如何将任务＆LT转换; TDerived＆GT;给任务＆LT; TBASE＆GT ;?

问题描述

由于C＃的任务是一个类，你显然不能投了任务＆LT; TDerived＆GT; 到任务＆LT; TBASE＆GT; 。

不过，你可以这样做：

 公共异步任务＆LT; TBASE＆GT;跑（） {
    返回等待MethodThatReturnsDerivedTask（）;
}
 

有没有一种静态任务的方法我可以打电话获得任务＆LT; TDerived＆GT; 实例，它本质上只是指向底层的任务，并投下的结果呢？我想是这样的：

 公共任务＆LT; TBASE＆GT;跑（） {
    返回Task.FromDerived（MethodThatReturnsDerivedTask（））;
}
 

请问这样的方法存在吗？是否有任何开销使用异步方法只用于此目的？

解决方案

No.

Yes. But it's the easiest solution.

Note that a more generic approach is an extension method for Task such as Then. Stephen Toub explored this in a blog post and I've recently incorporated it into AsyncEx.

Using Then, your code would look like:

public Task<TBase> Run()
{
  return MethodThatReturnsDerivedTask().Then(x => (TBase)x);
}

Another approach with slightly less overhead would be to create your own TaskCompletionSource<TBase> and have it completed with the derived result (using TryCompleteFromCompletedTask in my AsyncEx library):

public Task<TBase> Run()
{
  var tcs = new TaskCompletionSource<TBase>();
  MethodThatReturnsDerivedTask().ContinueWith(
      t => tcs.TryCompleteFromCompletedTask(t),
      TaskContinuationOptions.ExecuteSynchronously);
  return tcs.Task;
}

or (if you don't want to take a dependency on AsyncEx):

public Task<TBase> Run()
{
  var tcs = new TaskCompletionSource<TBase>();
  MethodThatReturnsDerivedTask().ContinueWith(t =>
  {
    if (t.IsFaulted)
      tcs.TrySetException(t.Exception.InnerExceptions);
    else if (t.IsCanceled)
      tcs.TrySetCanceled();
    else
      tcs.TrySetResult(t.Result);
  }, TaskContinuationOptions.ExecuteSynchronously);
  return tcs.Task;
}

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