[release/6.0-rc1] Support reference type custom converters in source gen

src/libraries/System.Text.Json/gen/JsonSourceGenerator.Emitter.cs

@@ -277,10 +277,12 @@ private string GenerateForTypeWithUnknownConverter(TypeGenerationSpec typeMetada
                 string typeCompilableName = typeMetadata.TypeRef;
                 string typeFriendlyName = typeMetadata.TypeInfoPropertyName;
 
-                StringBuilder sb = new();
+                string metadataInitSource;
 
                 // TODO (https://github.com/dotnet/runtime/issues/52218): consider moving this verification source to common helper.
-                string metadataInitSource = $@"{JsonConverterTypeRef} converter = {typeMetadata.ConverterInstantiationLogic};
+                if (typeMetadata.IsValueType)
+                {
+                    metadataInitSource = $@"{JsonConverterTypeRef} converter = {typeMetadata.ConverterInstantiationLogic};
                     {TypeTypeRef} typeToConvert = typeof({typeCompilableName});
                     if (!converter.CanConvert(typeToConvert))
                     {{
@@ -309,6 +311,18 @@ private string GenerateForTypeWithUnknownConverter(TypeGenerationSpec typeMetada
                     }}
 
                     _{typeFriendlyName} = {JsonMetadataServicesTypeRef}.{GetCreateValueInfoMethodRef(typeCompilableName)}({OptionsInstanceVariableName}, converter);";
+                }
+                else
+                {
+                    metadataInitSource = $@"{JsonConverterTypeRef} converter = {typeMetadata.ConverterInstantiationLogic};
+                    {TypeTypeRef} typeToConvert = typeof({typeCompilableName});
+                    if (!converter.CanConvert(typeToConvert))
+                    {{
+                        throw new {InvalidOperationExceptionTypeRef}($""The converter '{{converter.GetType()}}' is not compatible with the type '{{typeToConvert}}'."");
+                    }}
+
+                    _{typeFriendlyName} = {JsonMetadataServicesTypeRef}.{GetCreateValueInfoMethodRef(typeCompilableName)}({OptionsInstanceVariableName}, converter);";
+                }
 
                 return GenerateForType(typeMetadata, metadataInitSource);
             }

src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Tests/ContextClasses.cs

@@ -29,6 +29,10 @@ public interface ITestContext
         public JsonTypeInfo<object[]> ObjectArray { get; }
         public JsonTypeInfo<string> String { get; }
         public JsonTypeInfo<RealWorldContextTests.ClassWithEnumAndNullable> ClassWithEnumAndNullable { get; }
+        public JsonTypeInfo<ClassWithCustomConverter> ClassWithCustomConverter { get; }
+        public JsonTypeInfo<StructWithCustomConverter> StructWithCustomConverter { get; }
+        public JsonTypeInfo<ClassWithBadCustomConverter> ClassWithBadCustomConverter { get; }
+        public JsonTypeInfo<StructWithBadCustomConverter> StructWithBadCustomConverter { get; }
     }
 
     internal partial class JsonContext : JsonSerializerContext

src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Tests/MetadataAndSerializationContextTests.cs

@@ -27,6 +27,10 @@ namespace System.Text.Json.SourceGeneration.Tests
     [JsonSerializable(typeof(object[]))]
     [JsonSerializable(typeof(string))]
     [JsonSerializable(typeof(RealWorldContextTests.ClassWithEnumAndNullable))]
+    [JsonSerializable(typeof(ClassWithCustomConverter))]
+    [JsonSerializable(typeof(StructWithCustomConverter))]
+    [JsonSerializable(typeof(ClassWithBadCustomConverter))]
+    [JsonSerializable(typeof(StructWithBadCustomConverter))]
     internal partial class MetadataAndSerializationContext : JsonSerializerContext, ITestContext
     {
     }
@@ -58,6 +62,10 @@ public override void EnsureFastPathGeneratedAsExpected()
             Assert.Null(MetadataAndSerializationContext.Default.ObjectArray.Serialize);
             Assert.Null(MetadataAndSerializationContext.Default.String.Serialize);
             Assert.NotNull(MetadataAndSerializationContext.Default.ClassWithEnumAndNullable.Serialize);
+            Assert.NotNull(MetadataAndSerializationContext.Default.ClassWithCustomConverter);
+            Assert.NotNull(MetadataAndSerializationContext.Default.StructWithCustomConverter);
+            Assert.Throws<InvalidOperationException>(() => MetadataAndSerializationContext.Default.ClassWithBadCustomConverter);
+            Assert.Throws<InvalidOperationException>(() => MetadataAndSerializationContext.Default.StructWithBadCustomConverter);
         }
     }
 }

src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Tests/MetadataContextTests.cs

