Demo

看一个简单的例子

public async Task<string> MyTaskAsync(Uri address) 
{
    return await Task.Run(() =>
    {
        using (var client = new WebClient()) 
        {
            return client.DownloadString(address);
        }
    });
}

乍一眼看这个函数似乎很正常，用一个Task包裹一个耗时的WebClient.DownloadString函数。但是我们需要问一下自己:

WebClient.DownloadString 是CPU密集型，还是IO密集型
在Task.Run所在的托管线程里，是阻塞的还是非阻塞的

答案很明确：IO密集型，并且仍然是阻塞的，那么这意味着什么：我们只是换了一个地方阻塞，也就是类似于拆东墙补西墙，这样的代码确实可以使UI不再卡顿，但实际上对于性能的可拓展并没有什么好处。

Solve

解决方案很简单，就是用系统提供的Async函数即可。

public async Task<string> MyTaskAsync(Uri address)
{
    using (var client = new HttpClient())
    {
        return await client.GetStringAsync(address);
    }
}

区别

无论是真异步，还是假异步，都要访问Web，来获取数据，区别在哪呢？

Throughout this entire process, a key takeaway is that no thread is dedicated to running the task. Although work is executed in some context (that is, the OS does have to pass data to a device driver and respond to an interrupt), there is no thread dedicated to waiting for data from the request to come back. This allows the system to handle a much larger volume of work rather than waiting for some I/O call to finish. (From MSDN)

一句话：真异步是没有专用线程等待的。

我们知道IO是很慢的，如果让一个线程去等待他，显然是错误的做法，这也是假异步的做法。我们需要的是系统把数据准备好了，然后在通知托管线程继续处理。在系统处理IO的时候，托管线程就可以想干啥就干啥，而不是傻等着。

Ref

[1.] Why you shouldn’t create asynchronous wrappers with Task.Run()

[2.] MSDN: async in depth