[Breaking change]: BinaryReader.GetString() will return "\uFFFD" on malformed encoded string sequences.

[jozkee]

Description

dotnet/runtime#80331 introduced a minor breaking change only affecting malformed encoded payloads.

Prior to .NET 9, a malformed encoded string [0x01, 0xC2] parsed with BinaryReader.ReadString() would return an empty string.

ON .NET 9, it would return "\uFFFD" which is the REPLACEMENT CHARACTER used to replace an unknown, unrecognised, or unrepresentable character. We accepted this change because it only affected malformed payloads and matches Unicode standards.

Version

.NET 9 Preview 7

Previous behavior

var ms = new MemoryStream(new byte[] { 0x01, 0xC2 });
using (var br = new BinaryReader(ms))
{
    string s = br.ReadString();
    Console.WriteLine(s == "\uFFFD"); // false
    Console.WriteLine(s.Length); // 0
}

New behavior

var ms = new MemoryStream(new byte[] { 0x01, 0xC2 });
using (var br = new BinaryReader(ms))
{
    string s = br.ReadString();
    Console.WriteLine(s == "\uFFFD"); // true
    Console.WriteLine(s.Length); // 1
}

Type of breaking change

• Binary incompatible: Existing binaries might encounter a breaking change in behavior, such as failure to load or execute, and if so, require recompilation.
• Source incompatible: When recompiled using the new SDK or component or to target the new runtime, existing source code might require source changes to compile successfully.
• Behavioral change: Existing binaries might behave differently at run time.

Reason for change

Perf improvement affecting a rare scenario.

Recommended action

If you want to keep the previous behavior where incomplete byte sequence were being omitted at the end of the string, you can TrimEnd("\uFFFD") the result.

Feature area

Core .NET libraries

Affected APIs

BinaryReader.ReadString()

---

Associated WorkItem - 320280

[jozkee]

FWIW: This is somewhat undefined behavior and is inconsistent with other decoding APIs in BinaryReader. Using [0xC2], ReadChar() throws EndOfStreamException and ReadChars(1) returns an empty array.

cc @adamsitnik @GrabYourPitchforks @jeffhandley @teo-tsirpanis