Convert.FromHexString returning an OperationStatus throws unless destination buffer is precisely sized

[vcsjones]

Description

Convert.FromHexString that returns an OperationStatus will throw if

1. The input hex string is short enough that it does not go down the vector 128 path.
2. The destination is larger than needed.

Reproduction Steps

using System.Buffers;

string hex = Convert.ToHexString([255, 255, 255]);
byte[] buffer = new byte[100];
OperationStatus status = Convert.FromHexString(hex, buffer, out int charsConsumed, out int bytesWritten);
Console.WriteLine(status);
Console.WriteLine(bytesWritten);
Console.WriteLine(charsConsumed);

Expected behavior

The hex string decode successfully. The bytesWritten output value should be 3.

Actual behavior

Unhandled exception. System.IndexOutOfRangeException: Index was outside the bounds of the array.
   at System.HexConverter.TryDecodeFromUtf16_Scalar(ReadOnlySpan`1 chars, Span`1 bytes, Int32& charsProcessed)
   at System.HexConverter.TryDecodeFromUtf16(ReadOnlySpan`1 chars, Span`1 bytes, Int32& charsProcessed)
   at System.Convert.FromHexString(ReadOnlySpan`1 source, Span`1 destination, Int32& charsConsumed, Int32& bytesWritten)
   at System.Convert.FromHexString(String source, Span`1 destination, Int32& charsConsumed, Int32& bytesWritten)
   at Program.<Main>$(String[] args) in /Users/vcsjones/Projects/scratch/Program.cs:line 5

Regression?

No, this is a new API in .NET 9.

Known Workarounds

Slice the destination buffer to the required size.

Configuration

• .NET 9.0.0-preview.6.24327.7
• macOS
• Apple Silicon M1 / aarch64

Other information

A debug build of the runtime fails for me hitting this assert:

runtime/src/libraries/Common/src/System/HexConverter.cs

Line 334 in c72aa98

Debug.Assert(chars.Length / 2 == bytes.Length, "Target buffer not right-sized for provided characters");

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[hrrrrustic]

Introduced in my PR with initial implementation: #86556