Can't set DialogResult in WPF

Question

I show a WPF window using ShowDialog() from the calling window. The window opens and is modal as expected. However, in my OK and Cancel button's click events in the dialog window I set this.DialogResult = true (or false) respectively, and the value does not get set. The window closes as expected, but DialogResult is still null.

Is this a bug in WPF? Or is there a reason the DialogResult property cannot be set yet does not throw an exception? The window is not hosted in a browser.

Code in the calling window:

Window2 win = new Window2();
bool? result = win.ShowDialog();
if (result.HasValue && result.Value) {
   //never gets here because result is always null
}

Code in the dialog window:

this.DialogResult = true;

Answer 1

DialogResult is a nullable bool. However you do not have to cast it to get it's value.

bool? result = myWindow.ShowDialog();
if (result ?? false)
{
  // snip
}

The ?? sets the default value to return if the result is null. More information: Using Nullable Types (C# Programming Guide)

As for the original question, the only time I have seen and traced this issue is when the window was being disposed between setting the DialogResult and closing the window. Unfortunately the only advice that I can offer is for you step through your code and check the order of the operations. I believe that I "fixed" it by setting the DialogResult and then explicitly closing the window.

Answer 2

Well first of all you have to take into account that it returns a nullable bool (bool?), so in order to compare it or set it to another variable you have to cast it to a regular bool

bool result = (bool)myWindow.DialogResult;

As for it being null... I don't see why that should happen, unless it's somehow being set back to null AFTER being set to true or false. Can you show your code?

EDIT:

Your code worked fine for me, this is what I have in the second window:

private void button2_Click(object sender, RoutedEventArgs e)
{
    this.DialogResult = false;
}

private void button1_Click(object sender, RoutedEventArgs e)
{
    this.DialogResult = true;
}

And in Window1:

private void window1_Loaded(object sender, RoutedEventArgs e)
{
    Window2 win = new Window2();

    bool? result = win.ShowDialog();

    if (result.HasValue && result.Value)
    {
        //it DID get here
    }
}

Is there any big difference?

Answer 3

I have just had exactly the same problem and it seems to be caused by my overriding the OnClosing() method. I needed to override OnClosing() to stop the user closing the modal window via the close (X) button.

When I comment out the OnClosing() method, the problem goes away and the DialogResult is returned with the expected values of true or false, as set.

For interest here was my button click handlers and OnClosing method:

private void AlternateButton_Click(object sender, RoutedEventArgs e)
{
    this.DialogResult = false;
    buttonHasBeenClicked = true;
    this.Close();
}

private void DefaultButton_Click(object sender, RoutedEventArgs e)
{
    this.DialogResult = true;
    buttonHasBeenClicked = true;
    this.Close();
}

protected override void OnClosing(System.ComponentModel.CancelEventArgs e)
{
    base.OnClosing(e);
    if (!buttonHasBeenClicked)
    {
        // Prevent the user closing the window without pressing one of the buttons.
        e.Cancel = true;
    }
}

Answer 4

I have been into this problem too, and the only way i have found to fix it was using this code in my Class :

public new bool? DialogResult { get; set; }

and after setting my DialogResult it work out for me !! ( very strange issue ). this was the code i was using :

cmdCancel = new RelayCommand(() => { DataContact.Reload(); this.DialogResult = false; this.Close(); });

and to open my dialog :

public static MessageBoxResult ShowQuestionYesNo(string message)
        {
            POLMessageBox w = new POLMessageBox("سوال", MessageBoxType.QuestionYesNo, message);
            w.ShowDialog();
            var b = w.DialogResult;
            if (b == true) return MessageBoxResult.Yes;
            if (b == false) return MessageBoxResult.No;
            return MessageBoxResult.No;
        }