData` method to populate the
     `CloudEvent.Data` property appropriately.
-  - Return the result of `CloudEvent.ValidateForConversion` which
+  - Return the result of `Validation.CheckCloudEventArgument` which
     will validate the event, and either return the original reference if
     the event is valid, or throw an appropriate `ArgumentException`
     otherwise.
@@ -155,9 +155,10 @@ The conversion should follow the following steps of pseudo-code:
 
 - Parameter validation (which may be completed in any order):
   - `cloudEvent` and `formatter` should be non-null
+    (`CheckCloudEventArgument` will validate this for the `cloudEvent` parameter)
   - In a `CopyTo...` method, `destination` should be non-null
   - The `contentMode` should be a known, supported value
-  - Call `cloudEvent.ValidateForConversion(nameof(cloudEvent))`
+  - Call `Validation.CheckCloudEventArgument(cloudEvent, nameof(cloudEvent))`
     for validation of the original CloudEvent.
 - For structured mode encoding:
   - Call `formatter.EncodeStructuredModeMessage` to encode

docs/formatters.md

@@ -73,10 +73,10 @@ being non-null, and additionally perform CloudEvent validation on:
 - The `CloudEvent` accepted in `EncodeBinaryModeEventData` or
   `EncodeStructuredModeMessage`
 
-Validation should be performed using the `ValidateForConversion`
+Validation should be performed using the `Validation.CheckCloudEventArgument`
 method, so that an appropriate `ArgumentException` is thrown.
 
-The formatter should *not* perform validation on the
+The formatter should *not* perform validation on the `CloudEvent`
-`CloudEvent` accepted in `DecodeBinaryModeEventData`. This is
+accepted in `DecodeBinaryModeEventData`, beyond asserting that the
-typically called by a protocol binding which should perform
+argument is not null. This is typically called by a protocol binding
-validation itself later.
+which should perform validation itself later. 

src/CloudNative.CloudEvents.Amqp/AmqpClientExtensions.cs

@@ -5,6 +5,7 @@
 using Amqp;
 using Amqp.Framing;
 using Amqp.Types;
+using CloudNative.CloudEvents.Core;
 using System;
 using System.Collections.Generic;
 using System.IO;
@@ -50,8 +51,8 @@ namespace CloudNative.CloudEvents.Amqp
             CloudEventFormatter formatter,
             IEnumerable<CloudEventAttribute> extensionAttributes)
         {
-            message = message ?? throw new ArgumentNullException(nameof(message));
+            Validation.CheckNotNull(message, nameof(message));
-            formatter = formatter ?? throw new ArgumentNullException(nameof(formatter));
+            Validation.CheckNotNull(formatter, nameof(formatter));
 
             if (HasCloudEventsContentType(message, out var contentType))
             {
@@ -122,7 +123,7 @@ namespace CloudNative.CloudEvents.Amqp
                     throw new ArgumentException("Binary mode data in AMQP message must be in the application data section");
                 }
 
-                return cloudEvent.ValidateForConversion(nameof(message));
+                return Validation.CheckCloudEventArgument(cloudEvent, nameof(message));
             }
         }
 
@@ -141,9 +142,8 @@ namespace CloudNative.CloudEvents.Amqp
         /// <param name="formatter">The formatter to use within the conversion. Must not be null.</param>
         public static Message ToAmqpMessage(this CloudEvent cloudEvent, ContentMode contentMode, CloudEventFormatter formatter)
         {
-            cloudEvent = cloudEvent ?? throw new ArgumentNullException(nameof(cloudEvent));
+            Validation.CheckCloudEventArgument(cloudEvent, nameof(cloudEvent));
-            cloudEvent.ValidateForConversion(nameof(cloudEvent));
+            Validation.CheckNotNull(formatter, nameof(formatter));
-            formatter = formatter ?? throw new ArgumentNullException(nameof(formatter));
 
             var applicationProperties = MapHeaders(cloudEvent);
             RestrictedDescribed bodySection;

src/CloudNative.CloudEvents.AspNetCore/CloudEventJsonInputFormatter.cs

@@ -2,6 +2,7 @@
 // Licensed under the Apache 2.0 license.
 // See LICENSE file in the project root for full license information.
 
+using CloudNative.CloudEvents.Core;
 using Microsoft.AspNetCore.Mvc.Formatters;
 using Microsoft.Net.Http.Headers;
 using System;
@@ -30,15 +31,8 @@ namespace CloudNative.CloudEvents
 
         public override async Task<InputFormatterResult> ReadRequestBodyAsync(InputFormatterContext context, Encoding encoding)
         {
-            if (context == null)
+            Validation.CheckNotNull(context, nameof(context));
-            {
+            Validation.CheckNotNull(encoding, nameof(encoding));
-                throw new ArgumentNullException(nameof(context));
-            }
-
-            if (encoding == null)
-            {
-                throw new ArgumentNullException(nameof(encoding));
-            }
 
             var request = context.HttpContext.Request;
 

src/CloudNative.CloudEvents.AspNetCore/HttpRequestExtensions.cs

@@ -3,6 +3,7 @@
 // See LICENSE file in the project root for full license information.
 
 using CloudNative.CloudEvents.Http;
+using CloudNative.CloudEvents.Core;
 using Microsoft.AspNetCore.Http;
 using System;
 using System.Collections.Generic;
@@ -79,13 +80,10 @@ namespace CloudNative.CloudEvents
                 cloudEvent.DataContentType = httpRequest.ContentType;
                 if (httpRequest.Body is Stream body)
                 {
-                    // TODO: This is a bit ugly. We have code in BinaryDataUtilities to handle this, but
+                    byte[] data = await BinaryDataUtilities.ToByteArrayAsync(body).ConfigureAwait(false);
-                    // we'd rather not expose it...
+                    formatter.DecodeBinaryModeEventData(data, cloudEvent);
-                    var memoryStream = new MemoryStream();
-                    await body.CopyToAsync(memoryStream).ConfigureAwait(false);
-                    formatter.DecodeBinaryModeEventData(memoryStream.ToArray(), cloudEvent);
                 }
-                return cloudEvent.ValidateForConversion(nameof(httpRequest));
+                return Validation.CheckCloudEventArgument(cloudEvent, nameof(httpRequest));
             }
         }
 

src/CloudNative.CloudEvents.Avro/AvroEventFormatter.cs

@@ -5,6 +5,7 @@
 using Avro;
 using Avro.Generic;
 using Avro.IO;
+using CloudNative.CloudEvents.Core;
 using System;
 using System.Collections.Generic;
 using System.IO;
@@ -53,7 +54,7 @@ namespace CloudNative.CloudEvents
 
         public override CloudEvent DecodeStructuredModeMessage(Stream data, ContentType contentType, IEnumerable<CloudEventAttribute> extensionAttributes)
         {
-            data = data ?? throw new ArgumentNullException(nameof(data));
+            Validation.CheckNotNull(data, nameof(data));
 
             var decoder = new BinaryDecoder(data);
             var rawEvent = avroReader.Read<GenericRecord>(null, decoder);
@@ -62,7 +63,7 @@ namespace CloudNative.CloudEvents
 
         public override CloudEvent DecodeStructuredModeMessage(byte[] data, ContentType contentType, IEnumerable<CloudEventAttribute> extensionAttributes)
         {
-            data = data ?? throw new ArgumentNullException(nameof(data));
+            Validation.CheckNotNull(data, nameof(data));
             return DecodeStructuredModeMessage(new MemoryStream(data), contentType, extensionAttributes);
         }
 
