JSON source generator emitting source for types unexpectedly

[stephentoub]

Description

In the repo below, Exception should not be serialized as it's defined, but rather only via the ExceptionConverter that treats it as a string. However, the source generator is emitting code to handle all of the properties on Exception. While this is unnecessary code, it also results in trimmer warnings about members of Exception that aren't safe to serialized.

Reproduction Steps

using System.Text.Json;
using System.Text.Json.Serialization;

Console.WriteLine(JsonSerializer.Serialize(new MyClass { Error = new Exception("error") } ));

sealed class MyClass
{
    [JsonConverter(typeof(ExceptionConverter))]
    public Exception? Error { get; set; }
}

sealed class ExceptionConverter : JsonConverter<Exception>
{
    public override Exception Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options) =>
        new Exception(reader.GetString());

    public override void Write(Utf8JsonWriter writer, Exception? value, JsonSerializerOptions options) =>
        writer.WriteStringValue(value?.Message ?? string.Empty);
}

[JsonSerializable(typeof(MyClass))]
partial class MyContext : JsonSerializerContext;

Expected behavior

Code emitted for Exception related to its converter.

Actual behavior

Emits this:

// <auto-generated/>

#nullable enable annotations
#nullable disable warnings

// Suppress warnings about [Obsolete] member usage in generated code.
#pragma warning disable CS0612, CS0618

partial class MyContext
{
    private global::System.Text.Json.Serialization.Metadata.JsonTypeInfo<global::System.Exception>? _Exception;
    
    /// <summary>
    /// Defines the source generated JSON serialization contract metadata for a given type.
    /// </summary>
    #nullable disable annotations // Marking the property type as nullable-oblivious.
    public global::System.Text.Json.Serialization.Metadata.JsonTypeInfo<global::System.Exception> Exception
    #nullable enable annotations
    {
        get => _Exception ??= (global::System.Text.Json.Serialization.Metadata.JsonTypeInfo<global::System.Exception>)Options.GetTypeInfo(typeof(global::System.Exception));
    }
    
    private global::System.Text.Json.Serialization.Metadata.JsonTypeInfo<global::System.Exception> Create_Exception(global::System.Text.Json.JsonSerializerOptions options)
    {
        if (!TryGetTypeInfoForRuntimeCustomConverter<global::System.Exception>(options, out global::System.Text.Json.Serialization.Metadata.JsonTypeInfo<global::System.Exception> jsonTypeInfo))
        {
            var objectInfo = new global::System.Text.Json.Serialization.Metadata.JsonObjectInfoValues<global::System.Exception>
            {
                ObjectCreator = () => new global::System.Exception(),
                ObjectWithParameterizedConstructorCreator = null,
                PropertyMetadataInitializer = _ => ExceptionPropInit(options),
                ConstructorParameterMetadataInitializer = null,
                ConstructorAttributeProviderFactory = static () => typeof(global::System.Exception).GetConstructor(InstanceMemberBindingFlags, binder: null, global::System.Array.Empty<global::System.Type>(), modifiers: null),
                SerializeHandler = ExceptionSerializeHandler,
            };
            
            jsonTypeInfo = global::System.Text.Json.Serialization.Metadata.JsonMetadataServices.CreateObjectInfo<global::System.Exception>(options, objectInfo);
            jsonTypeInfo.NumberHandling = null;
        }
    
        jsonTypeInfo.OriginatingResolver = this;
        return jsonTypeInfo;
    }

    private static global::System.Text.Json.Serialization.Metadata.JsonPropertyInfo[] ExceptionPropInit(global::System.Text.Json.JsonSerializerOptions options)
    {
        var properties = new global::System.Text.Json.Serialization.Metadata.JsonPropertyInfo[8];

        var info0 = new global::System.Text.Json.Serialization.Metadata.JsonPropertyInfoValues<global::System.Collections.IDictionary>
        {
            IsProperty = true,
            IsPublic = true,
            IsVirtual = true,
            DeclaringType = typeof(global::System.Exception),
            Converter = null,
            Getter = static obj => ((global::System.Exception)obj).Data,
            Setter = null,
            IgnoreCondition = null,
            HasJsonInclude = false,
            IsExtensionData = false,
            NumberHandling = null,
            PropertyName = "Data",
            JsonPropertyName = null,
            AttributeProviderFactory = static () => typeof(global::System.Exception).GetProperty("Data", InstanceMemberBindingFlags, null, typeof(global::System.Collections.IDictionary), global::System.Array.Empty<global::System.Type>(), null),
        };
        
        properties[0] = global::System.Text.Json.Serialization.Metadata.JsonMetadataServices.CreatePropertyInfo<global::System.Collections.IDictionary>(options, info0);
        properties[0].IsGetNullable = false;

