如何：通过轮询侦听取消请求
下面的示例展示了一种方便用户代码定期轮询取消令牌，以确定是否已通过调用线程发出取消请求的方式。 此示例使用 System.Threading.Tasks.Task 类型，但相同的模式适用于 System.Threading.ThreadPool 类型或 System.Threading.Thread 类型直接创建的异步操作。
示例
若要轮询，必须有某种循环或递归代码，可用于定期读取布尔 IsCancellationRequested 属性的值。 如果使用的是 System.Threading.Tasks.Task 类型，且正在等待任务在调用线程上完成，可以使用 ThrowIfCancellationRequested 方法来检查属性并抛出异常。 通过使用此方法，可确保抛出正确的异常来响应请求。 如果使用的是 Task，那么调用此方法优于手动抛出 OperationCanceledException。 如果无需抛出异常，可以直接检查属性，并通过方法返回结果（如果属性是 true 的话）。
using System;
using System.Threading;
using System.Threading.Tasks;
public struct Rectangle
{
public int columns;
public int rows;
}
class CancelByPolling
{
static void Main()
{
var tokenSource = new CancellationTokenSource();
// Toy object for demo purposes
Rectangle rect = new Rectangle() { columns = 1000, rows = 500 };
// Simple cancellation scenario #1. Calling thread does not wait
// on the task to complete, and the user delegate simply returns
// on cancellation request without throwing.
Task.Run(() => NestedLoops(rect, tokenSource.Token), tokenSource.Token);
// Simple cancellation scenario #2. Calling thread does not wait
// on the task to complete, and the user delegate throws
// OperationCanceledException to shut down task and transition its state.
// Task.Run(() => PollByTimeSpan(tokenSource.Token), tokenSource.Token);
Console.WriteLine("Press 'c' to cancel");
if (Console.ReadKey(true).KeyChar == 'c') {
tokenSource.Cancel();
Console.WriteLine("Press any key to exit.");
}
Console.ReadKey();
tokenSource.Dispose();
}
static void NestedLoops(Rectangle rect, CancellationToken token)
{
for (int col = 0; col < rect.columns && !token.IsCancellationRequested; col++) {
// Assume that we know that the inner loop is very fast.
// Therefore, polling once per column in the outer loop condition
// is sufficient.
for (int row = 0; row < rect.rows; row++) {
// Simulating work.
Thread.SpinWait(5_000);
Console.Write("{0},{1} ", col, row);
}
}
if (token.IsCancellationRequested) {
// Cleanup or undo here if necessary...
Console.WriteLine("\r\nOperation canceled");
Console.WriteLine("Press any key to exit.");
// If using Task:
// token.ThrowIfCancellationRequested();
}
}
}
Imports System.Threading
Imports System.Threading.Tasks
Public Structure Rectangle
Public columns As Integer
Public rows As Integer
End Structure
Class CancelByPolling
Shared Sub Main11()
Dim tokenSource As New CancellationTokenSource()
' Toy object for demo purposes
Dim rect As New Rectangle()
rect.columns = 1000
rect.rows = 500
' Simple cancellation scenario #1. Calling thread does not wait
' on the task to complete, and the user delegate simply returns
' on cancellation request without throwing.
Task.Run(Sub() NestedLoops(rect, tokenSource.Token), tokenSource.Token)
' Simple cancellation scenario #2. Calling thread does not wait
' on the task to complete, and the user delegate throws
' OperationCanceledException to shut down task and transition its state.
' Task.Run(Sub() PollByTimeSpan(tokenSource.Token), tokenSource.Token)
Console.WriteLine("Press 'c' to cancel")
If Console.ReadKey(True).KeyChar = "c"c Then
tokenSource.Cancel()
Console.WriteLine("Press any key to exit.")
End If
Console.ReadKey()
tokenSource.Dispose()
End Sub
Shared Sub NestedLoops(ByVal rect As Rectangle, ByVal token As CancellationToken)
Dim col As Integer
For col = 0 To rect.columns - 1
' Assume that we know that the inner loop is very fast.
' Therefore, polling once per column in the outer loop condition
' is sufficient.
For row As Integer = 0 To rect.rows - 1
' Simulating work.
Thread.SpinWait(5000)
Console.Write("0',1' ", col, row)
Next
Next
If token.IsCancellationRequested = True Then
' Cleanup or undo here if necessary...
Console.WriteLine(vbCrLf + "Operation canceled")
Console.WriteLine("Press any key to exit.")
' If using Task:
' token.ThrowIfCancellationRequested()
End If
End Sub
End Class
调用 ThrowIfCancellationRequested 非常快，不会在循环中产生很大的开销。
如果调用的是 ThrowIfCancellationRequested，那么如果除了抛出异常之外，还要执行其他工作来响应取消，只需显式检查 IsCancellationRequested 属性即可。 在此示例中，可以看到代码实际访问属性两次：一次是显式访问，另一次是在 ThrowIfCancellationRequested 方法中。 不过，由于读取 IsCancellationRequested 属性在每次访问时只涉及一个易失读取指令，因此从性能角度来看，双重访问的意义并不大。 最好仍调用此方法，而不是手动抛出 OperationCanceledException。