@@ -119,13 +120,12 @@ namespace CloudNative.CloudEvents
                 }
             }
 
-            return cloudEvent.ValidateForConversion(nameof(record));
+            return Validation.CheckCloudEventArgument(cloudEvent, nameof(record));
         }
 
         public override byte[] EncodeStructuredModeMessage(CloudEvent cloudEvent, out ContentType contentType)
         {
-            cloudEvent = cloudEvent ?? throw new ArgumentNullException(nameof(cloudEvent));
+            Validation.CheckCloudEventArgument(cloudEvent, nameof(cloudEvent));
-            cloudEvent.ValidateForConversion(nameof(cloudEvent));
 
             contentType = new ContentType(CloudEvent.MediaType + MediaTypeSuffix);
 

src/CloudNative.CloudEvents.Kafka/KafkaClientExtensions.cs

@@ -3,6 +3,7 @@
 // See LICENSE file in the project root for full license information.
 
 using CloudNative.CloudEvents.Extensions;
+using CloudNative.CloudEvents.Core;
 using Confluent.Kafka;
 using System;
 using System.Collections.Generic;
@@ -50,8 +51,8 @@ namespace CloudNative.CloudEvents.Kafka
         public static CloudEvent ToCloudEvent(this Message<string, byte[]> message,
             CloudEventFormatter formatter, IEnumerable<CloudEventAttribute> extensionAttributes)
         {
-            message = message ?? throw new ArgumentNullException(nameof(message));
+            Validation.CheckNotNull(message, nameof(message));
-            formatter = formatter ?? throw new ArgumentNullException(nameof(formatter));
+            Validation.CheckNotNull(formatter, nameof(formatter));
 
             if (!IsCloudEvent(message))
             {
@@ -104,7 +105,7 @@ namespace CloudNative.CloudEvents.Kafka
             }
 
             InitPartitioningKey(message, cloudEvent);
-            return cloudEvent.ValidateForConversion(nameof(message));
+            return Validation.CheckCloudEventArgument(cloudEvent, nameof(message));
         }
 
         private static string ExtractContentType(Message<string, byte[]> message)
@@ -133,15 +134,11 @@ namespace CloudNative.CloudEvents.Kafka
         /// <param name="formatter">The formatter to use within the conversion. Must not be null.</param>
         public static Message<string, byte[]> ToKafkaMessage(this CloudEvent cloudEvent, ContentMode contentMode, CloudEventFormatter formatter)
         {
-            cloudEvent = cloudEvent ?? throw new ArgumentNullException(nameof(cloudEvent));
+            Validation.CheckCloudEventArgument(cloudEvent, nameof(cloudEvent));
-            cloudEvent.ValidateForConversion(nameof(cloudEvent));
+            Validation.CheckNotNull(formatter, nameof(formatter));
-            formatter = formatter ?? throw new ArgumentNullException(nameof(formatter));
 
             // TODO: Is this appropriate? Why can't we transport a CloudEvent without data in Kafka?
-            if (cloudEvent.Data == null)
+            Validation.CheckArgument(cloudEvent.Data is object, nameof(cloudEvent), "Only CloudEvents with data can be converted to Kafka messages");
-            {
-                throw new ArgumentNullException(nameof(cloudEvent.Data));
-            }
             var headers = MapHeaders(cloudEvent, formatter);
             string key = (string) cloudEvent[Partitioning.PartitionKeyAttribute];
             byte[] value;

src/CloudNative.CloudEvents.Mqtt/MqttClientExtensions.cs

@@ -2,6 +2,7 @@
 // Licensed under the Apache 2.0 license.
 // See LICENSE file in the project root for full license information.
 
+using CloudNative.CloudEvents.Core;
 using MQTTnet;
 using System;
 using System.Collections.Generic;
@@ -34,8 +35,8 @@ namespace CloudNative.CloudEvents.Mqtt
         public static CloudEvent ToCloudEvent(this MqttApplicationMessage message,
             CloudEventFormatter formatter, IEnumerable<CloudEventAttribute> extensionAttributes)
         {
-            message = message ?? throw new ArgumentNullException(nameof(message));
+            Validation.CheckNotNull(formatter, nameof(formatter));
-            formatter = formatter ?? throw new ArgumentNullException(nameof(formatter));
+            Validation.CheckNotNull(message, nameof(message));
 
             // TODO: Determine if there's a sensible content type we should apply.
             return formatter.DecodeStructuredModeMessage(message.Payload, contentType: null, extensionAttributes);
@@ -51,9 +52,8 @@ namespace CloudNative.CloudEvents.Mqtt
         /// <param name="topic">The MQTT topic for the message. May be null.</param>
         public static MqttApplicationMessage ToMqttApplicationMessage(this CloudEvent cloudEvent, ContentMode contentMode, CloudEventFormatter formatter, string topic)
         {
-            cloudEvent = cloudEvent ?? throw new ArgumentNullException(nameof(cloudEvent));
+            Validation.CheckCloudEventArgument(cloudEvent, nameof(cloudEvent));
-            cloudEvent.ValidateForConversion(nameof(cloudEvent));
+            Validation.CheckNotNull(formatter, nameof(formatter));
-            formatter = formatter ?? throw new ArgumentNullException(nameof(formatter));
 
             switch (contentMode)
             {

src/CloudNative.CloudEvents.NewtonsoftJson/JsonEventFormatter.cs

@@ -2,6 +2,7 @@
 // Licensed under the Apache 2.0 license.
 // See LICENSE file in the project root for full license information.
 
+using CloudNative.CloudEvents.Core;
 using Newtonsoft.Json;
 using Newtonsoft.Json.Linq;
 using System;
@@ -112,23 +113,23 @@ namespace CloudNative.CloudEvents.NewtonsoftJson
         /// </summary>
         public JsonEventFormatter(JsonSerializer serializer)
         {
-            Serializer = serializer ?? throw new ArgumentNullException(nameof(serializer));
+            Serializer = Validation.CheckNotNull(serializer, nameof(serializer));
         }
 
         public override async Task<CloudEvent> DecodeStructuredModeMessageAsync(Stream data, ContentType contentType, IEnumerable<CloudEventAttribute> extensionAttributes)
         {
-            data = data ?? throw new ArgumentNullException(nameof(data));
+            Validation.CheckNotNull(data, nameof(data));
 
-            var jsonReader = CreateJsonReader(data, contentType.GetEncoding());
+            var jsonReader = CreateJsonReader(data, MimeUtilities.GetEncoding(contentType));
             var jObject = await JObject.LoadAsync(jsonReader).ConfigureAwait(false);
             return DecodeJObject(jObject, extensionAttributes);
         }
 
         public override CloudEvent DecodeStructuredModeMessage(Stream data, ContentType contentType, IEnumerable<CloudEventAttribute> extensionAttributes)
         {
-            data = data ?? throw new ArgumentNullException(nameof(data));
+            Validation.CheckNotNull(data, nameof(data));
 
