除了其他答案，还有await (c#参考)

更具体地说，在包含的例子中，它解释了您的情况

下面的Windows窗体示例说明了await在 异步方法，WaitAsynchronouslyAsync。对比一下它的行为 方法使用waitsynchrontically的行为。没有等待 应用到任务的操作符，waitsynchronize同步运行 尽管在定义中使用了async修饰符，并且调用了 线程。睡在它的身体里。

private async void button1_Click(object sender, EventArgs e)
{
    // Call the method that runs asynchronously.
    string result = await WaitAsynchronouslyAsync();

    // Call the method that runs synchronously.
    //string result = await WaitSynchronously ();

    // Display the result.
    textBox1.Text += result;
}

// The following method runs asynchronously. The UI thread is not
// blocked during the delay. You can move or resize the Form1 window 
// while Task.Delay is running.
public async Task<string> WaitAsynchronouslyAsync()
{
    await Task.Delay(10000);
    return "Finished";
}

// The following method runs synchronously, despite the use of async.
// You cannot move or resize the Form1 window while Thread.Sleep
// is running because the UI thread is blocked.
public async Task<string> WaitSynchronously()
{
    // Add a using directive for System.Threading.
    Thread.Sleep(10000);
    return "Finished";
}

我认为你用System.Threading.Thread.Sleep选了一个不好的例子

异步任务的要点是让它在后台执行，而不锁定主线程，例如执行DownloadFileAsync

System.Threading.Thread.Sleep不是“正在完成”的事情，它只是休眠，因此你的下一行在5秒后到达……

阅读这篇文章，我认为它很好地解释了async和await概念:http://msdn.microsoft.com/en-us/library/vstudio/hh191443.aspx

除了其他答案，还有await (c#参考)

更具体地说，在包含的例子中，它解释了您的情况

下面的Windows窗体示例说明了await在 异步方法，WaitAsynchronouslyAsync。对比一下它的行为 方法使用waitsynchrontically的行为。没有等待 应用到任务的操作符，waitsynchronize同步运行 尽管在定义中使用了async修饰符，并且调用了 线程。睡在它的身体里。

private async void button1_Click(object sender, EventArgs e)
{
    // Call the method that runs asynchronously.
    string result = await WaitAsynchronouslyAsync();

    // Call the method that runs synchronously.
    //string result = await WaitSynchronously ();

    // Display the result.
    textBox1.Text += result;
}

// The following method runs asynchronously. The UI thread is not
// blocked during the delay. You can move or resize the Form1 window 
// while Task.Delay is running.
public async Task<string> WaitAsynchronouslyAsync()
{
    await Task.Delay(10000);
    return "Finished";
}

// The following method runs synchronously, despite the use of async.
// You cannot move or resize the Form1 window while Thread.Sleep
// is running because the UI thread is blocked.
public async Task<string> WaitSynchronously()
{
    // Add a using directive for System.Threading.
    Thread.Sleep(10000);
    return "Finished";
}

根据我的理解，async和await所做的主要事情之一是使代码易于编写和阅读。

它们是为了让异步代码易于编写和阅读。

这和生成后台线程来执行长时间逻辑是一样的吗?

一点也不。

//我不明白为什么这个方法必须被标记为'async'。

async关键字启用await关键字。所以任何使用await的方法都必须被标记为async。

// DoSomethingAsync()方法在5秒休眠后到达该行。难道不应该立即到达吗?

不会，因为异步方法默认情况下不会在另一个线程上运行。

//是否在后台线程上执行?

No.

你可能会发现我的async/await介绍很有用。官方MSDN文档也非常好(尤其是TAP部分)，异步团队还发布了一个很好的FAQ。

public static void Main(string[] args)
{
    string result = DownloadContentAsync().Result;
    Console.ReadKey();
}

// You use the async keyword to mark a method for asynchronous operations.
// The "async" modifier simply starts synchronously the current thread. 
// What it does is enable the method to be split into multiple pieces.
// The boundaries of these pieces are marked with the await keyword.
public static async Task<string> DownloadContentAsync()// By convention, the method name ends with "Async
{
    using (HttpClient client = new HttpClient())
    {
        // When you use the await keyword, the compiler generates the code that checks if the asynchronous operation is finished.
        // If it is already finished, the method continues to run synchronously.
        // If not completed, the state machine will connect a continuation method that must be executed WHEN the Task is completed.


        // Http request example. 
        // (In this example I can set the milliseconds after "sleep=")
        String result = await client.GetStringAsync("http://httpstat.us/200?sleep=1000");

        Console.WriteLine(result);

        // After completing the result response, the state machine will continue to synchronously execute the other processes.


        return result;
    }
}

这里有一个快速的控制台程序，让那些遵循。TaskToDo方法是你想让它异步的长期运行的方法。让它以异步方式运行是由TestAsync方法完成的。test loops方法只是运行TaskToDo任务，并异步运行它们。你可以在结果中看到这一点，因为它们在每次运行中完成的顺序不同——当它们完成时，它们会报告给控制台UI线程。简单，但我认为简单的例子比复杂的例子更好地揭示了模式的核心:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;

namespace TestingAsync
{
    class Program
    {
        static void Main(string[] args)
        {
            TestLoops();
            Console.Read();
        }

        private static async void TestLoops()
        {
            for (int i = 0; i < 100; i++)
            {
                await TestAsync(i);
            }
        }

        private static Task TestAsync(int i)
        {
            return Task.Run(() => TaskToDo(i));
        }

        private async static void TaskToDo(int i)
        {
            await Task.Delay(10);
            Console.WriteLine(i);
        }
    }
}