???? Task Parallel Library (TPL) 中，當你使用async/await?語法關鍵字時，你可能遇到以下異常處理的情況: Catch 塊只會處理第一個異常而忽略其它的異常。來看下面代碼塊:

1: private static async void TestCatchExceptionWithTPL() 2: { 3: try 4: { 5: await Task.Factory.StartNew(() => 6: { 7: Task.Factory.StartNew(() => 8: { throw new NullReferenceException(); }, 9: TaskCreationOptions.AttachedToParent); 10: Task.Factory.StartNew(() => 11: { throw new ArgumentException(); }, 12: TaskCreationOptions.AttachedToParent); 13: }); 14: } 15: catch (AggregateException aex) 16: { 17: // this catch will never be target 18: } 19: catch (Exception ex) 20: { 21: Console.WriteLine("## {0} ##", ex.GetType().Name); 22: } 23: } 執行上面的代碼，有注意8,11行分別有兩個Exception,而這時到第一個Catch塊中AggregateException只會捕捉第一個NullReferenceException異常，這是By Design的問題。有個解決方案是這樣的:
1: try 2: { 3: Task t = Task.Factory.StartNew(() => 4: { 5: Task.Factory.StartNew(() => 6: { throw new NullReferenceException(); }, 7: TaskCreationOptions.AttachedToParent); 8: Task.Factory.StartNew(() => 9: { throw new ArgumentException(); }, 10: TaskCreationOptions.AttachedToParent); 11: }); 12: await t.ContinueWith(tsk => { if (tsk.Exception != null) throw tsk.Exception; }); 13: } 14: catch (AggregateException ex) 15: { 16: foreach (var exc in ex.Flatten().InnerExceptions) 17: { 18: Console.WriteLine(exc.GetType().Name); 19: } 20: }

?

注意上面12行代碼，使用了CoutinueWith方法，為了讓代碼更加簡潔了，我們可以寫一個擴展方法：

/// <summary> /// ThrowMultiple /// </summary> /// <param name="t">Task</param> /// <returns>Task type</returns> public static Task ThrowMultiple(this Task t) { return t.ContinueWith(tsk => { if (tsk.Exception != null) throw tsk.Exception; }); } ? /// <summary> /// ThrowMultiple /// </summary> /// <typeparam name="T">Task this</typeparam> /// <param name="t">Task t</param> /// <returns>Task generic</returns> public static Task<T> ThrowMultiple<T>(this Task<T> t) { return t.ContinueWith(tsk => { if (tsk.Exception != null) throw tsk.Exception; return tsk.Result; }); }

然后代碼就是變成這樣了:

1: try 2: { 3: await Task.Factory.StartNew(() => 4: { 5: Task.Factory.StartNew(() => 6: { throw new NullReferenceException(); }, 7: TaskCreationOptions.AttachedToParent); 8: Task.Factory.StartNew(() => 9: { throw new ArgumentException(); }, 10: TaskCreationOptions.AttachedToParent); 11: }).ThrowMultiple(); 12: } 13: catch (AggregateException aex) 14: { 15: // this catch will never be target 16: } 17: catch (Exception ex) 18: { 19: Console.WriteLine("## {0} ##", ex.GetType().Name); 20: }

有看到上面代碼11行的變化為我們擴展方法。就這么簡單，以上代碼在Vs2012下 .net framework 4.5 version 下測試通過，希望對你開發有幫助。

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作者：Petter Liu
出處：http://www.cnblogs.com/wintersun/
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該文章也同時發布在我的獨立博客中-Petter Liu Blog。

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