        var info1 = new global::System.Text.Json.Serialization.Metadata.JsonPropertyInfoValues<string>
        {
            IsProperty = true,
            IsPublic = true,
            IsVirtual = true,
            DeclaringType = typeof(global::System.Exception),
            Converter = null,
            Getter = static obj => ((global::System.Exception)obj).HelpLink,
            Setter = static (obj, value) => ((global::System.Exception)obj).HelpLink = value!,
            IgnoreCondition = null,
            HasJsonInclude = false,
            IsExtensionData = false,
            NumberHandling = null,
            PropertyName = "HelpLink",
            JsonPropertyName = null,
            AttributeProviderFactory = static () => typeof(global::System.Exception).GetProperty("HelpLink", InstanceMemberBindingFlags, null, typeof(string), global::System.Array.Empty<global::System.Type>(), null),
        };
        
        properties[1] = global::System.Text.Json.Serialization.Metadata.JsonMetadataServices.CreatePropertyInfo<string>(options, info1);

        var info2 = new global::System.Text.Json.Serialization.Metadata.JsonPropertyInfoValues<int>
        {
            IsProperty = true,
            IsPublic = true,
            IsVirtual = false,
            DeclaringType = typeof(global::System.Exception),
            Converter = null,
            Getter = static obj => ((global::System.Exception)obj).HResult,
            Setter = static (obj, value) => ((global::System.Exception)obj).HResult = value!,
            IgnoreCondition = null,
            HasJsonInclude = false,
            IsExtensionData = false,
            NumberHandling = null,
            PropertyName = "HResult",
            JsonPropertyName = null,
            AttributeProviderFactory = static () => typeof(global::System.Exception).GetProperty("HResult", InstanceMemberBindingFlags, null, typeof(int), global::System.Array.Empty<global::System.Type>(), null),
        };
        
        properties[2] = global::System.Text.Json.Serialization.Metadata.JsonMetadataServices.CreatePropertyInfo<int>(options, info2);

        var info3 = new global::System.Text.Json.Serialization.Metadata.JsonPropertyInfoValues<global::System.Exception>
        {
            IsProperty = true,
            IsPublic = true,
            IsVirtual = false,
            DeclaringType = typeof(global::System.Exception),
            Converter = null,
            Getter = static obj => ((global::System.Exception)obj).InnerException,
            Setter = null,
            IgnoreCondition = null,
            HasJsonInclude = false,
            IsExtensionData = false,
            NumberHandling = null,
            PropertyName = "InnerException",
            JsonPropertyName = null,
            AttributeProviderFactory = static () => typeof(global::System.Exception).GetProperty("InnerException", InstanceMemberBindingFlags, null, typeof(global::System.Exception), global::System.Array.Empty<global::System.Type>(), null),
        };
        
        properties[3] = global::System.Text.Json.Serialization.Metadata.JsonMetadataServices.CreatePropertyInfo<global::System.Exception>(options, info3);

        var info4 = new global::System.Text.Json.Serialization.Metadata.JsonPropertyInfoValues<string>
        {
            IsProperty = true,
            IsPublic = true,
            IsVirtual = true,
            DeclaringType = typeof(global::System.Exception),
            Converter = null,
            Getter = static obj => ((global::System.Exception)obj).Message,
            Setter = null,
            IgnoreCondition = null,
            HasJsonInclude = false,
            IsExtensionData = false,
            NumberHandling = null,
            PropertyName = "Message",
            JsonPropertyName = null,
            AttributeProviderFactory = static () => typeof(global::System.Exception).GetProperty("Message", InstanceMemberBindingFlags, null, typeof(string), global::System.Array.Empty<global::System.Type>(), null),
        };
        
        properties[4] = global::System.Text.Json.Serialization.Metadata.JsonMetadataServices.CreatePropertyInfo<string>(options, info4);
        properties[4].IsGetNullable = false;

        var info5 = new global::System.Text.Json.Serialization.Metadata.JsonPropertyInfoValues<string>
        {
            IsProperty = true,
            IsPublic = true,
            IsVirtual = true,
            DeclaringType = typeof(global::System.Exception),
            Converter = null,
            Getter = static obj => ((global::System.Exception)obj).Source,
            Setter = static (obj, value) => ((global::System.Exception)obj).Source = value!,
            IgnoreCondition = null,
            HasJsonInclude = false,
            IsExtensionData = false,
            NumberHandling = null,
            PropertyName = "Source",
            JsonPropertyName = null,
            AttributeProviderFactory = static () => typeof(global::System.Exception).GetProperty("Source", InstanceMemberBindingFlags, null, typeof(string), global::System.Array.Empty<global::System.Type>(), null),
        };
        
        properties[5] = global::System.Text.Json.Serialization.Metadata.JsonMetadataServices.CreatePropertyInfo<string>(options, info5);

