[API Proposal]: Adding a Cardinality/PopCount property to BitArray

[polijp]

Background and motivation

You often need to know how many bits are set to 1 in a BitArray. Presently, there are two ways to achieve that:

• either you copy the BitArray instance to an int[] and then you count (eventually using intrinsics)
• or you access one by one the bits.

Adding a Cardinality property takes the advantage of the underlying int array. It could use the instrinsic BitOperations.PopCount() method.

API Proposal

namespace System.Collections;

public class BitArray ...
{
    public int PopCount => ...
}

API Usage

// Fancy the value
var c = new BitArray(10);
if (c.PopCount %2 ==0) do something;

### Alternative Designs

_No response_

### Risks

_No response_

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