同步等待异步方法在同一线程上完成

Question

有没有一种方法可以同步等待在同一线程上运行的异步方法？

理想的效果是

• 使Worker（）在UI线程上异步运行
• 并同时等待它完成，然后Close（）方法返回

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

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

有没有一种方法可以同步等待在同一线程上运行的异步方法？

您无需同步等待。 通过使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，但是我怀疑您可以不用它，而是通过Close()表单事件来取消工作程序。