c# - 延迟任务，超过延迟时间则返回空值

Question

如何延迟任务直到返回变量的值，如果超过延迟时间，则变量的值返回 null。例如:

TaskA.Delay(5000);

if variable not equal to empty string
continue with Task A

else
if delay time not exceeded yet

continue running the current task
else

set variable's value to empty string

Answer 1

诀窍是启动一个“延迟”任务，并与“实际”任务同时运行它，看看哪个任务先完成:

public async Task<int?> ValueOrNull( )
{
  var task = SomeAsyncMethod() //--> the work to wait on
  var timeout = Task.Delay( new TimeSpan( 0,0,5 ) );
  var first = await Task.WhenAny( task, timeout );
  if ( first == timeout ) return null;
  await first;  //--> It's already done, but let it throw any exception here
  return;
}

您可以通过将任务和超时传递到以下位置来概括这一点:

public async Task<T> ValueOrNull( Task<T> task, TimeSpan patience )
{
  var timeout = Task.Delay( patience );
  var first = await Task.WhenAny( task, timeout );
  if ( first == timeout ) return default( T );
  await first;  //--> It's already done, but let it throw any exception here
  return;
}