Fix regression or document breaking change in BinaryReader

[jozkee]

Follow-up to #80331 (comment).

On the following code:

var ms = new MemoryStream(new byte[] { 0x01, 0xC2 });
using (var br = new BinaryReader(ms))
{
    string s = br.ReadString();

    Console.WriteLine(s);
    Console.WriteLine(s.Length);
    Console.WriteLine(s == "\uFFFD");
}

ReadString returns empty string on .NET 8 or prior. After #80331, it returns "\uFFFD".

Thanks @GrabYourPitchforks for pointing the error after merge and share the affected scenario in question.
cc @teo-tsirpanis @adamsitnik

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Issue Details

---

Follow-up to #80331 (comment).

On the following code:

var ms = new MemoryStream(new byte[] { 0x01, 0xC2 });
using (var br = new BinaryReader(ms))
{
    string s = br.ReadString();

    Console.WriteLine(s);
    Console.WriteLine(s.Length);
    Console.WriteLine(s == "\uFFFD");
}

ReadString returns empty string on .NET 8 or prior. After #80331, it returns "\uFFFD".

Thanks @GrabYourPitchforks for pointing the error after merge and share the affected scenario in question.
cc @teo-tsirpanis @adamsitnik

Author:	Jozkee
Assignees:	-
Labels:	area-System.IO, regression-from-last-release
Milestone:	9.0.0

[adamsitnik]

I've missed the notification. @teo-tsirpanis PTAL

[teo-tsirpanis]

This new behavior seems the correct one, and matches Unicode standards, and a breaking change done in .NET Core 3.0.

[jeffhandley]

Thanks, @teo-tsirpanis! Closing this issue since the new behavior is correct and intentional, and it was documented.

[jozkee]

@jeffhandley that document is for 3.0, it may be surprising for customers jumping from 8.0 or lower to not find this documented as part of 9.0 breaking changes.

[dotnet-policy-service]

Added needs-breaking-change-doc-created label because this issue has the breaking-change label.

1. Create and link to this issue a matching issue in the dotnet/docs repo using the breaking change documentation template, then remove this needs-breaking-change-doc-created label.

Tagging @dotnet/compat for awareness of the breaking change.