PopCount<T> and Generic Bit Counting for IBinaryInteger<T>

[BoCoKeith]

I need to be able to get the number of bits as an integer needed for a generic integer data type (IBinaryInteger) inside a generic method. I can't find a property or method that gives me this directly, am I missing something?

Assuming not, it seems like a great answer would be ...

public int CountBits<T>() where T: IBinaryInteger<T> => (int)(T.PopCount(T.AllBitsSet));

... but the value returned from PopCount can't be cast to an integer.

My next choice is something like ...

public int CountBits<T>() where T: IBinaryInteger<T> => T.AllBitsSet.GetByteCount() * 8;

... which is mostly fine, except that it wouldn't work for a integer type that isn't a multiple of 8 (say a hypothetical UInt7).

I'm left with counting the bits myself...

public int CountBits<T>() where T: IBinaryInteger<T> {
    int count = 0;
    int value = T.AllBitsSet;
    while (value != T.Zero) {
        count++;
        value >>= 1;
    };
    return count;
}

... which I don't mind (it works great), but I don't want to go down this path if there's a built-in way to do it.

As an aside, what is the logic behind having PopCount return a T and not an int? It seems like an odd choice with limited use cases (particular since other PopCount methods, such as those in BitConverter, do return ints).

[huoyaoyuan]

... but the value returned from PopCount can't be cast to an integer.

Use int.CreateXXX to cast.