How Can I Close a WPF Form from the ViewModel Using an Attached Property in MVVM?

Close WPF form in MVVM using attached properties

In MVVM applications, managing form closing can be challenging because the ViewModel does not know about the actual view. This article explores an elegant solution using attached properties to seamlessly close a form from a ViewModel.

Dilemma

Consider a typical login form where the ViewModel holds user credentials. When submitting the login command, the ViewModel starts the login operation:

• After successful login, the login form needs to be closed and a valid DialogResult returned.
• When the login is invalid, an error message will be displayed and the form will remain open.

Break barriers with additional attributes

To address this challenge, we can leverage an additional property to bridge the gap between ViewModel and View:

<Window ... DialogResult="{Binding DialogResult, Source={StaticResource ViewModel}}"
       xmlns:xc="clr-namespace:ExCastle.Wpf">
</Window>

This property binds the ViewModel's DialogResult property to the window's DialogResult property. When the ViewModel modifies the DialogResult, the attached property automatically updates the window's DialogResult and closes the form accordingly.

Code implementation

The following is the code for the attached attribute:

using System.Windows;

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);
        }
    }
}

In the ViewModel, just implement INotifyPropertyChanged and update the DialogResult property when login is successful:

private bool? _dialogResult;
public bool? DialogResult
{
    get { return _dialogResult; }
    set
    {
        _dialogResult = value;
        OnPropertyChanged(nameof(DialogResult));
    }
}

// 登录命令逻辑
public ICommand LoginCommand { get; set; }

Simplicity and Purity

This approach elegantly solves the form closing problem without affecting MVVM principles. The ViewModel remains decoupled from the view while effectively controlling form closing through simple property binding.

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