Preserve Color.Empty when serializing

docfx/docs/built-in-converters.md

@@ -44,7 +44,7 @@ Enums, arrays and various dictionary types that utilize these types are implicit
 ## Other
 
 - @System.Boolean
-- @System.Drawing.Color
+- <xref:System.Drawing.Color> - represented by an ARGB integer, or the boolean `false` for `Color.Empty`.
 - @System.Globalization.CultureInfo
 - @System.Text.Encoding
 - @System.EventArgs

src/Nerdbank.MessagePack/Converters/BclConverters.cs

@@ -16,17 +16,39 @@ namespace Nerdbank.MessagePack.Converters;
 internal class SystemDrawingColorConverter : MessagePackConverter<Color>
 {
 	public override Color Read(ref MessagePackReader reader, SerializationContext context)
-		=> Color.FromArgb(reader.ReadInt32());
+	{
+		if (reader.NextMessagePackType == MessagePackType.Boolean)
+		{
+			if (!reader.ReadBoolean())
+			{
+				return Color.Empty;
+			}
+
+			throw new MessagePackSerializationException("Unexpected true value while deserializing System.Drawing.Color.");
+		}
+
+		return Color.FromArgb(reader.ReadInt32());
+	}
 
 	public override void Write(ref MessagePackWriter writer, in Color value, SerializationContext context)
-		=> writer.Write(value.ToArgb());
+	{
+		if (value.IsEmpty)
+		{
+			writer.Write(false);
+		}
+		else
+		{
+			writer.Write(value.ToArgb());
+		}
+	}
 
 	public override JsonObject? GetJsonSchema(JsonSchemaContext context, ITypeShape typeShape)
 		=> new()
 		{
-			["type"] = "integer",
-			["format"] = "int32",
-			["description"] = "An ARGB color value.",
+			["anyOf"] = new JsonArray(
+				new JsonObject { ["type"] = "boolean", ["enum"] = new JsonArray(false) },
+				new JsonObject { ["type"] = "integer", ["format"] = "int32" }),
+			["description"] = "An ARGB color value, or false for Color.Empty.",
 		};
 }
 

test/Nerdbank.MessagePack.TUnit/BuiltInConverterTests.cs

@@ -14,6 +14,40 @@ public partial class BuiltInConverterTests : MessagePackSerializerTestBase
 	[Test]
 	public void SystemDrawingColor() => this.AssertRoundtrip<Color, Witness>(Color.FromArgb(1, 2, 3, 4));
 
+	[Test]
+	public void SystemDrawingColor_Empty()
+	{
+		ReadOnlySequence<byte> msgpack = this.AssertRoundtrip<Color, Witness>(Color.Empty);
+		Assert.Equal(MessagePackType.Boolean, new MessagePackReader(msgpack).NextMessagePackType);
+		Assert.True(this.Serializer.Deserialize<Color, Witness>(msgpack, this.TimeoutToken).IsEmpty);
+	}
+
+	[Test]
+	public void SystemDrawingColor_TransparentBlack()
+	{
+		Color transparentBlack = Color.FromArgb(0);
+		ReadOnlySequence<byte> msgpack = this.AssertRoundtrip<Color, Witness>(transparentBlack);
+		Assert.Equal(MessagePackType.Integer, new MessagePackReader(msgpack).NextMessagePackType);
+		Assert.False(this.Serializer.Deserialize<Color, Witness>(msgpack, this.TimeoutToken).IsEmpty);
+	}
+
+	[Test]
+	public void NullableSystemDrawingColor_Null()
+	{
+		ReadOnlySequence<byte> msgpack = this.AssertRoundtrip<Color?, Witness>(null);
+		Assert.Equal(MessagePackType.Nil, new MessagePackReader(msgpack).NextMessagePackType);
+		Assert.Null(this.Serializer.Deserialize<Color?, Witness>(msgpack, this.TimeoutToken));
+	}
+
+	[Test]
+	public void NullableSystemDrawingColor_Empty()
+	{
+		ReadOnlySequence<byte> msgpack = this.AssertRoundtrip<Color?, Witness>(Color.Empty);
+		Color? result = this.Serializer.Deserialize<Color?, Witness>(msgpack, this.TimeoutToken);
+		Assert.True(result.HasValue);
+		Assert.True(result.Value.IsEmpty);
+	}
+
 	[Test]
 	public void SystemDrawingPoint() => this.AssertRoundtrip<Point, Witness>(new Point(1, 1));
 
@@ -596,6 +630,7 @@ public partial record HasDateTimeOffset(DateTimeOffset Value);
 	[GenerateShapeFor<Guid>]
 	[GenerateShapeFor<Point>]
 	[GenerateShapeFor<Color>]
+	[GenerateShapeFor<Color?>]
 	[GenerateShapeFor<byte[]>]
 	[GenerateShapeFor<CultureInfo>]
 	[GenerateShapeFor<EventArgs>]

test/Nerdbank.MessagePack.TUnit/SchemaTests.cs

@@ -9,6 +9,7 @@
 #endif
 
 using System.ComponentModel;
+using System.Drawing;
 using System.Globalization;
 using System.Reflection;
 using System.Runtime.CompilerServices;
@@ -42,6 +43,15 @@ internal enum Sex
 		Female,
 	}
 
+	[Test, MatrixDataSource]
+	public void NullableColorSchema(JsonSchemaDialect dialect)
+	{
+		JsonObject schema = this.Serializer.GetJsonSchema<HasNullableColors>(new JsonSchemaOptions { Dialect = dialect });
+		JSchema parsedSchema = JSchema.Parse(SchemaToString(schema));
+		JToken.Parse("""{"Color":null,"Colors":[null,false,0]}""").Validate(parsedSchema);
+		Assert.False(JToken.Parse("""{"Color":true,"Colors":[]}""").IsValid(parsedSchema));
+	}
+
 	[Test, MatrixDataSource]
 	public void BasicObject_Map(JsonSchemaDialect dialect) => this.AssertSchema(dialect, [new BasicObject { IntProperty = 3, StringProperty = "hi" }]);
 
@@ -514,4 +524,7 @@ public override void Write(ref MessagePackWriter writer, in CustomType? value, S
 			writer.WriteMapHeader(0);
 		}
 	}
+
+	[GenerateShape]
+	public partial record HasNullableColors(Color? Color, Color?[] Colors);
 }