-            var jsonReader = CreateJsonReader(data, contentType.GetEncoding());
+            var jsonReader = CreateJsonReader(data, MimeUtilities.GetEncoding(contentType));
             var jObject = JObject.Load(jsonReader);
             return DecodeJObject(jObject, extensionAttributes);
         }
@@ -151,7 +152,7 @@ namespace CloudNative.CloudEvents.NewtonsoftJson
             PopulateDataFromStructuredEvent(cloudEvent, jObject);
             // "data" is always the parameter from the public method. It's annoying not to be able to use
             // nameof here, but this will give the appropriate result.
-            return cloudEvent.ValidateForConversion("data");
+            return Validation.CheckCloudEventArgument(cloudEvent, "data");
         }
 
         private void PopulateAttributesFromStructuredEvent(CloudEvent cloudEvent, JObject jObject)
@@ -306,8 +307,7 @@ namespace CloudNative.CloudEvents.NewtonsoftJson
 
         public override byte[] EncodeStructuredModeMessage(CloudEvent cloudEvent, out ContentType contentType)
         {
-            cloudEvent = cloudEvent ?? throw new ArgumentNullException(nameof(cloudEvent));
+            Validation.CheckCloudEventArgument(cloudEvent, nameof(cloudEvent));
-            cloudEvent.ValidateForConversion(nameof(cloudEvent));
 
             contentType = new ContentType("application/cloudevents+json")
             {
@@ -389,8 +389,7 @@ namespace CloudNative.CloudEvents.NewtonsoftJson
 
         public override byte[] EncodeBinaryModeEventData(CloudEvent cloudEvent)
         {
-            cloudEvent = cloudEvent ?? throw new ArgumentNullException(nameof(cloudEvent));
+            Validation.CheckCloudEventArgument(cloudEvent, nameof(cloudEvent));
-            cloudEvent.ValidateForConversion(nameof(cloudEvent));
 
             if (cloudEvent.Data is null)
             {
@@ -405,11 +404,11 @@ namespace CloudNative.CloudEvents.NewtonsoftJson
                 // without a preamble (or rewrite StreamWriter...)
                 var stringWriter = new StringWriter();
                 Serializer.Serialize(stringWriter, cloudEvent.Data);
-                return contentType.GetEncoding().GetBytes(stringWriter.ToString());
+                return MimeUtilities.GetEncoding(contentType).GetBytes(stringWriter.ToString());
             }
             if (contentType.MediaType.StartsWith("text/") && cloudEvent.Data is string text)
             {
-                return contentType.GetEncoding().GetBytes(text);
+                return MimeUtilities.GetEncoding(contentType).GetBytes(text);
             }
             if (cloudEvent.Data is byte[] bytes)
             {
@@ -420,12 +419,12 @@ namespace CloudNative.CloudEvents.NewtonsoftJson
 
         public override void DecodeBinaryModeEventData(byte[] value, CloudEvent cloudEvent)
         {
-            value = value ?? throw new ArgumentNullException(nameof(value));
+            Validation.CheckNotNull(value, nameof(value));
-            cloudEvent = cloudEvent ?? throw new ArgumentNullException(nameof(cloudEvent));
+            Validation.CheckNotNull(cloudEvent, nameof(cloudEvent));
 
             ContentType contentType = new ContentType(cloudEvent.DataContentType ?? JsonMediaType);
 
-            Encoding encoding = contentType.GetEncoding();
+            Encoding encoding = MimeUtilities.GetEncoding(contentType);
 
             if (contentType.MediaType == JsonMediaType)
             {

src/CloudNative.CloudEvents.SystemTextJson/JsonEventFormatter.cs

@@ -2,6 +2,7 @@
 // Licensed under the Apache 2.0 license.
 // See LICENSE file in the project root for full license information.
 
+using CloudNative.CloudEvents.Core;
 using System;
 using System.Collections.Generic;
 using System.IO;
@@ -119,8 +120,9 @@ namespace CloudNative.CloudEvents.SystemTextJson
 
         private async Task<CloudEvent> DecodeStructuredModeMessageImpl(Stream data, ContentType contentType, IEnumerable<CloudEventAttribute> extensionAttributes, bool async)
         {
-            data = data ?? throw new ArgumentNullException(nameof(data));
+            Validation.CheckNotNull(data, nameof(data));
-            var encoding = contentType.GetEncoding();
+
+            var encoding = MimeUtilities.GetEncoding(contentType);
             JsonDocument document;
             if (encoding is UTF8Encoding)
             {
@@ -157,7 +159,7 @@ namespace CloudNative.CloudEvents.SystemTextJson
             PopulateDataFromStructuredEvent(cloudEvent, document);
             // "data" is always the parameter from the public method. It's annoying not to be able to use
             // nameof here, but this will give the appropriate result.
-            return cloudEvent.ValidateForConversion("data");
+            return Validation.CheckCloudEventArgument(cloudEvent, "data");
         }
 
         private void PopulateAttributesFromStructuredEvent(CloudEvent cloudEvent, JsonDocument document)
@@ -321,8 +323,7 @@ namespace CloudNative.CloudEvents.SystemTextJson
 
         public override byte[] EncodeStructuredModeMessage(CloudEvent cloudEvent, out ContentType contentType)
         {
-            cloudEvent = cloudEvent ?? throw new ArgumentNullException(nameof(cloudEvent));
+            Validation.CheckCloudEventArgument(cloudEvent, nameof(cloudEvent));
-            cloudEvent.ValidateForConversion(nameof(cloudEvent));
 
             contentType = new ContentType("application/cloudevents+json")
             {
@@ -404,8 +405,7 @@ namespace CloudNative.CloudEvents.SystemTextJson
 
         public override byte[] EncodeBinaryModeEventData(CloudEvent cloudEvent)
         {
-            cloudEvent = cloudEvent ?? throw new ArgumentNullException(nameof(cloudEvent));
+            Validation.CheckCloudEventArgument(cloudEvent, nameof(cloudEvent));
-            cloudEvent.ValidateForConversion(nameof(cloudEvent));
 
             if (cloudEvent.Data is null)
             {
@@ -414,19 +414,19 @@ namespace CloudNative.CloudEvents.SystemTextJson
             ContentType contentType = new ContentType(cloudEvent.DataContentType ?? JsonMediaType);
             if (contentType.MediaType == JsonMediaType)
             {
-                var encoding = contentType.GetEncoding();
+                var encoding = MimeUtilities.GetEncoding(contentType);
                 if (encoding is UTF8Encoding)
                 {
                     return JsonSerializer.SerializeToUtf8Bytes(cloudEvent.Data, SerializerOptions);
                 }
                 else
                 {
-                    return contentType.GetEncoding().GetBytes(JsonSerializer.Serialize(cloudEvent.Data, SerializerOptions));
+                    return MimeUtilities.GetEncoding(contentType).GetBytes(JsonSerializer.Serialize(cloudEvent.Data, SerializerOptions));
                 }
             }
             if (contentType.MediaType.StartsWith("text/") && cloudEvent.Data is string text)
             {
-                return contentType.GetEncoding().GetBytes(text);
+                return MimeUtilities.GetEncoding(contentType).GetBytes(text);
             }
             if (cloudEvent.Data is byte[] bytes)
             {
@@ -437,12 +437,12 @@ namespace CloudNative.CloudEvents.SystemTextJson
 
         public override void DecodeBinaryModeEventData(byte[] value, CloudEvent cloudEvent)
         {
-            value = value ?? throw new ArgumentNullException(nameof(value));
+            Validation.CheckNotNull(value, nameof(value));
-            cloudEvent = cloudEvent ?? throw new ArgumentNullException(nameof(cloudEvent));
+            Validation.CheckNotNull(cloudEvent, nameof(cloudEvent));
 
             ContentType contentType = new ContentType(cloudEvent.DataContentType ?? JsonMediaType);
 
-            Encoding encoding = contentType.GetEncoding();
+            Encoding encoding = MimeUtilities.GetEncoding(contentType);
 
             if (contentType.MediaType == JsonMediaType)
             {

src/CloudNative.CloudEvents/CloudEvent.cs

@@ -2,6 +2,7 @@
 // Licensed under the Apache 2.0 license.
 // See LICENSE file in the project root for full license information.
 
