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[英]Task.WhenAll().ContinueWith() throws unexpected Task Canceled exception
[英]Dataflow Task.WhenAll causes A task was canceled Exception
我是Dataflow的新手,並且按照此演練如何操作:取消Dataflow塊 。
我先單擊“添加”按鈕,然后單擊“取消”,但是單擊“ 取消 ”按鈕后,出現了有關“ 任務已取消異常 ”的異常 。 我找不到任何解決此錯誤的方法。
任何幫助,將不勝感激。
更新:演示代碼:
public partial class Form1 : Form
{
CancellationTokenSource cancellationTokenSource;
TransformBlock<WorkItem, WorkItem> startWork;
ActionBlock<WorkItem> completeWork;
ActionBlock<ToolStripProgressBar> incProgress;
ActionBlock<ToolStripProgressBar> decProgress;
TaskScheduler uiTaskScheduler;
public Form1()
{
InitializeComponent();
uiTaskScheduler = TaskScheduler.FromCurrentSynchronizationContext();
Cancel.Enabled = false;
}
private void Add_Click(object sender, EventArgs e)
{
if (!Cancel.Enabled)
{
CreatePipeline();
Cancel.Enabled = true;
}
for (int i = 0; i < 20; i++)
{
toolStripProgressBar1.Value++;
startWork.Post(new WorkItem());
}
}
private async void Cancel_Click(object sender, EventArgs e)
{
Add.Enabled = false;
Cancel.Enabled = false;
cancellationTokenSource.Cancel();
try
{
await Task.WhenAll(
completeWork.Completion,
incProgress.Completion,
decProgress.Completion);
}
catch (OperationCanceledException)
{
throw;
}
toolStripProgressBar4.Value += toolStripProgressBar1.Value;
toolStripProgressBar4.Value += toolStripProgressBar2.Value;
// Reset the progress bars that track the number of active work items.
toolStripProgressBar1.Value = 0;
toolStripProgressBar2.Value = 0;
// Enable the Add Work Items button.
Add.Enabled = true;
}
private void CreatePipeline()
{
cancellationTokenSource = new CancellationTokenSource();
startWork = new TransformBlock<WorkItem, WorkItem>(workItem =>
{
workItem.DoWork(250, cancellationTokenSource.Token);
decProgress.Post(toolStripProgressBar1);
incProgress.Post(toolStripProgressBar2);
return workItem;
},
new ExecutionDataflowBlockOptions
{
CancellationToken = cancellationTokenSource.Token
});
completeWork = new ActionBlock<WorkItem>(workItem =>
{
workItem.DoWork(1000, cancellationTokenSource.Token);
decProgress.Post(toolStripProgressBar2);
incProgress.Post(toolStripProgressBar3);
},
new ExecutionDataflowBlockOptions
{
CancellationToken = cancellationTokenSource.Token,
MaxDegreeOfParallelism = 2
});
startWork.LinkTo(completeWork);
startWork.Completion.ContinueWith(delegate { completeWork.Complete(); },cancellationTokenSource.Token);
incProgress = new ActionBlock<ToolStripProgressBar>(progress =>
{
progress.Value++;
},
new ExecutionDataflowBlockOptions
{
CancellationToken = cancellationTokenSource.Token,
TaskScheduler = uiTaskScheduler
});
decProgress = new ActionBlock<ToolStripProgressBar>(progress => progress.Value--,
new ExecutionDataflowBlockOptions
{
CancellationToken = cancellationTokenSource.Token,
TaskScheduler = uiTaskScheduler
});
}
class WorkItem
{
public void DoWork(int milliseconds, CancellationToken cancellationToken)
{
if (cancellationToken.IsCancellationRequested == false)
{
Thread.Sleep(milliseconds);
}
}
}
}
正如@SirRufo所指出的那樣,解決問題的方法就是根本不要在捕獲到異常后重新拋出該異常。 但是,為了突出顯示您可以與數據流一起使用的其他一些技術,如我在一個小樣本的注釋中所討論的。 我試圖保持原始代碼的精神和意圖。 為此; 原始代碼沒有顯示流程將如何正常完成,而不是取消流程,因此我也將其省略。
using System;
using System.Data;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using System.Threading.Tasks.Dataflow;