@@ -26,6 +26,12 @@ namespace System.Text.Json.SourceGeneration.Tests
     [JsonSerializable(typeof(object[]), GenerationMode = JsonSourceGenerationMode.Metadata)]
     [JsonSerializable(typeof(string), GenerationMode = JsonSourceGenerationMode.Metadata)]
     [JsonSerializable(typeof(RealWorldContextTests.ClassWithEnumAndNullable), GenerationMode = JsonSourceGenerationMode.Metadata)]
+    [JsonSerializable(typeof(ClassWithCustomConverter), GenerationMode = JsonSourceGenerationMode.Metadata)]
+    [JsonSerializable(typeof(StructWithCustomConverter), GenerationMode = JsonSourceGenerationMode.Metadata)]
+    [JsonSerializable(typeof(ClassWithCustomConverter), GenerationMode = JsonSourceGenerationMode.Metadata)]
+    [JsonSerializable(typeof(StructWithCustomConverter), GenerationMode = JsonSourceGenerationMode.Metadata)]
+    [JsonSerializable(typeof(ClassWithBadCustomConverter), GenerationMode = JsonSourceGenerationMode.Metadata)]
+    [JsonSerializable(typeof(StructWithBadCustomConverter), GenerationMode = JsonSourceGenerationMode.Metadata)]
     internal partial class MetadataWithPerTypeAttributeContext : JsonSerializerContext, ITestContext
     {
     }
@@ -55,6 +61,10 @@ public override void EnsureFastPathGeneratedAsExpected()
             Assert.Null(MetadataWithPerTypeAttributeContext.Default.ObjectArray.Serialize);
             Assert.Null(MetadataWithPerTypeAttributeContext.Default.String.Serialize);
             Assert.Null(MetadataWithPerTypeAttributeContext.Default.ClassWithEnumAndNullable.Serialize);
+            Assert.Null(MetadataWithPerTypeAttributeContext.Default.ClassWithCustomConverter.Serialize);
+            Assert.Null(MetadataWithPerTypeAttributeContext.Default.StructWithCustomConverter.Serialize);
+            Assert.Throws<InvalidOperationException>(() => MetadataWithPerTypeAttributeContext.Default.ClassWithBadCustomConverter.Serialize);
+            Assert.Throws<InvalidOperationException>(() => MetadataWithPerTypeAttributeContext.Default.StructWithBadCustomConverter.Serialize);
         }
     }
 
@@ -79,6 +89,10 @@ public override void EnsureFastPathGeneratedAsExpected()
     [JsonSerializable(typeof(object[]))]
     [JsonSerializable(typeof(string))]
     [JsonSerializable(typeof(RealWorldContextTests.ClassWithEnumAndNullable))]
+    [JsonSerializable(typeof(ClassWithCustomConverter))]
+    [JsonSerializable(typeof(StructWithCustomConverter))]
+    [JsonSerializable(typeof(ClassWithBadCustomConverter))]
+    [JsonSerializable(typeof(StructWithBadCustomConverter))]
     internal partial class MetadataContext : JsonSerializerContext, ITestContext
     {
     }
@@ -110,6 +124,10 @@ public override void EnsureFastPathGeneratedAsExpected()
             Assert.Null(MetadataContext.Default.ObjectArray.Serialize);
             Assert.Null(MetadataContext.Default.String.Serialize);
             Assert.Null(MetadataContext.Default.ClassWithEnumAndNullable.Serialize);
+            Assert.Null(MetadataContext.Default.ClassWithCustomConverter.Serialize);
+            Assert.Null(MetadataContext.Default.StructWithCustomConverter.Serialize);
+            Assert.Throws<InvalidOperationException>(() => MetadataContext.Default.ClassWithBadCustomConverter.Serialize);
+            Assert.Throws<InvalidOperationException>(() => MetadataContext.Default.StructWithBadCustomConverter.Serialize);
         }
     }
 }

src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Tests/MixedModeContextTests.cs