+using CloudNative.CloudEvents.Core;
 using System;
 using System.Collections.Generic;
 using System.Linq;
@@ -62,33 +63,32 @@ namespace CloudNative.CloudEvents
         /// to an empty sequence.</param>
         public CloudEvent(CloudEventsSpecVersion specVersion, IEnumerable<CloudEventAttribute> extensionAttributes)
         {
-            // TODO: Validate that all attributes are extension attributes.
             // TODO: Work out how to be more efficient, e.g. not creating a dictionary at all if there are no
             // extension attributes.
-            SpecVersion = Preconditions.CheckNotNull(specVersion, nameof(specVersion));
+            SpecVersion = Validation.CheckNotNull(specVersion, nameof(specVersion));
             if (extensionAttributes is object)
             {
                 foreach (var extension in extensionAttributes)
                 {
-                    Preconditions.CheckArgument(
+                    Validation.CheckArgument(
                         extension is object,
                         nameof(extensionAttributes),
                         "Extension attribute collection cannot contain null elements");
-                    Preconditions.CheckArgument(
+                    Validation.CheckArgument(
                         extension.Name != CloudEventsSpecVersion.SpecVersionAttributeName,
                         nameof(extensionAttributes),
                         "The 'specversion' attribute cannot be specified as an extension attribute");
-                    Preconditions.CheckArgument(
+                    Validation.CheckArgument(
                         SpecVersion.GetAttributeByName(extension.Name) is null,
                         nameof(extensionAttributes),
                         "'{0}' cannot be specified as the name of an extension attribute; it is already a context attribute",
                         extension.Name);
-                    Preconditions.CheckArgument(
+                    Validation.CheckArgument(
                         extension.IsExtension,
                         nameof(extensionAttributes),
                         "'{0}' is not an extension attribute",
                         extension.Name);
-                    Preconditions.CheckArgument(
+                    Validation.CheckArgument(
                         !this.extensionAttributes.ContainsKey(extension.Name),
                         nameof(extensionAttributes),
                         "'{0}' cannot be specified more than once as an extension attribute");
@@ -123,8 +123,8 @@ namespace CloudNative.CloudEvents
         {
             get
             {
-                Preconditions.CheckNotNull(attribute, nameof(attribute));
+                Validation.CheckNotNull(attribute, nameof(attribute));
-                Preconditions.CheckArgument(attribute.Name != CloudEventsSpecVersion.SpecVersionAttributeName, nameof(attribute), Strings.ErrorCannotIndexBySpecVersionAttribute);
+                Validation.CheckArgument(attribute.Name != CloudEventsSpecVersion.SpecVersionAttributeName, nameof(attribute), Strings.ErrorCannotIndexBySpecVersionAttribute);
 
                 // TODO: Is this validation definitely useful? It does mean we never return something
                 // that's invalid for the attribute, which is potentially good...
@@ -137,14 +137,14 @@ namespace CloudNative.CloudEvents
             }
             set
             {
-                Preconditions.CheckNotNull(attribute, nameof(attribute));
+                Validation.CheckNotNull(attribute, nameof(attribute));
-                Preconditions.CheckArgument(attribute.Name != CloudEventsSpecVersion.SpecVersionAttributeName, nameof(attribute), Strings.ErrorCannotIndexBySpecVersionAttribute);
+                Validation.CheckArgument(attribute.Name != CloudEventsSpecVersion.SpecVersionAttributeName, nameof(attribute), Strings.ErrorCannotIndexBySpecVersionAttribute);
 
                 string name = attribute.Name;
                 var knownAttribute = GetAttribute(name);
 
                 // TODO: Are we happy to add the extension in even if the value is null?
-                Preconditions.CheckArgument(knownAttribute is object || attribute.IsExtension,
+                Validation.CheckArgument(knownAttribute is object || attribute.IsExtension,
                     nameof(attribute),
                     "Cannot add an unknown non-extension attribute to an event.");
 
@@ -178,14 +178,14 @@ namespace CloudNative.CloudEvents
             get
             {
                 // TODO: Validate the attribute name is valid (e.g. not upper case)? Seems overkill.
-                Preconditions.CheckNotNull(attributeName, nameof(attributeName));
+                Validation.CheckNotNull(attributeName, nameof(attributeName));
-                Preconditions.CheckArgument(attributeName != CloudEventsSpecVersion.SpecVersionAttributeName, nameof(attributeName), Strings.ErrorCannotIndexBySpecVersionAttribute);
+                Validation.CheckArgument(attributeName != CloudEventsSpecVersion.SpecVersionAttributeName, nameof(attributeName), Strings.ErrorCannotIndexBySpecVersionAttribute);
-                return attributeValues.GetValueOrDefault(Preconditions.CheckNotNull(attributeName, nameof(attributeName)));
+                return attributeValues.GetValueOrDefault(Validation.CheckNotNull(attributeName, nameof(attributeName)));
             }            
             set
             {
-                Preconditions.CheckNotNull(attributeName, nameof(attributeName));
+                Validation.CheckNotNull(attributeName, nameof(attributeName));
-                Preconditions.CheckArgument(attributeName != CloudEventsSpecVersion.SpecVersionAttributeName, nameof(attributeName), Strings.ErrorCannotIndexBySpecVersionAttribute);
+                Validation.CheckArgument(attributeName != CloudEventsSpecVersion.SpecVersionAttributeName, nameof(attributeName), Strings.ErrorCannotIndexBySpecVersionAttribute);
 
                 var knownAttribute = GetAttribute(attributeName);
 
@@ -193,7 +193,7 @@ namespace CloudNative.CloudEvents
                 // (It's a simple way of populating extensions after the fact...)
                 if (knownAttribute is null)
                 {
-                    Preconditions.CheckArgument(value is null || value is string,
+                    Validation.CheckArgument(value is null || value is string,
                         nameof(value), "Cannot assign value of type {0} to unknown attribute '{1}'",
                         value.GetType(), attributeName);
                     knownAttribute = CloudEventAttribute.CreateExtension(attributeName, CloudEventAttributeType.String);
@@ -352,8 +352,8 @@ namespace CloudNative.CloudEvents
         /// <param name="value">The value of the attribute to set. Must not be null.</param>
         public void SetAttributeFromString(string name, string value)
         {
-            Preconditions.CheckNotNull(name, nameof(name));
+            Validation.CheckNotNull(name, nameof(name));
-            Preconditions.CheckNotNull(value, nameof(value));
+            Validation.CheckNotNull(value, nameof(value));
 
             var attribute = GetAttribute(name);
             if (attribute is null)
@@ -385,29 +385,6 @@ namespace CloudNative.CloudEvents
             throw new InvalidOperationException($"Missing required attributes: {joinedMissing}");
         }
 
