(C#) Tasks 中的异常处理（Exception Handling.）

多线程编程中要注意对线程异常的处理。首先写个例子。

一个线程用于显示信息(Show Messages)。主线程用于做其他工作(Do Works)。

using (Task taskShowMessages = new Task(ShowMessages))
{
try
{
taskShowMessages.Start();
DoWorks();
}
catch (DoWorkException ex)
{
Console.WriteLine("Error:{0}", ex.Message);
}
catch(Exception ex)
{
Console.WriteLine("Error:{0}", ex.Message);
}
}

public static void ShowMessages()
{
int i = 0;
while (true)
{
Console.WriteLine("{0} - Message_{1}", DateTime.Now.ToString(), i++);
Thread.Sleep(1000);

if ( i == 3)
{
throw new ShowMessageException(string.Format("User defined error occured during showing message_{0}!", i));
//throw new Exception(string.Format("Unhandled exception occured during showing message_{0}!", i));
}
}
}

public static void DoWorks()
{
int i = 0;
while (true)
{
Console.WriteLine("{0} - DoWork_{1}", DateTime.Now.ToString(), i++);
Thread.Sleep(2000);

if (i == 4)
{
throw new DoWorkException(string.Format("User defined error occured during doing work_{0}!", i));
//throw new Exception(string.Format("Unhandled exception occured during doing work{0}!", i));
}
}
}

public class ShowMessageException : Exception
{
public ShowMessageException(String message)
: base(message)
{
}
}

public class DoWorkException : Exception
{
public DoWorkException(String message)
: base(message)
{
}
}

输出：

12/27/2015 10:08:03 PM - DoWork_0
12/27/2015 10:08:03 PM - Message_0
12/27/2015 10:08:04 PM - Message_1
12/27/2015 10:08:05 PM - DoWork_1
12/27/2015 10:08:05 PM - Message_2
12/27/2015 10:08:07 PM - DoWork_2
12/27/2015 10:08:09 PM - DoWork_3
Error:User defined error occured during doing work_4!

可以看出来，Show Message 3 时发生一个异常，但是本程序无视它并继续DoWorks。知道DoWorks发生异常抛出。
在VS的debug环境下可以看到这个异常的发生：

A first chance exception of type 'ConsoleApplication2.Program.ShowMessageException' occurred in ConsoleApplication2.exe
A first chance exception of type 'ConsoleApplication2.Program.DoWorkException' occurred in ConsoleApplication2.exe

如果想正确的处理ShowMessage线程的异常，必须对其 Wait（）方法进行处理。

using (Task taskShowMessages = new Task(ShowMessages))
{
try
{
taskShowMessages.Start();
DoWorks();
}
catch (DoWorkException ex)
{
Console.WriteLine("Error is handled:{0}", ex.Message);
}
finally
{
try
{
taskShowMessages.Wait();
}
catch(AggregateException ae)
{
ae.Handle((x) =>
{
if (x is ShowMessageException)
{
Console.WriteLine("Error is handled:{0}", x.Message);
return true;
}
else
{
return false;
}
});
}
}
}

输出：

12/27/2015 10:16:37 PM - DoWork_0
12/27/2015 10:16:37 PM - Message_0
12/27/2015 10:16:38 PM - Message_1
12/27/2015 10:16:39 PM - DoWork_1
12/27/2015 10:16:39 PM - Message_2
12/27/2015 10:16:41 PM - DoWork_2
12/27/2015 10:16:43 PM - DoWork_3
Error is handled:User defined error occured during doing work_4!
Error is handled:User defined error occured during showing message_3!

那么现在如果其中一个线程出现异常的话，我们就希望抛出异常，结束程序，怎么做?

答案就是使用：

CancellationTokenSource

static CancellationTokenSource cancellationtoken = new CancellationTokenSource();
static void Main(string[] args)
{

using (Task taskShowMessages = new Task(ShowMessages, cancellationtoken.Token))
{
try
{
taskShowMessages.Start();
DoWorks();
}
catch (DoWorkException ex)
{
Console.WriteLine("Error is handled:{0}", ex.Message);
}
finally
{
cancellationtoken.Cancel();

try
{
taskShowMessages.Wait();
}
catch (AggregateException ae)
{
ae.Handle((x) =>
{
if (x is ShowMessageException)
{
Console.WriteLine("Error is handled:{0}", x.Message);
return true;
}
else
{
return false;
}
});
}
}
}
}

public static void ShowMessages()
{
int i = 0;
while (!cancellationtoken.IsCancellationRequested)
{
Console.WriteLine("{0} - Message_{1}", DateTime.Now.ToString(), i++);
Thread.Sleep(1000);

if ( i == 3)
{
//throw new ShowMessageException(string.Format("User defined error occured during showing message_{0}!", i));
//throw new Exception(string.Format("Unhandled exception occured during showing message_{0}!", i));
//break;
}
}
}

输出：

12/27/2015 10:40:56 PM - DoWork_0
12/27/2015 10:40:56 PM - Message_0
12/27/2015 10:40:57 PM - Message_1
12/27/2015 10:40:58 PM - DoWork_1
12/27/2015 10:40:58 PM - Message_2
12/27/2015 10:40:59 PM - Message_3
12/27/2015 10:41:00 PM - DoWork_2
12/27/2015 10:41:00 PM - Message_4
12/27/2015 10:41:01 PM - Message_5
12/27/2015 10:41:02 PM - DoWork_3
12/27/2015 10:41:02 PM - Message_6
12/27/2015 10:41:03 PM - Message_7
Error is handled:User defined error occured during doing work_4!

参考：
https://msdn.microsoft.com/en-us/library/dd997415(v=vs.110).aspx https://msdn.microsoft.com/en-us/library/dd537614(v=vs.110).aspx