Test Failure: Read_NonGeneric_NoPublicConstructor_Throws

[vcsjones]

Build Information

Build: https://dev.azure.com/dnceng-public/cbb18261-c48f-4abb-8651-8cdcb5474649/_build/results?buildId=1395596
Build error leg or test failing: System.Text.Json.Serialization.Tests.CollectionTestsDynamic_AsyncStreamWithSmallBuffer.Read_NonGeneric_NoPublicConstructor_Throws
Pull request: #127248

Error Message

Fill the error message using step by step known issues guidance.

{
  "ErrorMessage": "",
  "ErrorPattern": "CollectionTestsDynamic_AsyncStreamWithSmallBuffer\\.Read_NonGeneric_NoPublicConstructor_Throws.*\\[FAIL\\]",
  "BuildRetry": false,
  "ExcludeConsoleLog": false
}

Known issue validation

Build: 🔎 https://dev.azure.com/dnceng-public/public/_build/results?buildId=1395596
Error message validated: [CollectionTestsDynamic_AsyncStreamWithSmallBuffer\.Read_NonGeneric_NoPublicConstructor_Throws.*\[FAIL\]]
Result validation: ✅ Known issue matched with the provided build.
Validation performed at: 4/24/2026 9:34:43 PM UTC

Report

Build	Definition	Test	Pull Request
2960038	dotnet-runtime	System.Text.Json.Serialization.Tests.CollectionTestsDynamic_AsyncStreamWithSmallBuffer.Read_NonGeneric_NoPublicConstructor_Throws
1395596	dotnet/runtime	System.Text.Json.Serialization.Tests.CollectionTestsDynamic_AsyncStreamWithSmallBuffer.Read_NonGeneric_NoPublicConstructor_Throws	#127248

Summary

24-Hour Hit Count	7-Day Hit Count	1-Month Count
0	2	2

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[danmoseley]

No logs just
System.NullReferenceException : Object reference not set to an instance of an object.
Stack trace
at System.Text.Json.Serialization.Tests.CollectionTests.Read_NonGeneric_NoPublicConstructor_Throws(Type type, String json) in /_/src/libraries/System.Text.Json/tests/Common/CollectionTests/CollectionTests.NonGeneric.Read.cs:line 593
--- End of stack trace from previous location ---

AI says not much:

The interesting technical clue is that only the AsyncStreamWithSmallBuffer serializer wrapper failed. That wrapper forces
JsonSerializerOptions.DefaultBufferSize = 1 and inserts a UTF-8 BOM, which exercises the STJ async continuation path through
StreamReadBufferState and collection converter error handling. The expected path is ThrowNotSupportedException_*; instead a
NullReferenceException escapes from the deserialize call.