Get Custom Dialog Data via Messaging in WPF MVVM

Step 1: Define a Message Class with Reply Capability

Define a regular class that inherits from CommunityToolkit.Mvvm’s RequestMessage, enabling the Send method to have a return value. This return value is the data replied by the message subscriber.

public class OpenDialogMessage : RequestMessage<string> { }

At this point, the message receiver can also use the Reply method on the message to reply to the message sender.

For more message types, you can refer to this website.

Step 2: Main Window ViewModel Sends “Open Dialog” Message

Create a command bound to the corresponding button or other trigger. When triggered, it sends the “open dialog” message, receives the subscriber’s reply, and prints it to the console.

public partial class MainWindowViewModel : ObservableObject
{
    [RelayCommand]
    private void OpenDialog()
    {
        var result = WeakReferenceMessenger
            .Default.Send<OpenDialogMessage>();
        
        if (result != null)
        {
            Console.Out.WriteLine("result = {0}", result.Response);
        }
    }
}

Step 3: Main Window Code-Behind Subscribes to Message

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

        DataContext = new MainWindowViewModel();
        
        WeakReferenceMessenger.Default
            .Register<MainWindow, OpenDialogMessage>(this, (receiver, message) => { });
    }
}

Step 4: Add Close Event to Dialog and Display Dialog

Create an anonymous function for message handling, called when the message is received. Within it, create the dialog and add a window close event. When the window closes, reply the dialog data to the sender.

public partial class MainWindow : Window
{
    public MainWindow()
    {
        // --- Omit existing code ---
        
        WeakReferenceMessenger.Default
            .Register<MainWindow, OpenDialogMessage>(this, (receiver, message) =>
        {
            var dialog = new DialogWindow();
            
            // Add a window close event
            dialog.Closed += (s, e) =>
            {
                var w = s as DialogWindow;
                message.Reply(w?.InputText.Text);
            };
			
            // Opens the Modality dialog box
            dialog.ShowDialog();
        });
    }
}

Step 5: Create Dialog Interface

In this interface, there are three key points to note:

• The TextBox input content is the data to be retrieved, so it has been given a Name attribute for subsequent access.
• The two Button controls have IsDefault and IsCancel properties set respectively:
  • IsDefault: Sets whether the Button is the default button. Users invoke the default button by pressing Enter.
  • IsCancel: Sets whether the Button is the cancel button. Users can invoke the cancel button by pressing ESC.
• Click events are created for both Button controls to set the DialogResult value.

Since a modal dialog is opened here, as long as the dialog result value (DialogResult) is set, the window will automatically close.

<Window --- Omit existing code ---
        MinHeight="200" MinWidth="300"
        SizeToContent="WidthAndHeight"
        ResizeMode="NoResize"
        WindowStartupLocation="CenterOwner">
    
    <Grid RowDefinitions="Auto, *" 
          ColumnDefinitions="Auto, *" 
          Margin="20">
        <TextBlock Grid.Row="0" Grid.Column="0" 
                   Margin="0 0 5 0"
                   Text="Text:"/>
        <TextBox Grid.Row="0" Grid.Column="1" 
                 Name="InputText" Height="100"/>

        <StackPanel Grid.Row="2" Grid.Column="1" 
                    Orientation="Horizontal" HorizontalAlignment="Right"
                    VerticalAlignment="Bottom">
            <Button Name="OkBtn" Margin="0 0 5 0" 
                    IsDefault="True" Click="OkBtn_OnClick"
                    Content="OK"/>
            <Button Name="CancelBtn" IsCancel="True">Cancel</Button>
        </StackPanel>
    </Grid>
</Window>

private void OkBtn_OnClick(object sender, RoutedEventArgs e) =>
    DialogResult = true;

private void CancelBtn_OnClick(object sender, RoutedEventArgs e) =>
    DialogResult = false;