-        // TODO: consider moving this to an extension method in an implementation helper namespace?
-
-        /// <summary>
-        /// Validates that this CloudEvent is valid in the same way as <see cref="IsValid"/>,
-        /// but throwing an <see cref="ArgumentException"/> using the given parameter name
-        /// if the event is invalid. This is typically used within protocol bindings or event formatters
-        /// as the last step in decoding an event, or as the first step when encoding an event.
-        /// </summary>
-        /// <param name="paramName">The parameter name to use in the exception if the event is invalid.
-        /// May be null.</param>
-        /// <exception cref="ArgumentException">The event is invalid.</exception>
-        /// <returns>A reference to the same object, for simplicity of method chaining.</returns>
-        public CloudEvent ValidateForConversion(string paramName)
-        {
-            if (IsValid)
-            {
-                return this;
-            }
-            var missing = SpecVersion.RequiredAttributes.Where(attr => this[attr] is null).ToList();
-            string joinedMissing = string.Join(", ", missing);
-            throw new ArgumentException($"CloudEvent is missing required attributes: {joinedMissing}", paramName);
-        }
-
         /// <summary>
         /// Returns whether this CloudEvent is valid, i.e. whether all required attributes have
         /// values.

src/CloudNative.CloudEvents/CloudEventAttribute.cs

@@ -2,6 +2,7 @@
 // Licensed under the Apache 2.0 license.
 // See LICENSE file in the project root for full license information.
 
+using CloudNative.CloudEvents.Core;
 using System;
 
 namespace CloudNative.CloudEvents
@@ -44,7 +45,7 @@ namespace CloudNative.CloudEvents
         // TODO: Have a "mode" of Required/Optional/Extension?
 
         private CloudEventAttribute(string name, CloudEventAttributeType type, bool required, bool extension, Action<object> validator) =>
-            (Name, Type, IsRequired, IsExtension, this.validator) = (ValidateName(name), Preconditions.CheckNotNull(type, nameof(type)), required, extension, validator);
+            (Name, Type, IsRequired, IsExtension, this.validator) = (ValidateName(name), Validation.CheckNotNull(type, nameof(type)), required, extension, validator);
 
         internal static CloudEventAttribute CreateRequired(string name, CloudEventAttributeType type, Action<object> validator) =>
             new CloudEventAttribute(name, type, required: true, extension: false, validator: validator);
@@ -87,7 +88,7 @@ namespace CloudNative.CloudEvents
         /// <returns><paramref name="name"/>, for convenience.</returns>
         internal static string ValidateName(string name)
         {
-            Preconditions.CheckNotNull(name, nameof(name));
+            Validation.CheckNotNull(name, nameof(name));
             if (name.Length == 0)
             {
                 throw new ArgumentException("Attribute names must be non-empty", nameof(name));
@@ -106,7 +107,7 @@ namespace CloudNative.CloudEvents
 
         public object Parse(string text)
         {
-            Preconditions.CheckNotNull(text, nameof(text));
+            Validation.CheckNotNull(text, nameof(text));
             object value;
             // By wrapping every exception here, we always get an
             // ArgumentException (other than the ArgumentNullException above) and have the name in the message.
@@ -132,7 +133,7 @@ namespace CloudNative.CloudEvents
         /// <returns>The value, for simple method chaining.</returns>
         public object Validate(object value)
         {
-            Preconditions.CheckNotNull(value, nameof(value));
+            Validation.CheckNotNull(value, nameof(value));
             // By wrapping every exception, whether from the type or the custom validator, we always get an
             // ArgumentException (other than the ArgumentNullException above) and have the name in the message.
             try

src/CloudNative.CloudEvents/CloudEventAttributeType.cs

@@ -2,6 +2,7 @@
 // Licensed under the Apache 2.0 license.
 // See LICENSE file in the project root for full license information.
 
+using CloudNative.CloudEvents.Core;
 using System;
 using System.Globalization;
 
@@ -98,7 +99,7 @@ namespace CloudNative.CloudEvents
             {
             }
 
-            public override sealed object Parse(string value) => ParseImpl(Preconditions.CheckNotNull(value, nameof(value)));
+            public override sealed object Parse(string value) => ParseImpl(Validation.CheckNotNull(value, nameof(value)));
 
             public override sealed string Format(object value)
             {
@@ -109,13 +110,13 @@ namespace CloudNative.CloudEvents
 
             public override sealed void Validate(object value)
             {
-                Preconditions.CheckNotNull(value, nameof(value));
+                Validation.CheckNotNull(value, nameof(value));
                 if (!ClrType.IsInstanceOfType(value))
                 {
                     throw new ArgumentException($"Value of type {value.GetType()} is incompatible with expected type {ClrType}", nameof(value));
                 }
 
-                ValidateImpl((T)Preconditions.CheckNotNull(value, nameof(value)));
+                ValidateImpl((T)Validation.CheckNotNull(value, nameof(value)));
             }
 
             protected abstract T ParseImpl(string value);

src/CloudNative.CloudEvents/CloudEventFormatter.cs

@@ -2,6 +2,7 @@
 // Licensed under the Apache 2.0 license.
 // See LICENSE file in the project root for full license information.
 
+using CloudNative.CloudEvents.Core;
 using System;
 using System.Collections.Generic;
 using System.IO;

src/CloudNative.CloudEvents/Http/HttpClientExtensions.cs

@@ -2,6 +2,7 @@
 // Licensed under the Apache 2.0 license.
 // See LICENSE file in the project root for full license information.
 
+using CloudNative.CloudEvents.Core;
 using System;
 using System.Collections.Generic;
 using System.Linq;
@@ -60,7 +61,7 @@ namespace CloudNative.CloudEvents.Http
         /// <returns>true, if the response is a CloudEvent</returns>
         public static bool IsCloudEvent(this HttpRequestMessage httpRequestMessage)
         {
-            Preconditions.CheckNotNull(httpRequestMessage, nameof(httpRequestMessage));
+            Validation.CheckNotNull(httpRequestMessage, nameof(httpRequestMessage));
             return HasCloudEventsContentType(httpRequestMessage.Content) ||
                 httpRequestMessage.Headers.Contains(HttpUtilities.SpecVersionHttpHeader);
         }
@@ -72,7 +73,7 @@ namespace CloudNative.CloudEvents.Http
         /// <returns>true, if the response is a CloudEvent</returns>
         public static bool IsCloudEvent(this HttpResponseMessage httpResponseMessage)
         {
-            Preconditions.CheckNotNull(httpResponseMessage, nameof(httpResponseMessage));
+            Validation.CheckNotNull(httpResponseMessage, nameof(httpResponseMessage));
             return HasCloudEventsContentType(httpResponseMessage.Content) ||
                 httpResponseMessage.Headers.Contains(HttpUtilities.SpecVersionHttpHeader);
         }
@@ -109,7 +110,7 @@ namespace CloudNative.CloudEvents.Http
             CloudEventFormatter formatter,
             IEnumerable<CloudEventAttribute> extensionAttributes)
         {
-            Preconditions.CheckNotNull(httpResponseMessage, nameof(httpResponseMessage));
+            Validation.CheckNotNull(httpResponseMessage, nameof(httpResponseMessage));
             return ToCloudEventInternalAsync(httpResponseMessage.Headers, httpResponseMessage.Content, formatter, extensionAttributes, nameof(httpResponseMessage));
         }
 
