机遇屋-异步死锁处理（DeadLock)

异步死锁处理（DeadLock)

作者：ych

提倡异步编程旨在给用户更好的前端体验，但异步编程也让学习成本和犯错几率大大升高，其中最常见且最难处理的就是死锁。

何谓“死锁”，英文术语称“Deadlock”，当两个以上的运算单元，双方都在等待对方停止运行，以取得系统资源，但是没有一方提前退出时，这种状况，就称为死锁。

#### 1.Task与Await线程锁（控制台不会出现该问题）

winform
```
// My "library" method.public static async Task<JObject> GetJsonAsync(Uri uri)
{  using (var client = new HttpClient())
  {    var jsonString = await client.GetStringAsync(uri);    return JObject.Parse(jsonString);
  }
}

// My "top-level" method.
public void Button1_Click(...)
{  var jsonTask = GetJsonAsync(...);
  textBox1.Text = jsonTask.Result;
}
```
Asp.net下
```
// My "library" method.public static async Task<JObject> GetJsonAsync(Uri uri)
{  using (var client = new HttpClient())
  {    var jsonString = await client.GetStringAsync(uri);    return JObject.Parse(jsonString);
  }
}
 
// My "top-level" method.
public class MyController : ApiController
{  public string Get()
  {    var jsonTask = GetJsonAsync(...);    return jsonTask.Result.ToString();
  }
}
```
#### 避免这种情况有三种最佳实践

1.在“库”异步方法中，尽可能使用ConfigureAwait（false）。
```
<pre class="brush:c#;toolbar:false;" style="margin-top: 0px; margin-bottom: 0px; white-space: pre-wrap; overflow-wrap: break-word; font-family: "Courier New" !important;">public static async Task<JObject> GetJsonAsync(Uri uri)
{  using (var client = new HttpClient())
  {    var jsonString = await client.GetStringAsync(uri).ConfigureAwait(false);    return JObject.Parse(jsonString);
  }
}</pre>
```
2.不要阻止任务; 一直使用async。（即一用异步补补异步）
```
public async void Button1_Click(...)
{  var json = await GetJsonAsync(...);
  textBox1.Text = json;
}
 
public class MyController : ApiController
{ 
 public async Task<string> Get()
  {    var json = await GetJsonAsync(...);    return json.ToString();
  }
}
```
3.如果想结束async & await模式的调用，启动一个新的线程去await异步方法的返回结果
```
// My "library" method.public static async Task<JObject> GetJsonAsync(Uri uri)
{    using (var client = new HttpClient())
    {        var jsonString = await client.GetStringAsync(uri);        return JObject.Parse(jsonString);
    }
}
 
// My "top-level" method.
public string Get()
{    string jsonResultString = string.Empty;
 
    Task.Run(async () =>
    {
        jsonResultString = await GetJsonAsync(...);
　　　　　//await之后的代码
    }).Wait();//此处启动线程是为了防止Async & Await模式造成死锁
 
    return jsonResultString;
}
```

#### 2.经典线程锁（顺序问题）
```
int sharedResource1 = 1, sharedResource2 = 2;
var lockResource1 = newobject();
var lockResource2 = newobject();
var t1 = newThread(() =>{
    Console.WriteLine("thead 1 begin");   
    lock (lockResource1)
    {
        Thread.Sleep(10);        
        lock (lockResource2)
        {
            sharedResource1++;
            sharedResource2++;
        }
    }
    Console.WriteLine("thead 1 end");
});
var t2 = newThread(() =>{
    Console.WriteLine("thead 2 begin");    
    lock (lockResource2)
    {
        Thread.Sleep(10);        
        lock (lockResource1)
        {
            sharedResource1++;
            sharedResource2++;
        }
    }
 
    Console.WriteLine("thead 2 end");
});
 
t1.Start();
t2.Start();
```
#### 3.相互等待导致死锁
```
Task t1 = null, t2 = null;
t1 = Task.Factory.StartNew(() =>{
    Console.WriteLine("task 1 begin");
    Task.Delay(10);
    Task.WaitAll(t2);
    Console.WriteLine("task 1 end");
});
 
t2 = Task.Factory.StartNew(() =>{
    Console.WriteLine("task 2 begin");
    Task.Delay(10);
    Task.WaitAll(t1);
    Console.WriteLine("task 2 end");
});
 
Task.WaitAll(t1, t2);
Console.WriteLine("Done");
```
#### 4.WinForm Invoke抢夺UI线程死锁

```
privatevoid button1_Click(object sender, EventArgs e)
{    var t = Task.Factory.StartNew<string>(() =>
        {
 
            Thread.Sleep(0);            var text = Invoke(newFunc<string>(() =>
 
            {                // do some ui-dependent works
                return Text;
 
            }));            return text + " - new title";
 
        });
    Text = t.Result;
}
```
#### 5.WPF Dispatcher切换死锁

```
privatevoid Button_Click(object sender, RoutedEventArgs e)
{    var t = Task.Factory.StartNew<Brush>((state) =>
    {
        Task.Delay(10);        var clr = (Color)newColorConverter()
            .ConvertFromInvariantString(state asstring);        var brush = Dispatcher.Invoke<SolidColorBrush>(() =>
            {                // do some works
                returnnewSolidColorBrush() { Color = clr };
            });        return brush;
    }, "red");
    theButton.Background = t.Result;
}
```

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