using System.Windows.Forms;
namespace WindowsFormsApp1 {
public partial class Form1 : Form {
private CancellationTokenSource cancellationTokenSource;
private TransformBlock<WorkItem, WorkItem> startWork;
private ActionBlock<WorkItem> completeWork;
private IProgress<int> progressBar1Value;
private IProgress<int> progressBar2Value;
public Form1() {
InitializeComponent();
btnCancel.Enabled = false;
}
private async void btnAdd_Click(object sender, EventArgs e) {
if(!btnCancel.Enabled) {
CreatePipeline();
btnCancel.Enabled = true;
}
var data = Enumerable.Range(0, 20).Select(_ => new WorkItem());
foreach(var workItem in data) {
await startWork.SendAsync(workItem);
progressBar1.Value++;
}
}
private async void btnCancel_Click(object sender, EventArgs e) {
btnAdd.Enabled = false;
btnCancel.Enabled = false;
cancellationTokenSource.Cancel();
await completeWork.Completion.ContinueWith(tsk => this.Invoke(new Action(() => this.Text = "Flow Cancelled")),
TaskContinuationOptions.OnlyOnCanceled);
progressBar4.Value += progressBar1.Value;
progressBar4.Value += progressBar2.Value;
// Reset the progress bars that track the number of active work items.
progressBar1.Value = 0;
progressBar2.Value = 0;
// Enable the Add Work Items button.
btnAdd.Enabled = true;
}
private void CreatePipeline() {
cancellationTokenSource = new CancellationTokenSource();
progressBar1Value = new Progress<int>(_ => progressBar1.Value++);
progressBar2Value = new Progress<int>(_ => progressBar2.Value++);
startWork = new TransformBlock<WorkItem, WorkItem>(async workItem => {
await workItem.DoWork(250, cancellationTokenSource.Token);
progressBar1Value.Report(0); //Value is ignored since the progressbar value is simply incremented
progressBar2Value.Report(0); //Value is ignored since the progressbar value is simply incremented
return workItem;
},
new ExecutionDataflowBlockOptions {
CancellationToken = cancellationTokenSource.Token
});
completeWork = new ActionBlock<WorkItem>(async workItem => {
await workItem.DoWork(1000, cancellationTokenSource.Token);
progressBar1Value.Report(0); //Value is ignored since the progressbar value is simply incremented
progressBar2Value.Report(0); //Value is ignored since the progressbar value is simply incremented
},
new ExecutionDataflowBlockOptions {
CancellationToken = cancellationTokenSource.Token,
MaxDegreeOfParallelism = 2
});
startWork.LinkTo(completeWork, new DataflowLinkOptions() { PropagateCompletion = true });
}
}
public class WorkItem {
public async Task DoWork(int milliseconds, CancellationToken cancellationToken) {
if(cancellationToken.IsCancellationRequested == false) {
await Task.Delay(milliseconds);
}
}
}
}
檢查代碼后,我發布了,如果我點擊取消,任務將被取消。
await Task.WhenAll(
completeWork.Completion,
incProgress.Completion,
decProgress.Completion);
但是,上面的代碼Task.WhenAll需要所有任務都返回完成狀態,然后,如果返回的是已取消而不是已完成,則按預期方式拋出“ 任務已取消異常 ”。
為了解決此問題,如果取消了任務,我們應該返回任務完成,並且下面的代碼對我有用。
await Task.WhenAll(
completeWork.Completion.ContinueWith(task => cancelWork(task, "completeWork"), TaskContinuationOptions.OnlyOnCanceled),
incProgress.Completion.ContinueWith(task => cancelWork(task, "incProgress"), TaskContinuationOptions.OnlyOnCanceled),
decProgress.Completion.ContinueWith(task => cancelWork(task, "decProgress"), TaskContinuationOptions.OnlyOnCanceled));
合理嗎?
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