@@ -138,19 +139,19 @@ namespace CloudNative.CloudEvents.Http
             CloudEventFormatter formatter,
             IEnumerable<CloudEventAttribute> extensionAttributes)
         {
-            Preconditions.CheckNotNull(httpRequestMessage, nameof(httpRequestMessage));
+            Validation.CheckNotNull(httpRequestMessage, nameof(httpRequestMessage));
             return ToCloudEventInternalAsync(httpRequestMessage.Headers, httpRequestMessage.Content, formatter, extensionAttributes, nameof(httpRequestMessage));
         }
 
         private static async Task<CloudEvent> ToCloudEventInternalAsync(HttpHeaders headers, HttpContent content,
             CloudEventFormatter formatter, IEnumerable<CloudEventAttribute> extensionAttributes, string paramName)
         {
-            Preconditions.CheckNotNull(formatter, nameof(formatter));
+            Validation.CheckNotNull(formatter, nameof(formatter));
 
             if (HasCloudEventsContentType(content))
             {
                 var stream = await content.ReadAsStreamAsync().ConfigureAwait(false);
-                return await formatter.DecodeStructuredModeMessageAsync(stream, content.Headers.ContentType.ToContentType(), extensionAttributes).ConfigureAwait(false);
+                return await formatter.DecodeStructuredModeMessageAsync(stream, MimeUtilities.ToContentType(content.Headers.ContentType), extensionAttributes).ConfigureAwait(false);
             }
             else
             {
@@ -179,7 +180,7 @@ namespace CloudNative.CloudEvents.Http
                     var data = await content.ReadAsByteArrayAsync().ConfigureAwait(false);
                     formatter.DecodeBinaryModeEventData(data, cloudEvent);
                 }
-                return cloudEvent.ValidateForConversion(paramName);
+                return Validation.CheckCloudEventArgument(cloudEvent, paramName);
             }
         }
 

src/CloudNative.CloudEvents/Http/HttpContentExtensions.cs

@@ -2,6 +2,7 @@
 // Licensed under the Apache 2.0 license.
 // See LICENSE file in the project root for full license information.
 
+using CloudNative.CloudEvents.Core;
 using System;
 using System.Net.Http;
 using System.Net.Mime;
@@ -21,9 +22,8 @@ namespace CloudNative.CloudEvents.Http
         /// <param name="formatter">The formatter to use within the conversion. Must not be null.</param>
         public static HttpContent ToHttpContent(this CloudEvent cloudEvent, ContentMode contentMode, CloudEventFormatter formatter)
         {
-            Preconditions.CheckNotNull(cloudEvent, nameof(cloudEvent));
+            Validation.CheckCloudEventArgument(cloudEvent, nameof(cloudEvent));
-            cloudEvent.ValidateForConversion(nameof(cloudEvent));
+            Validation.CheckNotNull(formatter, nameof(formatter));
-            Preconditions.CheckNotNull(formatter, nameof(formatter));
 
             byte[] content;
             // The content type to include in the ContentType header - may be the data content type, or the formatter's content type.
@@ -43,7 +43,7 @@ namespace CloudNative.CloudEvents.Http
             var ret = new ByteArrayContent(content);
             if (contentType is object)
             {
-                ret.Headers.ContentType = contentType.ToMediaTypeHeaderValue();
+                ret.Headers.ContentType = MimeUtilities.ToMediaTypeHeaderValue(contentType);
             }
             else if (content.Length != 0)
             {

src/CloudNative.CloudEvents/Http/HttpListenerExtensions.cs

@@ -2,6 +2,7 @@
 // Licensed under the Apache 2.0 license.
 // See LICENSE file in the project root for full license information.
 
+using CloudNative.CloudEvents.Core;
 using System;
 using System.Collections.Generic;
 using System.Net;
@@ -27,10 +28,9 @@ namespace CloudNative.CloudEvents.Http
         public static Task CopyToHttpListenerResponseAsync(this CloudEvent cloudEvent, HttpListenerResponse destination,
             ContentMode contentMode, CloudEventFormatter formatter)
         {
-            Preconditions.CheckNotNull(cloudEvent, nameof(cloudEvent));
+            Validation.CheckCloudEventArgument(cloudEvent, nameof(cloudEvent));
-            cloudEvent.ValidateForConversion(nameof(cloudEvent));
+            Validation.CheckNotNull(destination, nameof(destination));
-            Preconditions.CheckNotNull(destination, nameof(destination));
+            Validation.CheckNotNull(formatter, nameof(formatter));
-            Preconditions.CheckNotNull(formatter, nameof(formatter));
 
             byte[] content;
             ContentType contentType;
@@ -142,8 +142,8 @@ namespace CloudNative.CloudEvents.Http
         public static CloudEvent ToCloudEvent(this HttpListenerRequest httpListenerRequest,
             CloudEventFormatter formatter, IEnumerable<CloudEventAttribute> extensionAttributes)
         {
-            Preconditions.CheckNotNull(httpListenerRequest, nameof(httpListenerRequest));
+            Validation.CheckNotNull(httpListenerRequest, nameof(httpListenerRequest));
-            Preconditions.CheckNotNull(formatter, nameof(formatter));
+            Validation.CheckNotNull(formatter, nameof(formatter));
 
             if (HasCloudEventsContentType(httpListenerRequest))
             {
@@ -176,7 +176,7 @@ namespace CloudNative.CloudEvents.Http
                 cloudEvent.DataContentType = httpListenerRequest.ContentType;
 
                 formatter.DecodeBinaryModeEventData(BinaryDataUtilities.ToByteArray(httpListenerRequest.InputStream), cloudEvent);
-                return cloudEvent.ValidateForConversion(nameof(httpListenerRequest));
+                return Validation.CheckCloudEventArgument(cloudEvent, nameof(httpListenerRequest));
             }
         }
 

src/CloudNative.CloudEvents/Http/HttpWebExtensions.cs

@@ -2,6 +2,7 @@
 // Licensed under the Apache 2.0 license.
 // See LICENSE file in the project root for full license information.
 
