Interlocked.CompareExchange missing for uint, ulong and (if possible) general structs.

[redknightlois]

I recently needed to use Interlocked.CompareExchange inside a function with the following form:

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        private bool TryClaimSlot(ref uint entryKey, int key)
        {
            var entryKeyValue = entryKey;
            //zero keys are claimed via hash
            if (entryKeyValue == 0 & key != 0)
            {
                entryKeyValue = Interlocked.CompareExchange(ref entryKey, key, 0);
                if (entryKeyValue == 0)
                {
                    // claimed a new slot
                    this.allocatedSlotCount.Increment();
                    return true;
                }
            }

            return key == entryKeyValue;
        }

problem is that it can´t be done because the Interlocked.CompareExchange<T> requires it to be a class.

Also if anyone knows a workaround for it, I would appreciate it.

[stephentoub]

workaround

For uint, you could do something like:

static unsafe uint InterlockedCompareExchange(ref uint location, uint value, uint comparand)
{
    fixed (uint* ptr = &location)
    {
        return (uint)Interlocked.CompareExchange(ref *(int*)ptr, (int)value, (int)comparand);
    }
}

For ulong, same but with long instead of int overload.

For structs, not sure how that would work given platform limitations on what sizes can be worked with atomically, alignment, etc.

[stephentoub]

(And with System.Runtime.CompilerServices.Unsafe, you could do the same thing for uint/ulong but without the unsafe pointer stuff: https://github.com/dotnet/corefx/blob/master/src/System.Runtime.CompilerServices.Unsafe/ref/System.Runtime.CompilerServices.Unsafe.cs#L19)

[redknightlois]

Great! Gonna use that.

[danmoseley]

Do we still need an issue? Is there a proposal to add such API?

[redknightlois]

It feels strange that uint and ulong are not treated as first class citizens in Interlocked methods. So yes, the proposal is to add them.

[stephentoub]

It feels strange that uint and ulong are not treated as first class citizens

I expect it's because they're not CLS compliant.

[jkotas]

This issue was moved to dotnet/corefx#26378