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[release/3.0] Fix BinaryReader.ReadChars for fragmented Streams (#26324) #26356
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…et#26324) BinaryReader.ReadChars incorrectly read more than necessary from the underlying Stream when multi-byte characters straddled the read chunks. Fixes https://github.com/dotnet/corefx/issues/40455
stephentoub
approved these changes
Aug 24, 2019
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Ports #26324 to 3.0
Fixes dotnet/corefx#40455
Description
BinaryReader.ReadChars may read more bytes than required from the underlying stream if the stream contain non-ASCII characters. It corrupts the data read by any subsequent BinaryReader.Read operation. The same bug was fixed in .NET Framework in 2010. The fix was accidentally removed during unification of BCL code between .NET Core and .NET Native.
Customer Impact
The bug was customer-reported as a regression of previous BinaryReader.ReadChars behavior. This API is used by ~7% of apps according to apisof.net.
Regression?
Yes, between .NET Core 2.2 (and earlier) and .NET Core 3.0.
Risk
Low. The fix is essentially adding back the previous code, with extra comments and correctness fix for UTF-16 encoding.