+using CloudNative.CloudEvents.Core;
 using System;
 using System.Net;
 using System.Net.Mime;
@@ -28,10 +29,9 @@ namespace CloudNative.CloudEvents.Http
         public static async Task CopyToHttpWebRequestAsync(this CloudEvent cloudEvent, HttpWebRequest destination,
             ContentMode contentMode, CloudEventFormatter formatter)
         {
-            Preconditions.CheckNotNull(cloudEvent, nameof(cloudEvent));
+            Validation.CheckCloudEventArgument(cloudEvent, nameof(cloudEvent));
-            cloudEvent.ValidateForConversion(nameof(cloudEvent));
+            Validation.CheckNotNull(destination, nameof(destination));
-            Preconditions.CheckNotNull(destination, nameof(destination));
+            Validation.CheckNotNull(formatter, nameof(formatter));
-            Preconditions.CheckNotNull(formatter, nameof(formatter));
 
             byte[] content;
             // The content type to include in the ContentType header - may be the data content type, or the formatter's content type.

src/CloudNative.CloudEvents/Impl/BinaryDataUtilities.cs

@@ -5,15 +5,15 @@
 using System.IO;
 using System.Threading.Tasks;
 
-namespace CloudNative.CloudEvents
+namespace CloudNative.CloudEvents.Core
 {
     /// <summary>
     /// Utilities methods for dealing with binary data, converting between
     /// streams, arrays, Memory{T} etc.
     /// </summary>
-    internal static class BinaryDataUtilities
+    public static class BinaryDataUtilities
     {
-        internal async static Task<byte[]> ToByteArrayAsync(Stream stream)
+        public async static Task<byte[]> ToByteArrayAsync(Stream stream)
         {
             // TODO: Optimize if it's already a MemoryStream?
             var memory = new MemoryStream();
@@ -21,7 +21,7 @@ namespace CloudNative.CloudEvents
             return memory.ToArray();
         }
 
-        internal static byte[] ToByteArray(Stream stream)
+        public static byte[] ToByteArray(Stream stream)
         {
             var memory = new MemoryStream();
             stream.CopyTo(memory);

src/CloudNative.CloudEvents/Impl/MimeUtilities.cs

@@ -6,13 +6,13 @@ using System.Net.Http.Headers;
 using System.Net.Mime;
 using System.Text;
 
-namespace CloudNative.CloudEvents
+namespace CloudNative.CloudEvents.Core
 {
     // TODO: Consider this name and namespace carefully. It really does need to be public, as all the event formatters are elsewhere.
     // But it's not ideal...
 
     /// <summary>
-    /// Utility and extension methods around MIME.
+    /// Utility methods around MIME.
     /// </summary>
     public static class MimeUtilities
     {
@@ -23,7 +23,7 @@ namespace CloudNative.CloudEvents
         /// <param name="contentType">The content type, or null if no content type is known.</param>
         /// <returns>An encoding suitable for the charset specified in <paramref name="contentType"/>,
         /// or UTF-8 if no charset has been specified.</returns>
-        public static Encoding GetEncoding(this ContentType contentType) =>
+        public static Encoding GetEncoding(ContentType contentType) =>
             contentType?.CharSet is string charSet ? Encoding.GetEncoding(charSet) : Encoding.UTF8;
 
         /// <summary>
@@ -31,7 +31,7 @@ namespace CloudNative.CloudEvents
         /// </summary>
         /// <param name="headerValue">The header value to convert. May be null.</param>
         /// <returns>The converted content type, or null if <paramref name="headerValue"/> is null.</returns>
-        public static ContentType ToContentType(this MediaTypeHeaderValue headerValue) =>
+        public static ContentType ToContentType(MediaTypeHeaderValue headerValue) =>
             headerValue is null ? null : new ContentType(headerValue.ToString());
 
         /// <summary>
@@ -39,7 +39,7 @@ namespace CloudNative.CloudEvents
         /// </summary>
         /// <param name="contentType">The content type to convert. May be null.</param>
         /// <returns>The converted media type header value, or null if <paramref name="contentType"/> is null.</returns>
-        public static MediaTypeHeaderValue ToMediaTypeHeaderValue(this ContentType contentType)
+        public static MediaTypeHeaderValue ToMediaTypeHeaderValue(ContentType contentType)
         {
             if (contentType is null)
             {

src/CloudNative.CloudEvents/Impl/Validation.cs

@@ -0,0 +1,98 @@
+// Copyright 2021 Cloud Native Foundation. 
+// Licensed under the Apache 2.0 license.
+// See LICENSE file in the project root for full license information.
+
+using System;
+using System.Linq;
+
+namespace CloudNative.CloudEvents.Core
+{
+    /// <summary>
+    /// Validation methods which are typically convenient for implementers of CloudEvent formatters
+    /// and protocol bindings.
+    /// </summary>
+    public static class Validation
+    {
+        /// <summary>
+        /// Validates that the given reference is non-null.
+        /// </summary>
+        /// <typeparam name="T">Type of the value to check</typeparam>
+        /// <param name="value">The reference to check for nullity</param>
+        /// <param name="paramName">The parameter name to use in the exception if <paramref name="value"/> is null.
+        /// May be null.</param>
+        /// <returns>The value of <paramref name="value"/>, for convenient method chaining or assignment.</returns>
+        public static T CheckNotNull<T>(T value, string paramName) where T : class =>
+            value ?? throw new ArgumentNullException(paramName);
+
+        /// <summary>
+        /// Validates an argument-dependent condition, throwing an exception if the check fails.
+        /// </summary>
+        /// <param name="condition">The condition to validate; this method will throw an <see cref="ArgumentException"/> if this is false.</param>
+        /// <param name="paramName">The name of the parameter being validated. May be null.</param>
+        /// <param name="message">The message to use in the exception, if one is thrown.</param>
+        public static void CheckArgument(bool condition, string paramName, string message)
+        {
+            if (!condition)
+            {
+                throw new ArgumentException(message, paramName);
+            }
+        }
+
+        /// <summary>
+        /// Validates an argument-dependent condition, throwing an exception if the check fails.
+        /// </summary>
+        /// <param name="condition">The condition to validate; this method will throw an <see cref="ArgumentException"/> if this is false.</param>
+        /// <param name="paramName">The name of the parameter being validated. May be null.</param>
+        /// <param name="messageFormat">The string format to use in the exception message, if one is thrown.</param>
+        /// <param name="arg1">The first argument in the string format.</param>
+        public static void CheckArgument(bool condition, string paramName, string messageFormat,
+            object arg1)
+        {
+            if (!condition)
+            {
+                throw new ArgumentException(string.Format(messageFormat, arg1), paramName);
+            }
+        }
+
+        /// <summary>
+        /// Validates an argument-dependent condition, throwing an exception if the check fails.
+        /// </summary>
+        /// <param name="condition">The condition to validate; this method will throw an <see cref="ArgumentException"/> if this is false.</param>
+        /// <param name="paramName">The name of the parameter being validated. May be null.</param>
+        /// <param name="messageFormat">The string format to use in the exception message, if one is thrown.</param>
+        /// <param name="arg1">The first argument in the string format.</param>
+        /// <param name="arg2">The first argument in the string format.</param>
+        public static void CheckArgument(bool condition, string paramName, string messageFormat,
+            object arg1, object arg2)
+        {
+            if (!condition)
+            {
+                throw new ArgumentException(string.Format(messageFormat, arg1, arg2), paramName);
+            }
+        }
+
+        /// <summary>
+        /// Validates that the specified CloudEvent is valid in the same way as <see cref="CloudEvent.IsValid"/>,
+        /// but throwing an <see cref="ArgumentException"/> using the given parameter name
+        /// if the event is invalid. This is typically used within protocol bindings or event formatters
+        /// as the last step in decoding an event, or as the first step when encoding an event.
+        /// </summary
+        /// <param name="cloudEvent">The event to validate.
+        /// <param name="paramName">The parameter name to use in the exception if <paramref name="cloudEvent"/> is null or invalid.
+        /// May be null.</param>
+        /// <exception cref="ArgumentNullException"><paramref name="cloudEvent"/> is null.</exception>
+        /// <exception cref="ArgumentException">The event is invalid.</exception>
+        /// <returns>A reference to the same object, for simplicity of method chaining.</returns>
+        public static CloudEvent CheckCloudEventArgument(CloudEvent cloudEvent, string paramName)
+        {
+            CheckNotNull(cloudEvent, paramName);
+            if (cloudEvent.IsValid)
+            {
+                return cloudEvent;
+            }
+            var missing = cloudEvent.SpecVersion.RequiredAttributes.Where(attr => cloudEvent[attr] is null).ToList();
+            string joinedMissing = string.Join(", ", missing);
+            throw new ArgumentException($"CloudEvent is missing required attributes: {joinedMissing}", paramName);
+        }
+    }
+}

