带参数的异步lambda表达式

Question

I am trying to call my async function from a wrapper. But I am getting this compiler error. In the following code, I would like to get string x value with most of awaits abstracted/pushed into CallAsyncFunc code. What is the right way to do this in c# 4.5? Thanks!

error CS4010: Cannot convert async lambda expression to delegate type ' System.Func<int,string> '. An async lambda expression may return void , Task or Task<T> , none of which are convertible to ' System.Func<int,string> '.

public async Task<T2> CallAsyncFunc<T1,T2>(T1 a, Func<T1, T2> func)
{
    return func.Invoke(a);
}

public async Task<string> GetString(int value)
{
    await Task.Run(() => Thread.Sleep(2000));
    return "" + value;
}

public async Task MainAsyncEntry()
{
    string x = await CallAsyncFunc<int, string>(30, async(x) => await GetString(x));
}

Answer 1

In:

string x = await CallAsyncFunc<int, string>(30, async(x) => await GetString(x));

You have two x that the first issue. Try:

string y = await CallAsyncFunc<int, string>(30, async(x) => await GetString(x));

Here the output of the lambda is string but because you use async it should be a Task or Task<T> .

You can write:

string y = await CallAsyncFunc<int, string>(30, x => GetString(x).Result);
// Equivalent to
string y = await CallAsyncFunc(30, x => GetString(x).Result);

Or

string y = await CallAsyncFunc<int, Task<string>>(30, x => GetString(x)).Result;
// Equivalent to
string y = await CallAsyncFunc(30, GetString).Result;

But in either cases, your CallAsyncFunc will run synchronously because it did not contain any await .

Here the good way to do it:

// This
public async Task<TOut> CallAsyncFunc<TIn, TOut>(TIn input, Func<TIn, Task<TOut>> func)
{
    return await func.Invoke(input);
}

// Or those
public Task<TOut> CallAsyncFunc<TIn, TOut>(TIn input, Func<TIn, Task<TOut>> func)
{
    return func.Invoke(input);
}

public async Task<string> GetString(int value)
{
    await Task.Run(() => Thread.Sleep(2000));
    return value.ToString(CultureInfo.InvariantCulture);
}

public async Task MainAsyncEntry()
{
    string y = await CallAsyncFunc(30, GetString);
}

Answer 2

Remark 1: await GetString(x) unwraps the Task<string> to string , then CallAsyncFunc wraps again into Task<string> . You should rather keep the wrapped expression all the way long of your calling stack. It is the recommanded way as far as I know.

Remark 2: Resharper warns about CallAsyncFunc:

This async method lacks ' await' operators and will run synchronously. Consider using the 'await' operator to await non-blocking API calls, or ' await TaskEx.Run(…)' to do CPU-bound work on a background thread

Indeed you can ommit the async keyword and avoid the re-wrapping into Task<string> .

As a result of Remark 1 and Remark 2, you could write following colde:

// REALLY ASYNC METHOD, WRAPS THE RESULT INTO Task<string>
public async Task<string> GetString(int value)
{
    await Task.Run(() => Thread.Sleep(2000));
    return "" + value;
}

// NOT ASYNC ANY MORE: DOES NOT WRAP THE RESULT INTO Task<T2> any more
public T2 CallAsyncFunc<T1, T2>(T1 a, Func<T1, T2> func)
{
    return func.Invoke(a);
}

// WAIT FOR THE ASYNC RESULT ONLY IN THE OUTER SCOPE (UNWRAPS Task<string> BY THE WAY)
string y = await CallAsyncFunc(30, GetString);
string z = await CallAsyncFunc<int, Task<string>>(30, GetString);