async_handle.cs

class C
{
  [DllImport...]
  private static extern IntPtr GetAHandle();
  
  private static Task WrapHandle()
  {
    var handle = GetAHandle();
    var tcs = new TaskCompletionSource();
    Task.Run(() =>
    {
      var e = new ManualResetEventSlim();
      using (var safeHandle = new SafeWaitHandle(handle, true))
      {
        e.SafeWaitHandle = 
        e.Wait();
      }
      tcs.TrySetResult(true);
    });
    return tcs.Task;
  }
}