        var info6 = new global::System.Text.Json.Serialization.Metadata.JsonPropertyInfoValues<string>
        {
            IsProperty = true,
            IsPublic = true,
            IsVirtual = true,
            DeclaringType = typeof(global::System.Exception),
            Converter = null,
            Getter = static obj => ((global::System.Exception)obj).StackTrace,
            Setter = null,
            IgnoreCondition = null,
            HasJsonInclude = false,
            IsExtensionData = false,
            NumberHandling = null,
            PropertyName = "StackTrace",
            JsonPropertyName = null,
            AttributeProviderFactory = static () => typeof(global::System.Exception).GetProperty("StackTrace", InstanceMemberBindingFlags, null, typeof(string), global::System.Array.Empty<global::System.Type>(), null),
        };
        
        properties[6] = global::System.Text.Json.Serialization.Metadata.JsonMetadataServices.CreatePropertyInfo<string>(options, info6);

        var info7 = new global::System.Text.Json.Serialization.Metadata.JsonPropertyInfoValues<global::System.Reflection.MethodBase>
        {
            IsProperty = true,
            IsPublic = true,
            IsVirtual = false,
            DeclaringType = typeof(global::System.Exception),
            Converter = null,
            Getter = static obj => ((global::System.Exception)obj).TargetSite,
            Setter = null,
            IgnoreCondition = null,
            HasJsonInclude = false,
            IsExtensionData = false,
            NumberHandling = null,
            PropertyName = "TargetSite",
            JsonPropertyName = null,
            AttributeProviderFactory = static () => typeof(global::System.Exception).GetProperty("TargetSite", InstanceMemberBindingFlags, null, typeof(global::System.Reflection.MethodBase), global::System.Array.Empty<global::System.Type>(), null),
        };
        
        properties[7] = global::System.Text.Json.Serialization.Metadata.JsonMetadataServices.CreatePropertyInfo<global::System.Reflection.MethodBase>(options, info7);

        return properties;
    }

    // Intentionally not a static method because we create a delegate to it. Invoking delegates to instance
    // methods is almost as fast as virtual calls. Static methods need to go through a shuffle thunk.
    private void ExceptionSerializeHandler(global::System.Text.Json.Utf8JsonWriter writer, global::System.Exception? value)
    {
        if (value is null)
        {
            writer.WriteNullValue();
            return;
        }
        
        writer.WriteStartObject();

        writer.WritePropertyName(PropName_Data);
        global::System.Text.Json.JsonSerializer.Serialize(writer, ((global::System.Exception)value).Data, IDictionary);
        writer.WriteString(PropName_HelpLink, ((global::System.Exception)value).HelpLink);
        writer.WriteNumber(PropName_HResult, ((global::System.Exception)value).HResult);
        writer.WritePropertyName(PropName_InnerException);
        ExceptionSerializeHandler(writer, ((global::System.Exception)value).InnerException);
        writer.WriteString(PropName_Message, ((global::System.Exception)value).Message);
        writer.WriteString(PropName_Source, ((global::System.Exception)value).Source);
        writer.WriteString(PropName_StackTrace, ((global::System.Exception)value).StackTrace);
        writer.WritePropertyName(PropName_TargetSite);
        global::System.Text.Json.JsonSerializer.Serialize(writer, ((global::System.Exception)value).TargetSite, MethodBase);

        writer.WriteEndObject();
    }
}

Regression?

n/a

Known Workarounds

n/a

Configuration

.NET 9

Other information

No response

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[eiriktsarpalis]

This happens because the source generator will attempt to source generate for every type that is found within the transitive closure of JsonSerializable types, even if those types are only defined on properties that define custom converters:

// A JsonTypeInfo<Bar> has been generated even if it isn't being used by Foo
Console.WriteLine(JsonSerializer.Serialize(new Bar("Value"), MyContext.Default.Bar)); // {"Value":"Value"}

class Foo
{
    [JsonConverter(typeof(CustomBarConverter))]
    public Bar? PropertyWithCustomConverter { get; set; }

    // Serializes Bar as a plain old string
    public class CustomBarConverter : JsonConverter<Bar>
    {
        public override Bar Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
            => new Bar(reader.GetString()!);

        public override void Write(Utf8JsonWriter writer, Bar value, JsonSerializerOptions options) =>
            writer.WriteStringValue(value.Value);
    }
}

record Bar(string Value);

[JsonSerializable(typeof(Foo))]
partial class MyContext : JsonSerializerContext;

I don't think we could change this today without incurring breaking changes.

[stephentoub]

How can trimmer warnings be suppressed just in the generated files? Should the source generator be suppressing them with pragmas? Otherwise, it seems you need to suppress them project-wide.