src/CloudNative.CloudEvents/Preconditions.cs

@@ -1,43 +0,0 @@
-// Copyright 2021 Cloud Native Foundation. 
-// Licensed under the Apache 2.0 license.
-// See LICENSE file in the project root for full license information.
-
-using System;
-
-namespace CloudNative.CloudEvents
-{
-    /// <summary>
-    /// Convenient precondition methods.
-    /// </summary>
-    internal static class Preconditions
-    {
-        internal static T CheckNotNull<T>(T value, string paramName) where T : class =>
-            value ?? throw new ArgumentNullException(paramName);
-
-        internal static void CheckArgument(bool condition, string paramName, string message)
-        {
-            if (!condition)
-            {
-                throw new ArgumentException(message, paramName);
-            }
-        }
-
-        internal static void CheckArgument(bool condition, string paramName, string messageFormat,
-            object arg1)
-        {
-            if (!condition)
-            {
-                throw new ArgumentException(string.Format(messageFormat, arg1), paramName);
-            }
-        }
-
-        internal static void CheckArgument(bool condition, string paramName, string messageFormat,
-            object arg1, object arg2)
-        {
-            if (!condition)
-            {
-                throw new ArgumentException(string.Format(messageFormat, arg1, arg2), paramName);
-            }
-        }
-    }
-}

src/CloudNative.CloudEvents/Timestamps.cs

@@ -2,6 +2,7 @@
 // Licensed under the Apache 2.0 license.
 // See LICENSE file in the project root for full license information.
 
+using CloudNative.CloudEvents.Core;
 using System;
 using System.Globalization;
 
@@ -49,10 +50,8 @@ namespace CloudNative.CloudEvents
         public static bool TryParse(string input, out DateTimeOffset result)
         {
             // TODO: Check this and add a test
-            if (input is null)
+            Validation.CheckNotNull(input, nameof(input));
-            {
+            
-                throw new ArgumentNullException(nameof(input));
-            }
             if (input.Length < MinLength) // "yyyy-MM-ddTHH:mm:ssZ" is the shortest possible value.
             {
                 result = default;

test/CloudNative.CloudEvents.UnitTests/CloudEventTest.cs

@@ -2,6 +2,7 @@
 // Licensed under the Apache 2.0 license.
 // See LICENSE file in the project root for full license information.
 
+using CloudNative.CloudEvents.Core;
 using System;
 using System.Linq;
 using System.Net.Mime;
@@ -112,7 +113,7 @@ namespace CloudNative.CloudEvents.UnitTests
             Assert.Contains(CloudEventsSpecVersion.Default.SourceAttribute.Name, exception1.Message);
             Assert.DoesNotContain(CloudEventsSpecVersion.Default.TypeAttribute.Name, exception1.Message);
 
-            var exception2 = Assert.Throws<ArgumentException>(() => cloudEvent.ValidateForConversion("param"));
+            var exception2 = Assert.Throws<ArgumentException>(() => Validation.CheckCloudEventArgument(cloudEvent, "param"));
             Assert.Equal("param", exception2.ParamName);
             Assert.Contains(CloudEventsSpecVersion.Default.IdAttribute.Name, exception1.Message);
             Assert.Contains(CloudEventsSpecVersion.Default.SourceAttribute.Name, exception1.Message);
@@ -130,7 +131,7 @@ namespace CloudNative.CloudEvents.UnitTests
             };
             Assert.True(cloudEvent.IsValid);
             Assert.Same(cloudEvent, cloudEvent.Validate());
-            Assert.Same(cloudEvent, cloudEvent.ValidateForConversion("param"));
+            Assert.Same(cloudEvent, Validation.CheckCloudEventArgument(cloudEvent, "param"));
         }
 
         [Fact]

test/CloudNative.CloudEvents.UnitTests/Impl/MimeUtilitiesTest.cs

@@ -8,7 +8,7 @@ using System.Net.Mime;
 using System.Text;
 using Xunit;
 
-namespace CloudNative.CloudEvents.UnitTests.Http
+namespace CloudNative.CloudEvents.Core.UnitTests
 {
     public class MimeUtilitiesTest
     {
@@ -21,9 +21,9 @@ namespace CloudNative.CloudEvents.UnitTests.Http
         public void ContentTypeConversions(string text)
         {
             var originalContentType = new ContentType(text);
-            var header = originalContentType.ToMediaTypeHeaderValue();
+            var header = MimeUtilities.ToMediaTypeHeaderValue(originalContentType);
             AssertEqualParts(text, header.ToString());
-            var convertedContentType = header.ToContentType();
+            var convertedContentType = MimeUtilities.ToContentType(header);
             AssertEqualParts(originalContentType.ToString(), convertedContentType.ToString());
 
             // Conversions can end up reordering the parameters. In reality we're only
@@ -40,8 +40,8 @@ namespace CloudNative.CloudEvents.UnitTests.Http
         [Fact]
         public void ContentTypeConversions_Null()
         {
-            Assert.Null(default(ContentType).ToMediaTypeHeaderValue());
+            Assert.Null(MimeUtilities.ToMediaTypeHeaderValue(default(ContentType)));
-            Assert.Null(default(MediaTypeHeaderValue).ToContentType());
+            Assert.Null(MimeUtilities.ToContentType(default(MediaTypeHeaderValue)));
         }
 
         [Theory]
@@ -50,7 +50,7 @@ namespace CloudNative.CloudEvents.UnitTests.Http
         public void ContentTypeGetEncoding(string charSet)
         {
             var contentType = new ContentType($"text/plain; charset={charSet}");
-            Encoding encoding = contentType.GetEncoding();
+            Encoding encoding = MimeUtilities.GetEncoding(contentType);
             Assert.Equal(charSet, encoding.WebName);
         }
 
@@ -58,7 +58,7 @@ namespace CloudNative.CloudEvents.UnitTests.Http
         public void ContentTypeGetEncoding_NoContentType()
         {
             ContentType contentType = null;
-            Encoding encoding = contentType.GetEncoding();
+            Encoding encoding = MimeUtilities.GetEncoding(contentType);
             Assert.Equal(Encoding.UTF8, encoding);
         }
 
@@ -66,7 +66,7 @@ namespace CloudNative.CloudEvents.UnitTests.Http
         public void ContentTypeGetEncoding_NoCharSet()
         {
             ContentType contentType = new ContentType("text/plain");
-            Encoding encoding = contentType.GetEncoding();
+            Encoding encoding = MimeUtilities.GetEncoding(contentType);
             Assert.Equal(Encoding.UTF8, encoding);
         }