如何：取消 Parallel.For 或 ForEach Loop
Parallel.For 和 Parallel.ForEach 方法支持通过使用取消标记进行取消。 有关常规取消的更多信息，请参见取消。 在并行循环中，您在 ParallelOptions 参数中向方法提供 CancellationToken，然后将并行调用置于 try-catch 块中。
示例
下面的示例演示如何取消对 Parallel.ForEach 的调用。 您可以将同样的方法应用于 Parallel.For 调用。
' How to: Cancel a Parallel.For or ForEach Loop
Imports System.Threading
Imports System.Threading.Tasks
Module CancelParallelLoops
Sub Main()
Dim nums() As Integer = Enumerable.Range(0, 10000000).ToArray()
Dim cts As New CancellationTokenSource
' Use ParallelOptions instance to store the CancellationToken
Dim po As New ParallelOptions
po.CancellationToken = cts.Token
po.MaxDegreeOfParallelism = System.Environment.ProcessorCount
Console.WriteLine("Press any key to start. Press 'c' to cancel.")
Console.ReadKey()
' Run a task so that we can cancel from another thread.
Dim t As Task = Task.Factory.StartNew(Sub()
If Console.ReadKey().KeyChar = "c"c Then
cts.Cancel()
End If
Console.WriteLine(vbCrLf & "Press any key to exit.")
End Sub)
Try
' The error "Exception is unhandled by user code" will appear if "Just My Code"
' is enabled. This error is benign. You can press F5 to continue, or disable Just My Code.
Parallel.ForEach(nums, po, Sub(num)
Dim d As Double = Math.Sqrt(num)
Console.CursorLeft = 0
Console.Write("{0:##.##} on {1}", d, Thread.CurrentThread.ManagedThreadId)
po.CancellationToken.ThrowIfCancellationRequested()
End Sub)
Catch e As OperationCanceledException
Console.WriteLine(e.Message)
End Try
Console.ReadKey()
End Sub
End Module
namespace CancelParallelLoops
{
using System;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
class Program
{
static void Main()
{
int[] nums = Enumerable.Range(0, 10000000).ToArray();
CancellationTokenSource cts = new CancellationTokenSource();
// Use ParallelOptions instance to store the CancellationToken
ParallelOptions po = new ParallelOptions();
po.CancellationToken = cts.Token;
po.MaxDegreeOfParallelism = System.Environment.ProcessorCount;
Console.WriteLine("Press any key to start. Press 'c' to cancel.");
Console.ReadKey();
// Run a task so that we can cancel from another thread.
Task.Factory.StartNew(() =>
{
if (Console.ReadKey().KeyChar == 'c')
cts.Cancel();
Console.WriteLine("press any key to exit");
});
try
{
Parallel.ForEach(nums, po, (num) =>
{
double d = Math.Sqrt(num);
Console.WriteLine("{0} on {1}", d, Thread.CurrentThread.ManagedThreadId);
po.CancellationToken.ThrowIfCancellationRequested();
});
}
catch (OperationCanceledException e)
{
Console.WriteLine(e.Message);
}
Console.ReadKey();
}
}
}
如果发出取消信号的标记与 ParallelOptions 实例中指定的标记相同，则并行循环将对取消引发单一 OperationCanceledException。 如果某个其他标记导致取消，则循环将引发 AggregateException 以及 OperationCanceledException 作为 InnerException。