无法隐式转换类型'字符串'到'System.Threading.Tasks.Task＆LT;串GT;' [英] Cannot implicitly convert type 'string' to 'System.Threading.Tasks.Task<string>'

问题描述

我是新来的异步编程，因此通过一些异步采样codeS去后，我想编写一个简单的异步code的

I am new to asynchronous programming, so after going through some async sample codes, I thought of writing a simple async code

我创建了一个简单的WinForm应用程序和表单里面我写了下面code。但它只是不工作

I created a simple Winform application and inside the Form I wrote the following code. But its just not working

private Task<string> methodAsync() {
    Thread.Sleep(10000);
    return "Hello"; //Error: Cannot implicitly convert type 'string' to 'System.Threading.Tasks.Task<string>'
}

private async void button1_Click(object sender, EventArgs e)
{
    string s = await methodAsync();
    MessageBox.Show(s);
}

可能有人请把一些光在这里..

Could someone please put some light here..

推荐答案

该方法的列出的返回类型是任务＆LT;串GT; 。你试图返回字符串。他们是不一样的，也没有从字符串隐式转换为任务＆LT;字符串方式＆gt; ，因此误差

The listed return type of the method is Task<string>. You're trying to return a string. They are not the same, nor is there an implicit conversion from string to Task<string>, hence the error.

你很可能与异步方法，其返回值是自动包装在这混乱工作由编译器。目前，这种方法不能异步方法。你几乎可以肯定，为了做到这一点：

You're likely confusing this with an async method in which the return value is automatically wrapped in a Task by the compiler. Currently that method is not an async method. You almost certainly meant to do this:

private async Task<string> methodAsync() 
{
    await Task.Delay(10000);
    return "Hello";
}

有两个重要的变化。首先，该方法被标记为异步，这意味着返回类型包裹在一个工作，使得该方法编译。接下来，我们不想做一个阻塞等待。作为一般规则，使用时的await 模式总是避免时，你可以阻止等待。 Task.Delay 是将指定的毫秒数后可以完成的任务。通过等待 -ing这一任务，我们有效地在这个时间进行非阻塞的等待（实际上该方法的其余部分是工作的延续）。

There are two key changes. First, the method is marked as async, which means the return type is wrapped in a Task, making the method compile. Next, we don't want to do a blocking wait. As a general rule, when using the await model always avoid blocking waits when you can. Task.Delay is a task that will be completed after the specified number of milliseconds. By await-ing that task we are effectively performing a non-blocking wait for that time (in actuality the remainder of the method is a continuation of that task).

如果您preFER做这件事的方式4.0，而无需使用伺机，你可以这样做：

If you prefer a 4.0 way of doing it, without using await , you can do this:

private Task<string> methodAsync() 
{
    return Task.Delay(10000)
        .ContinueWith(t => "Hello");
}

第一个版本将编译下来的东西，或多或少是这样，但它会产生一些额外的样板code在他们的支持错误处理和的其他功能等待我们不充分利用这里。

如果你的 Thread.sleep代码（10000）是真正的意思仅仅是对一些长期运行的方法的占位符，而不是仅仅等待了一段时间的方式，那么你需要确保工作在另一个线程中完成的，而不是当前的背景下，。这样做的最简单的方法就是通过 Task.Run ：

If your Thread.Sleep(10000) is really meant to just be a placeholder for some long running method, as opposed to just a way of waiting for a while, then you'll need to ensure that the work is done in another thread, instead of the current context. The easiest way of doing that is through Task.Run:

private Task<string> methodAsync() 
{
    return Task.Run(()=>
        {
            SomeLongRunningMethod();
            return "Hello";
        });
}

或更可能是：

private Task<string> methodAsync() 
{
    return Task.Run(()=>
        {
            return SomeLongRunningMethodThatReturnsAString();
        });
}

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