Error on TryComputeHash

[Zetanova]

The generic HashAlgorithm.TryComputeHash(ReadOnlySpan<byte> source, Span<byte> destination, out int bytesWritten) implementation of this function gives an error on the FinalBlock call.

It would be nice to overload this method

[daiplusplus]

I experienced the same problem today when I was reimplementing MurmurHash myself and was running tests that compared my implementation to this library's implementations as a reference.

• The root cause is (what I consider) a poor API design in .NET's HashAlgorithm class: when you subclass HashAlgorithm you need to set the hash length; this can be done in two ways:
  • By either setting the protected Int32 HashSizeValue field in your constructor - or anywhere in your subclass (because the field is fully mutable and not readonly).
  • Or by overriding the public virtual Int32 HashSize { get; } property.
  • However, if you only override HashSize and never set HashSizeValue then you'll get this error.
• When HashAlgorithm added support for Span<Byte> in the new virtual methods TryComputeHash and TryHashFinal, the default implementations only check the protected Int32 HashSizeValue field and not the property.
• ...but this MurmurHashXX library only overrides the properties, it never sets the HashSizeValue field.
• So the default implementations throw the InvalidOperationException "The algorithm's implementation is incorrect." because it thinks the hash length is 0 bytes, instead of 4 or 16.
• Subclassing MurmurHash32 and MurmurHash128 isn't really a feasible option, but the good news is that you can use reflection to correctly set the field right-after you get an instance of the objects.

Like so:

private static readonly FieldInfo _hashAlgorithm_HashSizeValue_Field = typeof(HashAlgorithm).GetField( "HashSizeValue", BindingFlags.NonPublic | BindingFlags.Instance ) ?? throw new InvalidOperationException( "Couldn't find HashAlgorithm.HashSizeValue field." );

private static void FixHashSizeValueField( this HashAlgorithm hashAlgo )
{
	_hashAlgorithm_HashSizeValue_Field.SetValue( obj: hashAlgo, value: hashAlgo.HashSize );
}