Make ActorId deserializable (#457)

Fixes: #444

ActorId doesn't define a default constructor, and so it's not
deserializable via System.Text.Json.

This change implements a converter so that ActorId's API shape can work
with the serializer properly.

I added an integration test for ActorReference as well, but no library
changes we needed for that, ActorId was the blocker.

src/Dapr.Actors/ActorId.cs

@@ -7,10 +7,13 @@ namespace Dapr.Actors
 {
     using System;
     using System.Runtime.Serialization;
+    using System.Text.Json.Serialization;
+    using Dapr.Actors.Seralization;
 
     /// <summary>
     /// The ActorId represents the identity of an actor within an actor service.
     /// </summary>
+    [JsonConverter(typeof(ActorIdJsonConverter))]
     [DataContract(Name = "ActorId")]
     public class ActorId
     {

src/Dapr.Actors/Serialization/ActorIdJsonConverter.cs

@@ -0,0 +1,51 @@
+// ------------------------------------------------------------
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT License.
+// ------------------------------------------------------------
+
+namespace Dapr.Actors.Seralization
+{
+    using System;
+    using System.Text.Json;
+    using System.Text.Json.Serialization;
+
+    // Converter for ActorId - will be serialized as a JSON string
+    internal class ActorIdJsonConverter : JsonConverter<ActorId>
+    {
+        public override ActorId Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
+        {
+            if (typeToConvert != typeof(ActorId))
+            {
+                throw new ArgumentException( $"Conversion to the type '{typeToConvert}' is not supported.", nameof(typeToConvert));
+            }
+
+            // Note - we generate random Guids for Actor Ids when we're generating them randomly
+            // but we don't actually enforce a format. Ids could be a number, or a date, or whatever,
+            // we don't really care. However we always **represent** Ids in JSON as strings.
+            if (reader.TokenType == JsonTokenType.String && 
+                reader.GetString() is string text && 
+                !string.IsNullOrWhiteSpace(text))
+            {
+                return new ActorId(text);
+            }
+            else if (reader.TokenType == JsonTokenType.Null)
+            {
+                return null;
+            }
+
+            throw new JsonException(); // The serializer will provide a default error message.
+        }
+
+        public override void Write(Utf8JsonWriter writer, ActorId value, JsonSerializerOptions options)
+        {
+            if (value is null)
+            {
+                writer.WriteNullValue();
+            }
+            else
+            {
+                writer.WriteStringValue(value.GetId());
+            }
+        }
+    }
+}

test/Dapr.Actors.Test/Serialization/ActorIdJsonConverterTest.cs

@@ -0,0 +1,108 @@
+// ------------------------------------------------------------
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT License.
+// ------------------------------------------------------------
+
+using System.Text.Json;
+using System.Text.Json.Serialization;
+using Xunit;
+
+namespace Dapr.Actors.Serialization
+{
+    public class ActorIdJsonConverterTest
+    {
+        [Fact]
+        public void CanSerializeActorId()
+        {
+            var id = ActorId.CreateRandom();
+            var document = new { actor = id, };
+
+            // We use strings for ActorId - the result should be the same as passing the Id directly.
+            var expected = JsonSerializer.Serialize(new { actor = id.GetId(), });
+            
+            var serialized = JsonSerializer.Serialize(document);
+
+            Assert.Equal(expected, serialized);
+        }
+
+        [Fact]
+        public void CanSerializeNullActorId()
+        {
+            var document = new { actor = (ActorId)null, };
+
+            var expected = JsonSerializer.Serialize(new { actor = (string)null, });
+            
+            var serialized = JsonSerializer.Serialize(document);
+
+            Assert.Equal(expected, serialized);
+        }
+
+        [Fact]
+        public void CanDeserializeActorId()
+        {
+            var id = ActorId.CreateRandom().GetId();
+            var document = $@"
+{{
+    ""actor"": ""{id}""
+}}";
+            
+            var deserialized = JsonSerializer.Deserialize<ActorHolder>(document);
+
+            Assert.Equal(id, deserialized.Actor.GetId());
+        }
+
+        [Fact]
+        public void CanDeserializeNullActorId()
+        {
+            var id = ActorId.CreateRandom().GetId();
+            var document = $@"
+{{
+    ""actor"": null
+}}";
+            
+            var deserialized = JsonSerializer.Deserialize<ActorHolder>(document);
+
+            Assert.Null(deserialized.Actor);
+        }
+
+        [Theory]
+        [InlineData("{ \"actor\": ")]
+        [InlineData("{ \"actor\": \"hi")]
+        [InlineData("{ \"actor\": }")]
+        [InlineData("{ \"actor\": 3 }")]
+        [InlineData("{ \"actor\": \"\"}")]
+        [InlineData("{ \"actor\": \"        \"}")]
+        public void CanReportErrorsFromInvalidData(string document)
+        {
+            // The error messages are provided by the serializer, don't test them here
+            // that would be fragile.
+            Assert.Throws<JsonException>(() =>
+            {
+                JsonSerializer.Deserialize<ActorHolder>(document);
+            });
+        }
+
+        // Regression test for #444
+        [Fact]
+        public void CanRoundTripActorReference()
+        {
+            var reference = new ActorReference()
+            {
+                ActorId = ActorId.CreateRandom(),
+                ActorType = "TestActor",
+            };
+
+            var serialized = JsonSerializer.Serialize(reference);
+            var deserialized = JsonSerializer.Deserialize<ActorReference>(serialized);
+
+            Assert.Equal(reference.ActorId.GetId(), deserialized.ActorId.GetId());
+            Assert.Equal(reference.ActorType, deserialized.ActorType);
+        }
+
+        private class ActorHolder
+        {
+            [JsonPropertyName("actor")]
+            public ActorId Actor { get; set; }
+        }
+    }
+}