Fix configuration sources (#1724)

src/OpenTelemetry.AutoInstrumentation/Configuration/CompositeConfigurationSource.cs

@@ -61,7 +61,7 @@ internal class CompositeConfigurationSource : IConfigurationSource, IEnumerable<
     public string GetString(string key)
     {
         return _sources.Select(source => source.GetString(key))
-            .FirstOrDefault(value => value != null);
+             .FirstOrDefault(value => !string.IsNullOrEmpty(value));
     }
 
     /// <summary>

src/OpenTelemetry.AutoInstrumentation/Configuration/EnvironmentConfigurationSource.cs

@@ -24,8 +24,7 @@ namespace OpenTelemetry.AutoInstrumentation.Configuration;
 /// </summary>
 internal class EnvironmentConfigurationSource : StringConfigurationSource
 {
-    /// <inheritdoc />
-    public override string GetString(string key)
+    protected override string GetStringInternal(string key)
     {
         try
         {

src/OpenTelemetry.AutoInstrumentation/Configuration/NameValueConfigurationSource.cs

@@ -36,8 +36,7 @@ internal class NameValueConfigurationSource : StringConfigurationSource
         _nameValueCollection = nameValueCollection;
     }
 
-    /// <inheritdoc />
-    public override string GetString(string key)
+    protected override string GetStringInternal(string key)
     {
         return _nameValueCollection[key];
     }

src/OpenTelemetry.AutoInstrumentation/Configuration/StringConfigurationSource.cs

@@ -25,7 +25,16 @@ namespace OpenTelemetry.AutoInstrumentation.Configuration;
 internal abstract class StringConfigurationSource : IConfigurationSource
 {
     /// <inheritdoc />
-    public abstract string GetString(string key);
+    public virtual string GetString(string key)
+    {
+        var value = GetStringInternal(key);
+        if (string.IsNullOrEmpty(value))
+        {
+            return null;
+        }
+
+        return value;
+    }
 
     /// <inheritdoc />
     public virtual int? GetInt32(string key)
@@ -53,4 +62,6 @@ internal abstract class StringConfigurationSource : IConfigurationSource
         var value = GetString(key);
         return bool.TryParse(value, out bool result) ? result : null;
     }
+
+    protected abstract string GetStringInternal(string key);
 }

test/OpenTelemetry.AutoInstrumentation.Tests/Util/ConfigurationSourceExtensionsTests.cs

@@ -17,6 +17,8 @@
 using System;
 using System.Collections.Specialized;
 using FluentAssertions;
+using FluentAssertions.Execution;
+using Moq;
 using OpenTelemetry.AutoInstrumentation.Configuration;
 using OpenTelemetry.AutoInstrumentation.Util;
 using Xunit;
@@ -126,4 +128,51 @@ public class ConfigurationSourceExtensionsTests
         act.Should().Throw<FormatException>()
             .WithMessage("Invalid enum value: invalid");
     }
+
+    [Fact]
+    public void ParseEmptyAsNull_StringConfigurationSource()
+    {
+        var source = new DummyStringConfigurationSource();
+
+        using (new AssertionScope())
+        {
+            source.GetString("TEST_NULL_VALUE").Should().BeNull();
+            source.GetString("TEST_EMPTY_VALUE").Should().BeNull();
+        }
+    }
+
+    [Fact]
+    public void ParseEmptyAsNull_CompositeConfigurationSource()
+    {
+        var mockSource = new Mock<IConfigurationSource>();
+        var compositeSource = new CompositeConfigurationSource();
+
+        mockSource.Setup(x => x.GetString(It.Is<string>(x => x == "TEST_NULL_VALUE"))).Returns<string>(k => null);
+        mockSource.Setup(x => x.GetString(It.Is<string>(x => x == "TEST_EMPTY_VALUE"))).Returns<string>(k => string.Empty);
+
+        compositeSource.Add(mockSource.Object);
+
+        using (new AssertionScope())
+        {
+            compositeSource.GetString("TEST_NULL_VALUE").Should().BeNull();
+            compositeSource.GetString("TEST_EMPTY_VALUE").Should().BeNull();
+        }
+    }
+
+    private class DummyStringConfigurationSource : StringConfigurationSource
+    {
+        protected override string GetStringInternal(string key)
+        {
+            if (key == "TEST_NULL_VALUE")
+            {
+                return null;
+            }
+            else if (key == "TEST_EMPTY_VALUE")
+            {
+                return string.Empty;
+            }
+
+            throw new NotImplementedException($"Invalid key '{key}'");
+        }
+    }
 }