异步方法的返回类型

Question

I have an async method in C# as follows:

public async Task<string> GetData(int id)
        {
        Task<string> inp =  CommonMethod(id);
        return inp;
}

  public async Task<string> CommonMethod(int id)
        {
        string output ;
        output = await service.GetSomeDummyData(id);
        return output;
}

I am getting an error message as "Since this is an async method,the return expression must be of type string rather than 'Task'

SO, I converted the return type as :

public async Task<string> GetData(int id)
        {
        Task<string> inp =  CommonMethod(id);
        return inp.ToString();
}

The code is getting complied successfully .

I am new to asynchronous programming, is the above conversion method is a best practice of returning string ? Because from the CommonMethod , I am collecting the return type in Task<string> , and in the next statement I am using inp.Tostring() . Little bit of Dilemma in using the correct return types.

Answer 1

When you add the async modifier to the method, you're telling the compiler to automatically wrap its return value as a Task. Or a sequence of Tasks that continue each other on each await statement, loosely speaking. So your code:

public async Task<string> GetData(int id)
{
    Task<string> inp =  CommonMethod(id);
    return inp;
}

would compile to something resembling :

public Task<string> GetData(int id)
{
     return Task.Run() => 
     {
         Task<string> inp =  CommonMethod(id);
         return inp; 
     }
}

So what you're returning there isn't a Task<string> , but a Task<Task<string>> - a Task which returns another Task (which then returns string), which doesn't match the method signature.

So what you want to do is either drop the async modifier and return the internal task directly:

public Task<string> GetData(int id)
{
    Task<string> inp =  CommonMethod(id);
    return inp;
} 

which will work, and return the internal CommonMethod task, or add an internal await statement to return a new Task over the results of the internal task:

public async Task<string> GetData(int id)
{
    string inp =  await CommonMethod(id);
    return inp;
}

which, again loosely, returns a task immediately when the await statement is reached, with the code after it (the return statement) in a ContinueWith block over the internal task. When the internal task completes, the code after await runs, and returns the result as the Result of the original Task that was returned earlier.

Answer 2

You should rather do it this way:

public async Task<string> GetData(int id)
{
    string inp = await CommonMethod(id);
    return inp;
}

The same way you do it in your second call. The returned value will get wrapped up in a Task automatically.

Because you had Task as your return type, and you had packed this in Task manually in your code, it was throwing an error there.