How Can a ViewModel Close a Form While Adhering to MVVM Principles?

How can a ViewModel close a form while maintaining MVVM architecture?

The core principle of MVVM (Model-View-ViewModel) dictates that the ViewModel should remain unaware of the View's specifics. However, scenarios arise where the ViewModel needs to signal form closure. Let's explore two solutions that uphold MVVM best practices:

Method 1: Attached Property

This method uses an attached property to bridge the communication gap between the ViewModel and the View. The attached property directly sets the DialogResult of the form.

Implementation:

using System.Windows;
using System.Windows.Interactivity;

namespace ExCastle.Wpf
{
    public static class DialogCloser
    {
        public static readonly DependencyProperty DialogResultProperty =
            DependencyProperty.RegisterAttached(
                "DialogResult",
                typeof(bool?),
                typeof(DialogCloser),
                new PropertyMetadata(DialogResultChanged));

        private static void DialogResultChanged(
            DependencyObject d,
            DependencyPropertyChangedEventArgs e)
        {
            var window = d as Window;
            if (window != null)
                window.DialogResult = e.NewValue as bool?;
        }
        public static void SetDialogResult(Window target, bool? value)
        {
            target.SetValue(DialogResultProperty, value);
        }
    }
}

XAML Integration:

<Window ... xc:DialogCloser.DialogResult="{Binding DialogResult}" xmlns:xc="clr-namespace:ExCastle.Wpf">
    ...
</Window>

The ViewModel sets the DialogResult property, triggering the attached property's logic to close the window.

Method 2: Interaction Element (Behavior or Command)

This approach utilizes a Behavior or Command within the View to act as an intermediary. The ViewModel triggers the Behavior or Command, which then closes the form. This offers more flexibility but adds complexity.

Conclusion:

Both techniques enable ViewModel-driven form closure without violating MVVM. The attached property approach is simpler and more direct, while the interaction element approach provides greater extensibility. The optimal choice depends on the application's specific needs and complexity.

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