@@ -26,6 +26,10 @@ namespace System.Text.Json.SourceGeneration.Tests
     [JsonSerializable(typeof(object[]), GenerationMode = JsonSourceGenerationMode.Metadata)]
     [JsonSerializable(typeof(string), GenerationMode = JsonSourceGenerationMode.Metadata | JsonSourceGenerationMode.Serialization)]
     [JsonSerializable(typeof(RealWorldContextTests.ClassWithEnumAndNullable), GenerationMode = JsonSourceGenerationMode.Metadata | JsonSourceGenerationMode.Serialization)]
+    [JsonSerializable(typeof(ClassWithCustomConverter), GenerationMode = JsonSourceGenerationMode.Metadata | JsonSourceGenerationMode.Serialization)]
+    [JsonSerializable(typeof(StructWithCustomConverter), GenerationMode = JsonSourceGenerationMode.Metadata | JsonSourceGenerationMode.Serialization)]
+    [JsonSerializable(typeof(ClassWithBadCustomConverter), GenerationMode = JsonSourceGenerationMode.Metadata | JsonSourceGenerationMode.Serialization)]
+    [JsonSerializable(typeof(StructWithBadCustomConverter), GenerationMode = JsonSourceGenerationMode.Metadata | JsonSourceGenerationMode.Serialization)]
     internal partial class MixedModeContext : JsonSerializerContext, ITestContext
     {
     }
@@ -56,6 +60,10 @@ public override void EnsureFastPathGeneratedAsExpected()
             Assert.Null(MixedModeContext.Default.ObjectArray.Serialize);
             Assert.Null(MixedModeContext.Default.String.Serialize);
             Assert.NotNull(MixedModeContext.Default.ClassWithEnumAndNullable.Serialize);
+            Assert.Null(MixedModeContext.Default.ClassWithCustomConverter.Serialize);
+            Assert.Null(MixedModeContext.Default.StructWithCustomConverter.Serialize);
+            Assert.Throws<InvalidOperationException>(() => MixedModeContext.Default.ClassWithBadCustomConverter.Serialize);
+            Assert.Throws<InvalidOperationException>(() => MixedModeContext.Default.StructWithBadCustomConverter.Serialize);
         }
 
         [Fact]

src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Tests/RealWorldContextTests.cs

@@ -110,6 +110,64 @@ public virtual void RoundTripTypeNameClash()
             VerifyRepeatedLocation(expected, obj);
         }
 
+        [Fact]
+        public virtual void RoundTripWithCustomConverter_Class()
+        {
+            const string Json = "{\"MyInt\":142}";
+
+            ClassWithCustomConverter obj = new ClassWithCustomConverter()
+            {
+                MyInt = 42
+            };
+
+            string json = JsonSerializer.Serialize(obj, DefaultContext.ClassWithCustomConverter);
+            Assert.Equal(Json, json);
+
+            obj = JsonSerializer.Deserialize(Json, DefaultContext.ClassWithCustomConverter);
+            Assert.Equal(42, obj.MyInt);
+        }
+
+        [Fact]
+        public virtual void RoundTripWithCustomConverter_Struct()
+        {
+            const string Json = "{\"MyInt\":142}";
+
+            StructWithCustomConverter obj = new StructWithCustomConverter()
+            {
+                MyInt = 42
+            };
+
+            string json = JsonSerializer.Serialize(obj, DefaultContext.StructWithCustomConverter);
+            Assert.Equal(Json, json);
+
+            obj = JsonSerializer.Deserialize(Json, DefaultContext.StructWithCustomConverter);
+            Assert.Equal(42, obj.MyInt);
+        }
+
+        [Fact]
+        public virtual void BadCustomConverter_Class()
+        {
+            const string Json = "{\"MyInt\":142}";
+
+            Assert.Throws<InvalidOperationException>(() =>
+                JsonSerializer.Serialize(new ClassWithBadCustomConverter(), DefaultContext.ClassWithBadCustomConverter));
+
+            Assert.Throws<InvalidOperationException>(() =>
+                JsonSerializer.Deserialize(Json, DefaultContext.ClassWithBadCustomConverter));
+        }
+
+        [Fact]
+        public virtual void BadCustomConverter_Struct()
+        {
+            const string Json = "{\"MyInt\":142}";
+
+            Assert.Throws<InvalidOperationException>(() =>
+                JsonSerializer.Serialize(new StructWithBadCustomConverter(), DefaultContext.StructWithBadCustomConverter));
+
+            Assert.Throws<InvalidOperationException>(() =>
+                JsonSerializer.Deserialize(Json, DefaultContext.StructWithBadCustomConverter));
+        }
+
         protected static Location CreateLocation()
         {
             return new Location

src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Tests/SerializationContextTests.cs

