Task.Run global Messagebox which blocks main window

Question

I run an awaited function in the constructor of my WPF application:

  Task.Run(async () =>
  {
      var r = await checkValidLicenseAsync(hardwareid);
      if (r == false)
        MessageBox.Show("no License.");
  }

The messagebox is not blocking my main window. I think this is because I run in a task? What is a workaround?

Invoke it on GUI thread. Throw an exception, handle it later and show the message box. — Karel Frajták, Jan 23 '19 at 15:09
"how can I invoke on the GUI thread?" Please, Google that, thanks. — Karel Frajták, Jan 23 '19 at 15:16

Olivier Jacot-Descombes · Accepted Answer · 2019-01-23T15:53:20.207

0

What are you expecting from running this code as a task? It does not provide any advantage here. The possibly long running license validation is already being called with await making it non-UI blocking (unless its implementation is messed up).

To make the message box blocking, i.e. modal, simply write

  bool valid = await checkValidLicenseAsync(hardwareid); // non-blocking
  if (!valid) {
      MessageBox.Show("no License."); // blocking
  }

Assuming that this is the System.Windows.MessageBox and not its winforms pendant.

A possible approach is to use a factory method to return the object. Another one is to use an AsyncLazy<T> as described here by Stephen Cleary

See also: - Svick's answer to Can constructors be async?.

edited Jan 23 '19 at 15:53

answered Jan 23 '19 at 15:30

Olivier Jacot-Descombes

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i run this code in a constructor. In a constructor I can't await a method – daniel Jan 23 '19 at 15:34
A constructor should always leave the object fully initialized. Also a constructor should never fail. – Olivier Jacot-Descombes Jan 23 '19 at 15:39
ok, I think I have to move my code to the loaded event of my main window – daniel Jan 23 '19 at 15:55

score -1 · Answer 2 · answered Jan 23 '19 at 16:48

-1

I don't think you can call an awaitable method into an constructor. Since constructor can't be added an async descriptor. You can call an delegate in an constructor. Just wrap your logic into a async delegate

answered Jan 23 '19 at 16:48

Big Cash

1

Task.Run global Messagebox which blocks main window

2 Answers2