Displaying a custom dialog window during a long-running task

Question

Let's say I have a very simple ProgressBar with IsIndeterminate=true :

<Window x:Class="My.Controls.IndeterminateProgressDialog"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" Name="Window"
        Width="300" Height="110" ResizeMode="NoResize" Topmost="True"
        WindowStartupLocation="CenterScreen" WindowStyle="None">

        <Grid>            
            <ProgressBar Width="200" Height="20" IsIndeterminate="True" />
        </Grid>

</Window>

I want to show this dialog during a Task that may take a while. I don't care about progress (I can't determine it), I just want to inform the user that I do something that may take some seconds.

 public void GetResult()
 {
   string result = DoWhileShowingDialogAsync().Result;
   //...
 }

 private async Task<string> DoWhileShowingDialogAsync()
 {
   var pd = new IndeterminateProgressDialog();

   pd.Show();   
   string ret = await Task.Run(() => DoSomethingComplex()));            
   pd.Close();

   return ret;
}

However the UI just freezes infinitely and the Task seems to never return. The problem doesn't lie within DoSomethingComplex(), it completes without issues if I run it synchronously. I'm pretty sure it's because I misunderstood something with await/async, can someone point me in the right direction?

Answer 1

.Result

That's a classic UI thread deadlock. Use await. Use it all the way up the call tree.

Answer 2

Just for some clarification, 'use it all the way up the call tree' means you need to call it from the UI thread. Something like this:

private Task<string> DoWhileShowingDialogAsync()
{
    return Task.Run(() => DoSomethingComplex());
} 

private string DoSomethingComplex()
{
    // wait a noticeable time
    for (int i = 0; i != 1000000000; ++i)
    ; // do nothing, just wait
}

private async void GetResult()
{
    pd.Show();
    string result = await DoWhileShowingDialogAsync();
    pd.Close();
}