How to make modal dialog in WPF?

Question

Did you try showing your window using the ShowDialog method?

Don't forget to set the Owner property on the dialog window to the main window. This will avoid weird behavior when Alt+Tabbing, etc.

A lot of these answers are simplistic, and if someone is beginning WPF, they may not know all of the "ins-and-outs", as it is more complicated than just telling someone "Use .ShowDialog()!". But that is the method (not .Show()) that you want to use in order to block use of the underlying window and to keep the code from continuing until the modal window is closed.

First, you need 2 WPF windows. (One will be calling the other.)

From the first window, let's say that was called MainWindow.xaml, in its code-behind will be:

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
    }
}

Then add your button to your XAML:

<Button Name="btnOpenModal" Click="btnOpenModal_Click" Content="Open Modal" />

And right-click the Click routine, select "Go to definition". It will create it for you in MainWindow.xaml.cs:

private void btnOpenModal_Click(object sender, RoutedEventArgs e)
{
}

Within that function, you have to specify the other page using its page class. Say you named that other page "ModalWindow", so that becomes its page class and is how you would instantiate (call) it:

private void btnOpenModal_Click(object sender, RoutedEventArgs e)
{
    ModalWindow modalWindow = new ModalWindow();
    modalWindow.ShowDialog();
}

Say you have a value you need set on your modal dialog. Create a textbox and a button in the ModalWindow XAML:

<StackPanel Orientation="Horizontal">
    <TextBox Name="txtSomeBox" />
    <Button Name="btnSaveData" Click="btnSaveData_Click" Content="Save" /> 
</StackPanel>

Then create an event handler (another Click event) again and use it to save the textbox value to a public static variable on ModalWindow and call this.Close().

public partial class ModalWindow : Window
{
    public static string myValue = String.Empty;        
    public ModalWindow()
    {
        InitializeComponent();
    }

    private void btnSaveData_Click(object sender, RoutedEventArgs e)
    {
        myValue = txtSomeBox.Text;
        this.Close();
    }
}

Then, after your .ShowDialog() statement, you can grab that value and use it:

private void btnOpenModal_Click(object sender, RoutedEventArgs e)
{
    ModalWindow modalWindow = new ModalWindow();
    modalWindow.ShowDialog();

    string valueFromModalTextBox = ModalWindow.myValue;
}

Window.Show will show the window, and continue execution -- it's a non-blocking call.

Window.ShowDialog will block the calling thread (kinda ^[1]), and show the dialog. It will also block interaction with the parent/owning window. When the dialog is dismissed (for whatever reason), ShowDialog will return to the caller, and will allow you to access DialogResult (if you want it).

^[1] It will keep the dispatcher pumping by pushing a dispatcher frame onto the WPF dispatcher. This will cause the message pump to keep pumping.

Given a Window object myWindow, myWindow.Show() will open it modelessly and myWindow.ShowDialog() will open it modally. However, even the latter doesn't block, from what I remember.