BinaryReader.GetString() returns "\uFFFD" on malformed sequences
A a minor breaking change was introduced that only affects malformed encoded payloads.
Prior to .NET 9, a malformed encoded string [0x01, 0xC2] that was parsed with BinaryReader.ReadString() returned an empty string.
Starting in .NET 9, BinaryReader.ReadString() returns "\uFFFD", which is the REPLACEMENT CHARACTER used to replace an unknown, unrecognized, or unrepresentable character. This change only affects malformed payloads and matches Unicode standards.
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
Starting in .NET 9, the same code snippet produces different results for s == "\uFFFD" and s.Length, as shown in the code comments:
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
}
Version introduced
.NET 9 Preview 7
Type of breaking change
This change is a behavioral change.
Reason for change
This change was made as a performance improvement that affects a rare scenario.
Recommended action
If you want to keep the previous behavior where incomplete byte sequence were omitted at the end of the string, call TrimEnd("\uFFFD") on the result.
