await运算符只能在async方法中使用-但方法是异步的

Question

I want to use a MessageDialog in VS12. Until now I have used:

MessageDialog msgdlg = new MessageDialog("Choose a color", "How To Async #1");
msgdlg.DefaultCommandIndex = 1;
msgdlg.Commands.Add(new UICommand("Red", null, Colors.Red));
msgdlg.Commands.Add(new UICommand("Green", null, Colors.Green));
msgdlg.Commands.Add(new UICommand("Blue", null, Colors.Blue));

IAsyncOperation<IUICommand> asyncOp = msgdlg.ShowAsync();
asyncOp.Completed = OnMessageDialogShowAsyncCompleted;

Now I want to eliminate the callbacks and use an anonymous methode with await. For test purpose i used:

MessageDialog msgdlg = new MessageDialog("Choose a color", "#3");
msgdlg.Commands.Add(new UICommand("Red", null, Colors.Red));
msgdlg.Commands.Add(new UICommand("Green", null, Colors.Green));
msgdlg.Commands.Add(new UICommand("Blue", null, Colors.Blue));

// Show the MessageDialog
IAsyncOperation<IUICommand> asyncOp = msgdlg.ShowAsync();
IUICommand command = await asyncOp;

Problem is, await produces an error, even if ShowAsync() is obviously asynchron. "The 'await' operator can only be used within an async method. Consider marking this method with the 'async' modifier and changing its return type to 'Task'."

Whats the problem here?

---

Ok, thanks to your comments I now doing this:

Loaded += async (sender, args) =>
        {
            #region Using await (from C# 5.0 on)
            MessageDialog msgdlg = new MessageDialog("Choose a color", "#3");
            msgdlg.Commands.Add(new UICommand("Red", null, Colors.Red));
            msgdlg.Commands.Add(new UICommand("Green", null, Colors.Green));
            msgdlg.Commands.Add(new UICommand("Blue", null, Colors.Blue));

            // Show the MessageDialog
            IAsyncOperation<IUICommand> asyncOp = msgdlg.ShowAsync();
            IUICommand command = await asyncOp;

            #endregion
        };

Now it works - thanks a lot!

Answer 1

But the method which uses the source from above is a constructor (page).

You cannot have asynchronous constructors. Move the async work out of the constructor. Maybe move it into a Load -like event. I don't know what GUI framework you're using but they always have a Load event.

Answer 2

您应该使方法async 。不能在non-async函数内部使用await 。

Answer 3

The function which calls those codes should be async as well. See details in this MS official sample bellow. Attention that the return of ForgotPassword is async Task< ActionResult > instead of ActionResult.

public async Task<ActionResult> ForgotPassword(ForgotPasswordViewModel model)
{
    if (ModelState.IsValid)
    {
        var user = await UserManager.FindByNameAsync(model.Email);
        if (user == null || !(await UserManager.IsEmailConfirmedAsync(user.Id)))
        {
        // Don't reveal that the user does not exist or is not confirmed
        return View("ForgotPasswordConfirmation");
    }

    var code = await UserManager.GeneratePasswordResetTokenAsync(user.Id);
    var callbackUrl = Url.Action("ResetPassword", "Account", 
    new { UserId = user.Id, code = code }, protocol: Request.Url.Scheme);
    await UserManager.SendEmailAsync(user.Id, "Reset Password", 
    "Please reset your password by clicking here: <a href=\"" + callbackUrl + "\">link</a>");        
    return View("ForgotPasswordConfirmation");
}

// If we got this far, something failed, redisplay form
return View(model);
}