@@ -27,6 +27,10 @@ namespace System.Text.Json.SourceGeneration.Tests
     [JsonSerializable(typeof(object[]))]
     [JsonSerializable(typeof(string))]
     [JsonSerializable(typeof(RealWorldContextTests.ClassWithEnumAndNullable))]
+    [JsonSerializable(typeof(ClassWithCustomConverter))]
+    [JsonSerializable(typeof(StructWithCustomConverter))]
+    [JsonSerializable(typeof(ClassWithBadCustomConverter))]
+    [JsonSerializable(typeof(StructWithBadCustomConverter))]
     internal partial class SerializationContext : JsonSerializerContext, ITestContext
     {
     }
@@ -51,6 +55,12 @@ internal partial class SerializationContext : JsonSerializerContext, ITestContex
     [JsonSerializable(typeof(object[]), GenerationMode = JsonSourceGenerationMode.Serialization)]
     [JsonSerializable(typeof(string), GenerationMode = JsonSourceGenerationMode.Serialization)]
     [JsonSerializable(typeof(RealWorldContextTests.ClassWithEnumAndNullable), GenerationMode = JsonSourceGenerationMode.Serialization)]
+    [JsonSerializable(typeof(ClassWithCustomConverter), GenerationMode = JsonSourceGenerationMode.Serialization)]
+    [JsonSerializable(typeof(StructWithCustomConverter), GenerationMode = JsonSourceGenerationMode.Serialization)]
+    [JsonSerializable(typeof(ClassWithCustomConverter), GenerationMode = JsonSourceGenerationMode.Serialization)]
+    [JsonSerializable(typeof(StructWithCustomConverter), GenerationMode = JsonSourceGenerationMode.Serialization)]
+    [JsonSerializable(typeof(ClassWithBadCustomConverter), GenerationMode = JsonSourceGenerationMode.Serialization)]
+    [JsonSerializable(typeof(StructWithBadCustomConverter), GenerationMode = JsonSourceGenerationMode.Serialization)]
     internal partial class SerializationWithPerTypeAttributeContext : JsonSerializerContext, ITestContext
     {
     }
@@ -76,6 +86,10 @@ internal partial class SerializationWithPerTypeAttributeContext : JsonSerializer
     [JsonSerializable(typeof(object[]), GenerationMode = JsonSourceGenerationMode.Serialization)]
     [JsonSerializable(typeof(string), GenerationMode = JsonSourceGenerationMode.Serialization)]
     [JsonSerializable(typeof(RealWorldContextTests.ClassWithEnumAndNullable), GenerationMode = JsonSourceGenerationMode.Serialization)]
+    [JsonSerializable(typeof(ClassWithCustomConverter), GenerationMode = JsonSourceGenerationMode.Serialization)]
+    [JsonSerializable(typeof(StructWithCustomConverter), GenerationMode = JsonSourceGenerationMode.Serialization)]
+    [JsonSerializable(typeof(ClassWithBadCustomConverter), GenerationMode = JsonSourceGenerationMode.Serialization)]
+    [JsonSerializable(typeof(StructWithBadCustomConverter), GenerationMode = JsonSourceGenerationMode.Serialization)]
     internal partial class SerializationContextWithCamelCase : JsonSerializerContext, ITestContext
     {
     }
@@ -112,6 +126,10 @@ public override void EnsureFastPathGeneratedAsExpected()
             Assert.Null(SerializationContext.Default.ObjectArray.Serialize);
             Assert.Null(SerializationContext.Default.String.Serialize);
             Assert.NotNull(SerializationContext.Default.ClassWithEnumAndNullable.Serialize);
+            Assert.Null(SerializationContext.Default.ClassWithCustomConverter.Serialize);
+            Assert.Null(SerializationContext.Default.StructWithCustomConverter.Serialize);
+            Assert.Throws<InvalidOperationException>(() => SerializationContext.Default.ClassWithBadCustomConverter.Serialize);
+            Assert.Throws<InvalidOperationException>(() => SerializationContext.Default.StructWithBadCustomConverter.Serialize);
         }
 
