Binary writer fix

[koszeggy]

Fixes #107265

I see that you already set the milestone to v10 but hoping that some contribution may help to add this to .NET 9 I prepared a possible solution. Of course, I understand if it's too late.

Feel free to adjust it.

[huoyaoyuan]

The original issue was marked as "needs further triage" and we haven't decided what's the expected behavior. Since it's not a recent change happened from 9.0, it's not likely to change for 9.0.

[koszeggy]

Since it's not a recent change happened from 9.0, it's not likely to change for 9.0.

Understood. Actually I can live with the (far from optimized) workaround I applied as it's just a unit test helper class:

public override void Write(string value)
{
#if !NET6_0_OR_GREATER // https://github.com/dotnet/runtime/issues/107265
    base.Write(value);
#else
    var bytes = Encoding.UTF8.GetBytes(value);
    Write7BitEncodedInt(bytes.Length);
    OutStream.Write(bytes, 0, bytes.Length);
#endif
    // [...] actual logging, stack trace dump
}

[kasperk81]

add a unit test in src/libraries/System.Runtime/tests/System.IO.Tests/BinaryWriter/BinaryWriter.WriteTests.cs

[koszeggy]

add a unit test in src/libraries/System.Runtime/tests/System.IO.Tests/BinaryWriter/BinaryWriter.WriteTests.cs

In the end I did not add a new test case but modified the WriteTest<T> method, executing the passed delegates on derived and non-derived BinaryWriter/BinaryReader instances, with UTF8 and non-UTF8 encodings.

Now the already existing BinaryWriter_WriteStringTest fails on .NET 9.0 but succeeds on .NET 5.0

[adamsitnik]

@koszeggy First of all please excuse me for such a delay in the code review process. For some reason I've missed the GH notification.

The fix LGTM, but I need to ensure that it's not going to regress the performance before merging it.

[Copilot]

Pull request overview

This PR fixes a breaking change in BinaryWriter.Write(string) introduced in .NET 6.0, where an optimization for short UTF-8 strings bypassed the Write7BitEncodedInt method, preventing derived classes from tracking writes through their overridden Write(byte) method. The fix ensures that derived BinaryWriter classes maintain consistent virtual method dispatch by adding a type check before applying the optimization.

Changes:

• Added GetType() == typeof(BinaryWriter) check to ensure the fast-path string optimization is only used for non-derived BinaryWriter instances
• Added comprehensive tests with derived BinaryWriter/BinaryReader classes that track internal positions to verify virtual method calls are symmetric
• Tests cover both UTF-8 and UTF-16 encodings with both base and derived classes

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/libraries/System.Private.CoreLib/src/System/IO/BinaryWriter.cs	Added type check to preserve virtual method dispatch for derived BinaryWriter classes when writing strings
src/libraries/System.Runtime/tests/System.IO.Tests/BinaryWriter/BinaryWriter.WriteTests.cs	Added TestWriter and TestReader classes that track position, expanded WriteTest to run with 4 scenarios (base/derived × UTF-8/UTF-16), added position synchronization validation

[jeffhandley]

@koszeggy / @adamsitnik I tasked Copilot with Add BinaryWriter benchmarks for _useFastUtf8 true and false code paths (dotnet/performance#5121) to unblock this PR. @huoyaoyuan's comment long ago reassured us the pattern taken here should be OK but for this change I do want to make sure we have benchmarks that would catch any salient perf changes.

@adamsitnik Can you re-review though too please?

[jeffhandley]

/ba-g BadExit on an unrelated test