AsyncCompletion.cs

public class AsyncCompletion
{
    private static readonly TaskCompletionSource<Unit> CompletedSource;
 
    static AsyncCompletion()
    {
        CompletedSource = new TaskCompletionSource<Unit>();
        CompletedSource.SetResult(Unit.Default);
    }
    
    private readonly object _gate = new object();
    private volatile TaskCompletionSource<Unit> _cts;
 
    public Task Completion
    {
        get
        {
            if (_cts != null)
                return _cts.Task;
 
            lock (_gate)
            {
                if (_cts != null)
                    return _cts.Task;
 
                _cts = new TaskCompletionSource<Unit>();
            }
 
            return _cts.Task;
        }
    }
 
    public void Complete()
    {
        if (_cts == CompletedSource)
            return;
 
        TaskCompletionSource<Unit> cts;
 
        lock (_gate)
        {
            if (_cts == CompletedSource)
                return;
 
            cts = _cts;
            _cts = CompletedSource;
        }
 
        if (cts != null)
        {
            cts.SetResult(Unit.Default);
        }
    }
}