同步等待异步方法在同一线程上完成

Question

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

The desired effect is

• to have the Worker() run asynchronously on the UI thread
• and at the same time wait for it to finish before the Close() method returns

The example below enters in a deadlock, and if I make Form1_FormClosing() async I don't satisfy the second condition.

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?

You don't need to synchronously wait. By making Worker async Task instead of async void you can get the desired behavior and remove the useless 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
}

I'm missing the implementation of Close() , but i suspect you could do without it and relay on the form closing event to cancel the worker.