Parallel.For 和 Parallel.ForEach 多載沒有任何特殊機制可用來處理可能擲回的例外狀況。 其在這一方面類似於一般 for 和 foreach 迴圈 (在 Visual Basic 中為 For 和 For Each)；未處理的例外狀況會導致此迴圈在目前執行反覆項目完成時立即終止。
當您將自己的例外狀況處理邏輯加入平行迴圈時，請處理多個執行緒上可能同時擲回類似例外狀況的情況，以及某個執行緒上擲回的例外狀況導致在另一個執行緒上擲回另一個例外狀況的情況。 您可以來自此迴圈的所有例外狀況包裝在 System.AggregateException 中，同時處理這兩種情況。 下列範例會示範一種可能的方式。
注意
啟用 [Just My Code] 時，Visual Studio 在某些情況下會在擲回例外狀況的字行上中斷，並顯示錯誤訊息，指出「使用者程式碼未處理例外狀況」。這個錯誤是良性的。 您可以按 F5 從中斷的地方繼續，並查看下面範例中示範的例外處理行為。 若要防止 Visual Studio 在遇到第一個錯誤時就中斷，只要取消核取 [工具]、[選項]、[偵錯]、[一般] 下的 [Just My Code] 核取方塊即可。
範例
在此範例中，會攔截所有例外狀況，然後包裝於擲回的 System.AggregateException 之中。 呼叫端可決定要處理哪一個例外狀況。
public static partial class Program
{
public static void ExceptionTwo()
{
// Create some random data to process in parallel.
// There is a good probability this data will cause some exceptions to be thrown.
byte[] data = new byte[5_000];
Random r = Random.Shared;
r.NextBytes(data);
try
{
ProcessDataInParallel(data);
}
catch (AggregateException ae)
{
var ignoredExceptions = new List<Exception>();
// This is where you can choose which exceptions to handle.
foreach (var ex in ae.Flatten().InnerExceptions)
{
if (ex is ArgumentException) Console.WriteLine(ex.Message);
else ignoredExceptions.Add(ex);
}
if (ignoredExceptions.Count > 0)
{
throw new AggregateException(ignoredExceptions);
}
}
Console.WriteLine("Press any key to exit.");
Console.ReadKey();
}
private static void ProcessDataInParallel(byte[] data)
{
// Use ConcurrentQueue to enable safe enqueueing from multiple threads.
var exceptions = new ConcurrentQueue<Exception>();
// Execute the complete loop and capture all exceptions.
Parallel.ForEach(data, d =>
{
try
{
// Cause a few exceptions, but not too many.
if (d < 3) throw new ArgumentException($"Value is {d}. Value must be greater than or equal to 3.");
else Console.Write(d + " ");
}
// Store the exception and continue with the loop.
catch (Exception e)
{
exceptions.Enqueue(e);
}
});
Console.WriteLine();
// Throw the exceptions here after the loop completes.
if (!exceptions.IsEmpty)
{
throw new AggregateException(exceptions);
}
}
}
' How to: Handle Exceptions in Parallel Loops
Imports System.Collections.Concurrent
Imports System.Collections.Generic
Imports System.Threading.Tasks
Module ExceptionsInLoops
Sub Main()
' Create some random data to process in parallel.
' There is a good probability this data will cause some exceptions to be thrown.
Dim data(1000) As Byte
Dim r As New Random()
r.NextBytes(data)
Try
ProcessDataInParallel(data)
Catch ae As AggregateException
Dim ignoredExceptions As New List(Of Exception)
' This is where you can choose which exceptions to handle.
For Each ex As Exception In ae.Flatten().InnerExceptions
If (TypeOf (ex) Is ArgumentException) Then
Console.WriteLine(ex.Message)
Else
ignoredExceptions.Add(ex)
End If
Next
If ignoredExceptions.Count > 0 Then
Throw New AggregateException(ignoredExceptions)
End If
End Try
Console.WriteLine("Press any key to exit.")
Console.ReadKey()
End Sub
Sub ProcessDataInParallel(ByVal data As Byte())
' Use ConcurrentQueue to enable safe enqueueing from multiple threads.
Dim exceptions As New ConcurrentQueue(Of Exception)
' Execute the complete loop and capture all exceptions.
Parallel.ForEach(Of Byte)(data, Sub(d)
Try
' Cause a few exceptions, but not too many.
If d < 3 Then
Throw New ArgumentException($"Value is {d}. Value must be greater than or equal to 3")
Else
Console.Write(d & " ")
End If
Catch ex As Exception
' Store the exception and continue with the loop.
exceptions.Enqueue(ex)
End Try
End Sub)
Console.WriteLine()
' Throw the exceptions here after the loop completes.
If exceptions.Count > 0 Then
Throw New AggregateException(exceptions)
End If
End Sub
End Module
