awaiting inside an awaitable function in .net-CodePudding

I have a function that returns a Task

    class Sample
    {
       TaskCompletionSource<int> _tcs;
    ..
       public Task<int> DoIt(){
         StartDoingStuff();
         return _tcs.Task;
       }
      ..
    
      private void FinshedStuffCallBack(){
        _tsc.SetResult(42);
      }
    }

Caller goes

 var sample = new Sample();
  var result = await Sample.DoIt();

works fine

Now I need to do something in addition in DoIt, this is itself awaitable

I naively tried

       public async Task<int> DoIt(){
        
         await DoOtherAsyncStuff();
          StartDoingStuff();
         return _tcs.Task;
       }

but this is not allowed

CS4016 Since this is an async method, the return expression must be of type 'int' rather than 'Task'

OK I get what its trying to say, but thats not my intention, I dont have the return value yet, it comes once StartDoingStuff triggers the callback.

Not sure what to try next.

CodePudding user response：

Most likely you just need (note the await on the last line):

       public async Task<int> DoIt()
       {
           await DoOtherAsyncStuff();
           StartDoingStuff();
           return await _tcs.Task;
       }

await is needed on the last line because an async function returning Task<int> needs to return int, while _tcs.Task is a Task<int>. Using await will wait for the Task's completion and return the int which is what we need.

However, depending on your complete code you may want something else. For example if you're doing more complicated things with TaskCompletionSource you may need to remove async for this definition and do something like

       public Task<int> DoIt()
       {
           return DoOtherAsyncStuff().ContinueWith(_ => 
               {
                   StartDoingStuff();
                   return _tcs.Task;
               }, TaskCompletionOptions.ExecuteSynchronously);
       }

In general it's best not to mess with TaskCompletionSource unless you're doing something more advanced, for example providing an async abstraction of something synchronous/callback based. Hence a complete code example may change my answer (for example what's the body of StartDoingStuff DoOtherAsyncStuff?).