         [Fact]
@@ -370,6 +388,10 @@ public override void EnsureFastPathGeneratedAsExpected()
             Assert.Null(SerializationWithPerTypeAttributeContext.Default.ObjectArray.Serialize);
             Assert.Null(SerializationWithPerTypeAttributeContext.Default.String.Serialize);
             Assert.NotNull(SerializationWithPerTypeAttributeContext.Default.ClassWithEnumAndNullable.Serialize);
+            Assert.Null(SerializationWithPerTypeAttributeContext.Default.ClassWithCustomConverter.Serialize);
+            Assert.Null(SerializationWithPerTypeAttributeContext.Default.StructWithCustomConverter.Serialize);
+            Assert.Throws<InvalidOperationException>(() => SerializationWithPerTypeAttributeContext.Default.ClassWithBadCustomConverter.Serialize);
+            Assert.Throws<InvalidOperationException>(() => SerializationWithPerTypeAttributeContext.Default.StructWithBadCustomConverter.Serialize);
         }
     }
 }

src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Tests/TestClasses.cs

@@ -151,4 +151,110 @@ public class JsonMessage
     }
 
     internal struct MyStruct { }
+
+    /// <summary>
+    /// Custom converter that adds\substract 100 from MyIntProperty.
+    /// </summary>
+    public class CustomConverterForClass : JsonConverter<ClassWithCustomConverter>
+    {
+        public override ClassWithCustomConverter Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
+        {
+            if (reader.TokenType != JsonTokenType.StartObject)
+            {
+                throw new JsonException("No StartObject");
+            }
+
+            ClassWithCustomConverter obj = new();
+
+            reader.Read();
+            if (reader.TokenType != JsonTokenType.PropertyName &&
+                reader.GetString() != "MyInt")
+            {
+                throw new JsonException("Wrong property name");
+            }
+
+            reader.Read();
+            obj.MyInt = reader.GetInt32() - 100;
+
+            reader.Read();
+            if (reader.TokenType != JsonTokenType.EndObject)
+            {
+                throw new JsonException("No EndObject");
+            }
+
+            return obj;
+        }
+
+        public override void Write(Utf8JsonWriter writer, ClassWithCustomConverter value, JsonSerializerOptions options)
+        {
+            writer.WriteStartObject();
+            writer.WriteNumber(nameof(ClassWithCustomConverter.MyInt), value.MyInt + 100);
+            writer.WriteEndObject();
+        }
+    }
+
+    [JsonConverter(typeof(CustomConverterForClass))]
+    public class ClassWithCustomConverter
+    {
+        public int MyInt { get; set; }
+    }
+
+    [JsonConverter(typeof(CustomConverterForStruct))] // Invalid
+    public struct ClassWithBadCustomConverter
+    {
+        public int MyInt { get; set; }
+    }
+
+    /// <summary>
+    /// Custom converter that adds\substract 100 from MyIntProperty.
+    /// </summary>
+    public class CustomConverterForStruct : JsonConverter<StructWithCustomConverter>
+    {
+        public override StructWithCustomConverter Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
+        {
+            if (reader.TokenType != JsonTokenType.StartObject)
+            {
+                throw new JsonException("No StartObject");
+            }
+
+            StructWithCustomConverter obj = new();
+
+            reader.Read();
+            if (reader.TokenType != JsonTokenType.PropertyName &&
+                reader.GetString() != "MyInt")
+            {
+                throw new JsonException("Wrong property name");
+            }
+
+            reader.Read();
+            obj.MyInt = reader.GetInt32() - 100;
+
+            reader.Read();
+            if (reader.TokenType != JsonTokenType.EndObject)
+            {
+                throw new JsonException("No EndObject");
+            }
+
+            return obj;
+        }
+
+        public override void Write(Utf8JsonWriter writer, StructWithCustomConverter value, JsonSerializerOptions options)
+        {
+            writer.WriteStartObject();
+            writer.WriteNumber(nameof(StructWithCustomConverter.MyInt), value.MyInt + 100);
+            writer.WriteEndObject();
+        }
+    }
+
+    [JsonConverter(typeof(CustomConverterForStruct))]
+    public struct StructWithCustomConverter
+    {
+        public int MyInt { get; set; }
+    }
+
+    [JsonConverter(typeof(CustomConverterForClass))] // Invalid
+    public struct StructWithBadCustomConverter
+    {
+        public int MyInt { get; set; }
+    }
 }