[API Proposal]: add overloads to support IAsyncEnumerable with source-gen

[steveharter]

Background and motivation

Add overloads to support IAsyncEnumerable with source-gen. See #58221 for more information.

Once implemented, source-gen scenarios using should no longer throw NotSupportedException when using IAsyncEnumerable<T>.

API Proposal

namespace System.Text.Json
{
     public static class JsonSerializer
     {
         public static IAsyncEnumerable<TValue?> DeserializeAsyncEnumerable<TValue>(Stream utf8Json, JsonSerializerOptions options, CancellationToken cancellationToken = default);
+        public static IAsyncEnumerable<TValue?> DeserializeAsyncEnumerable<TValue>(Stream utf8Json, JsonTypeInfo<TValue> jsonTypeInfo, CancellationToken cancellationToken = default);
     }
}

namespace System.Text.Json.Serialization.Metadata
{
    [EditorBrowsable(EditorBrowsableState.Never)]
    public static class JsonMetadataServices
    {
        public static JsonTypeInfo<TCollection> CreateIEnumerableInfo<TCollection, TElement>(JsonSerializerOptions options, JsonCollectionInfoValues<TCollection> collectionInfo) where TCollection : IEnumerable<TElement> {}
+       public static JsonTypeInfo<TAsyncEnumerable> CreateIAsyncEnumerableInfo<TAsyncEnumerable, TElement>(JsonSerializerOptions options, JsonCollectionInfoValues<TAsyncEnumerable> collectionInfo) where TAsyncEnumerable : IAsyncEnumerable<TElement> {}
    }
}

API Usage

JsonSerializer.Serialize(new MyPoco { Data = ... }, MyContext.MyPoco); // Currently not supported

[JsonSerializable(typeof(typeof(MyPoco))]
public class MyContext : JsonSerializerContext {}

public class MyPoco
{
    public IAsyncEnumerable<int> Data { get; set; }
}

Risks

No response

[ghost]

Tagging subscribers to this area: @eiriktsarpalis, @layomia
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Background and motivation

Add overloads to support IAsyncEnumerable with source-gen. See #58221 for more information.

Once implemented, source-gen scenarios using should no longer throw NotSupportedException when using IAsyncEnumerable<T>.

API Proposal

namespace System.Text.Json
{
     public static class JsonSerializer
     {
+        public static System.Collections.Generic.IAsyncEnumerable<TValue?> DeserializeAsyncEnumerable<TValue>(System.IO.Stream utf8Json, System.Text.Json.Serialization.Metadata.JsonTypeInfo<TValue> jsonTypeInfo, System.Threading.CancellationToken cancellationToken = default(System.Threading.CancellationToken));

     }
}

namespace System.Text.Json.Serialization.Metadata
{
    public static class JsonMetadataServices
    {
+        public static System.Text.Json.Serialization.JsonConverter<T?> GetAsyncEnumerableConverter<T>(System.Text.Json.Serialization.Metadata.JsonTypeInfo<T> underlyingTypeInfo);
    }
}

API Usage

// fast-path serialization will generate calls to serialize. For example on a property on a Foo POCO class:
private static void FooSerialize(global::System.Text.Json.Utf8JsonWriter writer, Foo value)
{
    ...
    writer.WritePropertyName(PropName_MyAsyncEnumerable);
    global::System.Text.Json.JsonSerializer.Serialize(writer, value.MyAsyncEnumerable, MySerializationContext.Default.MyAsyncEnumerable);
    ...
}

// and manual calls to deserialize are supported:
System.IO.Stream utf8Json = ...;
IAsyncEnumerable<MyAsyncEnumerable> value = JsonSerializer.DeserializeAsyncEnumerable(stream, MySerializationContext.Default.MyAsyncEnumerable);

Risks

No response

Author:	steveharter
Assignees:	-
Labels:	api-suggestion, area-System.Text.Json
Milestone:	7.0.0

[jeffhandley]

@pranavkm / @davidfowl -- Would ASP.NET use this in .NET 7 if we included it in one of the later previews? If not, we're leaning toward punting this to Future.

[eiriktsarpalis]

Exposing top-level methods for IAsyncEnumerable serialization is one thing, but for completeness we also need to consider exposing the IAsyncEnumerable converter in the JsonMetadataServices namespace to ensure parity in scenaria like the ones described in #58221

[eiriktsarpalis]

fast-path serialization will generate calls to serialize. For example on a property on a Foo POCO class

FWIW I don't think we would be able to support fast-path serialization for IAsyncEnumerable easily. It would require exposing a SerializeHandler delegate that supports asynchrony, e.g. something like Func<Utf8JsonWriter, TAsyncEnumerable, Task>, but I'm not sure we'd be able to plug this in easily into our existing serialization infrastructure.

[eiriktsarpalis]

I've updated the OP, believe we can take this to API review now.

[terrajobst]

Video

Looks good as proposed

namespace System.Text.Json
{
     public partial static class JsonSerializer
     {
        // Existing
        // public static IAsyncEnumerable<TValue?> DeserializeAsyncEnumerable<TValue>(Stream utf8Json, JsonSerializerOptions options, CancellationToken cancellationToken = default);
        public static IAsyncEnumerable<TValue?> DeserializeAsyncEnumerable<TValue>(Stream utf8Json, JsonTypeInfo<TValue> jsonTypeInfo, CancellationToken cancellationToken = default);
     }
}

namespace System.Text.Json.Serialization.Metadata
{
    [EditorBrowsable(EditorBrowsableState.Never)]
    public static class JsonMetadataServices
    {
        // Existing
        // public static JsonTypeInfo<TCollection> CreateIEnumerableInfo<TCollection, TElement>(JsonSerializerOptions options, JsonCollectionInfoValues<TCollection> collectionInfo) where TCollection : IEnumerable<TElement>;
        public static JsonTypeInfo<TAsyncEnumerable> CreateIAsyncEnumerableInfo<TAsyncEnumerable, TElement>(JsonSerializerOptions options, JsonCollectionInfoValues<TAsyncEnumerable> collectionInfo) where TAsyncEnumerable : IAsyncEnumerable<TElement>;
    }
}