System.InvalidProgramException : Cannot create boxed ByRef-like values.

[RohitBinjola]

Getting below issue in MOQ unit test after upgrading the framework from 3.1 to 6.

Message: 
System.InvalidProgramException : Cannot create boxed ByRef-like values.

Stack Trace: 

StreamProxy.Write(ReadOnlySpan`1 buffer)
StreamWriter.Flush(Boolean flushStream, Boolean flushEncoder)
StreamWriter.Flush()

Test Code: (Same code is working fine in .net core 3.1 but failing with below exception in .net 6)

public void StreamWriter_Test()    
{
            var stream = new Mock<Stream>();
            stream.Setup(s => s.CanWrite).Returns(true);
            using var sw = new StreamWriter(stream.Object);
            sw.WriteLine("Test");
            sw.Flush();
}

[stakx]

It would appear that StreamWriter.WriteLine calls its underlying stream's Write(ReadOnlySpan<> buffer) method, which Moq cannot mock (because its underlying library, DynamicProxy, cannot intercept it). That's an architectural limitation in both DynamicProxy and Moq; there's nothing that can be done about it. You'll likely need to create a mock Stream manually.