Fix #8302 redis subscriptions with union payload fields

[michaelstaib]

Fixes #8302

Summary

• Adds a Redis integration regression test that reproduces a subscription payload containing a union-typed field on a parent object.
• Adds a runtime-polymorphic JSON converter used by DefaultJsonMessageSerializer so interface/abstract members are serialized with runtime type metadata and deserialized back to concrete types.
• Keeps deserialization guarded by assignability checks before materialization.

Repro Test Code

[Fact]
public async Task Subscribe_Union_Field_In_Payload()
{
    using var cts = new CancellationTokenSource(Timeout);
    await using var services = CreateServer(
        builder => builder
            .AddSubscriptionType<UnionPayloadSubscription>()
            .AddType<UnionTextMessage>()
            .AddType<UnionCodeMessage>()
            .ModifyOptions(o => o.StrictValidation = false));
    var sender = services.GetRequiredService<ITopicEventSender>();

    var result = await services.ExecuteRequestAsync(
        """
        subscription {
          onUnionPayload {
            message {
              __typename
              ... on UnionTextMessage {
                text
              }
            }
          }
        }
        """,
        cancellationToken: cts.Token);

    await using var responseStream = result.ExpectResponseStream();
    var responses = responseStream.ReadResultsAsync().ConfigureAwait(false);

    await sender.SendAsync(
        "OnUnionPayload",
        new UnionPayloadEnvelope
        {
            Message = new UnionTextMessage { Text = "from-redis" }
        },
        cts.Token);
    await sender.CompleteAsync("OnUnionPayload");

    var snapshot = new Snapshot();

    await foreach (var response in responses.WithCancellation(cts.Token).ConfigureAwait(false))
    {
        snapshot.Add(response);
    }

    snapshot.MatchInline(
        """
        {
          "data": {
            "onUnionPayload": {
              "message": {
                "__typename": "UnionTextMessage",
                "text": "from-redis"
              }
            }
          }
        }
        """);
}

Tests

• dotnet test src/HotChocolate/Core/test/Subscriptions.Tests/HotChocolate.Subscriptions.Tests.csproj -f net8.0 --filter "FullyQualifiedName~DefaultJsonMessageSerializerTests.Serialize_And_Deserialize_DefaultMessage_With_Interface_Property" (Passed)
• dotnet test src/HotChocolate/Core/test/Subscriptions.Redis.Tests/HotChocolate.Subscriptions.Redis.Tests.csproj -f net8.0 --filter "FullyQualifiedName~RedisIntegrationTests.Subscribe_Union_Field_In_Payload" (Passed)
• dotnet test src/HotChocolate/Core/test/Subscriptions.Tests/HotChocolate.Subscriptions.Tests.csproj -f net9.0 --filter "FullyQualifiedName~DefaultJsonMessageSerializerTests.Serialize_And_Deserialize_DefaultMessage_With_Interface_Property" (Passed)
• dotnet test src/HotChocolate/Core/test/Subscriptions.Redis.Tests/HotChocolate.Subscriptions.Redis.Tests.csproj -f net9.0 --filter "FullyQualifiedName~RedisIntegrationTests.Subscribe_Union_Field_In_Payload" (Passed)
• dotnet test src/HotChocolate/Core/test/Subscriptions.Tests/HotChocolate.Subscriptions.Tests.csproj -f net10.0 --filter "FullyQualifiedName~DefaultJsonMessageSerializerTests.Serialize_And_Deserialize_DefaultMessage_With_Interface_Property" (Passed)
• dotnet test src/HotChocolate/Core/test/Subscriptions.Redis.Tests/HotChocolate.Subscriptions.Redis.Tests.csproj -f net10.0 --filter "FullyQualifiedName~RedisIntegrationTests.Subscribe_Union_Field_In_Payload" (Passed)

[github-actions]

Fusion Gateway Performance Results

Progress: 0/6 benchmarks completed — this report updates as each job finishes.

Simple Composite Query

	Req/s	Err%
Constant 2 (50 VUs)	pending	pending
Ramping 2 (0-500-0 VUs)	pending	pending

Response Times

	Min	Med	Avg	P90	P95	Max
Constant 2	pending	pending	pending	pending	pending	pending
Ramping 2	pending	pending	pending	pending	pending	pending

---

Deep Recursion Query

	Req/s	Err%
Constant 2 (50 VUs)	pending	pending
Ramping 2 (0-500-0 VUs)	pending	pending

Response Times

	Min	Med	Avg	P90	P95	Max
Constant 2	pending	pending	pending	pending	pending	pending
Ramping 2	pending	pending	pending	pending	pending	pending

---

Variable Batching Throughput

	Req/s	Err%
Constant 2 (50 VUs)	pending	pending
Ramping 2 (0-500-0 VUs)	pending	pending

Response Times

	Min	Med	Avg	P90	P95	Max
Constant 2	pending	pending	pending	pending	pending	pending
Ramping 2	pending	pending	pending	pending	pending	pending

---

Runner 2 = benchmarking-2

Run 22506364038 • Commit 287df5d • Fri, 27 Feb 2026 22:31:03 GMT