c# - 同步等待异步方法在同一线程上完成

Question

有没有办法同步等待在同一线程上运行的异步方法？

想要的效果是

• 让 Worker() 在 UI 线程上异步运行
• 同时在 Close() 方法返回之前等待它完成

下面的示例进入死锁，如果我使 Form1_FormClosing() 异步，则不满足第二个条件。

public partial class Form1 : Form
{
    TaskCompletionSource<bool> tcs = new TaskCompletionSource<bool>();
    CancellationTokenSource cts = new CancellationTokenSource();
    public Form1()
    {
        InitializeComponent();
        Show();
        Worker(cts.Token); // async worker started on UI thread
    }

    async void Worker(CancellationToken ct)
    {
        while (!ct.IsCancellationRequested)
            await TaskEx.Delay(1000);
        tcs.SetResult(true); // signal completition
    }

    private void button1_Click(object sender, EventArgs e)
    {
        Close();
        MessageBox.Show("This is supposed to be second");
    }

    private async void Form1_FormClosing(object sender, FormClosingEventArgs e)
    {
        cts.Cancel(); // request cancel 
        tcs.Task.Wait(); // deadlock
        await tcs.Task; // button1_Click() gets control back instead of Worker()
        MessageBox.Show("This is supposed to be first");
    }
}

Answer 1

Is there a way to wait synchronously for an async method that runs on the same thread?

您不需要同步等待。通过使用 Worker async Task 而不是 async void，您可以获得所需的行为并删除无用的 TaskCompletionSource:

private Task workerTask;
public Form()
{
     workerTask = Worker(cts.Token);
}

private async Task Worker(CancellationToken ct)
{
    while (!ct.IsCancellationRequested)
        await TaskEx.Delay(1000);
}

private async void Form1_FormClosing(object sender, FormClosingEventArgs e)
{
    cts.Cancel(); // request cancel
    await workerTask; // Wait for worker to finish before closing
}

我缺少 Close() 的实现，但我怀疑您可以不使用它并中继表单关闭事件来取消工作人员。