Task.Delay（0）不是异步的

Question

I have the following code (yes, I could be simulating JavaScript setTimeout api)

    async void setTimeout(dynamic callback, int timeout)
    {
        await Task.Delay(timeout);
        callback();
    }

It looks like for timeout > 0 , the setTimeout works asynchronously wherein the control is returned back to the callee on await and callback is invoked after the task runs asynchronously. BUT when timeout == 0 , the function behaves in a synchronous manner (code always run past the await line in the same thread without a context switch). On further digging, it turns out that Task.Delay is implemented to behave this way ( Task.Yield() versus Task.Delay(0) )

Wondering if there is a way to make Task.Delay(0) asynchronous or an alternate solution to make my setTimeout function asynchronous when timeout is 0 ? ( so that I could mimic JavaScript setTimeout functionality ) I see discussions on using Task.FromResult(true) or Task.WhenAll , but they don't seem to work.

Indeed I can use Task.Delay(1) instead of Task.Delay(0) but it does not look organic.

Answer 1

The reason Task.Delay(0) does not run asynchronously is because the async-await state machine explicitly checks whether the task is completed and if it is, it runs synchronously.

You can try using Task.Yield() , which will force the method to return immediately and resume the rest of the method on the current SynchornizationContext . eg:

async void setTimeout(dynamic callback, int timeout)
{
    if(timeout > 0)
    {
        await Task.Delay(timeout);
    }
    else
    { 
        await Task.Yield();
    }

    callback();
}