没有等待的尾调用异步

Question

Take this async method:

public async Task<string> ReadStringFromUrlAsync(string url)
{
    WebRequest request = WebRequest.Create(url);
    WebResponse response = request.GetResponse();
    Stream dataStream = response.GetResponseStream();
    var reader = new StreamReader(dataStream);
    return await reader.ReadToEndAsync();
}

Because it returns the result of another task I believe I can do away with async and await :

public Task<string> ReadStringFromUrlAsync(string url)
{
    WebRequest request = WebRequest.Create(url);
    WebResponse response = request.GetResponse();
    Stream dataStream = response.GetResponseStream();
    var reader = new StreamReader(dataStream);
    return reader.ReadToEndAsync();
}

I noticed that the call stack in the event of an exception won't mention ReadStringFromUrlAsync , are there any other drawbacks to removing aysnc / await on tail calls in this way.

Answer 1

It's fine in most cases, but there is an important difference in exception behavior: any exceptions before the task is returned will be thrown directly rather than placed on the task. In your example, if WebRequest.Create , GetResponse , GetResponseStream , or StreamReader(..) throws, then in an async method that exception would be placed on the task, while in a non- async method that exception would be thrown directly.

BTW, if that's representative of your real code, I'd recommend using HttpClient instead, which was designed with await in mind:

public Task<string> ReadStringFromUrlAsync(string url)
{
  return new HttpClient().GetStringAsync(url);
}

Answer 2

You should be fine with removing this await and async . In fact, Roslyn (new C# Compiler) team is thinking about making it an compiler optimization .

Answer 3

Yes, that is correct. When you're just returning a task, you can just omit the async and return the Task directly, without await ing.