Opening Windows in WPF Using Behaviors: A Guide to OpenWindowBehavior

In WPF (Windows Presentation Foundation), behaviors provide a powerful way to encapsulate and reuse functionality across multiple controls without writing code-behind logic. In this blog post, we'll explore a custom behavior called OpenWindowBehavior, which allows you to open a new window when attached to a ButtonBase or MenuItem (or any FrameworkElement). We'll also delve into how to add the Microsoft.Xaml.Behaviors library to your project and explain its role in WPF applications.

Introduction to Behaviors in WPF

Behaviors in WPF are reusable pieces of code that can be attached to UI elements to extend their functionality without modifying their code-behind files. They promote cleaner code and better separation of concerns, making your application more maintainable.

Understanding OpenWindowBehavior

The OpenWindowBehavior is a custom behavior that, when attached to a ButtonBase (like a Button) or MenuItem, opens a specified window. It supports opening the window either as a normal window or as a dialog, and you can set the startup location of the window.

Key Features:

WindowType: Specifies the type of the window to open.
IsDialog: Determines whether the window should be opened as a dialog.
WindowStartupLocation: Sets the startup location of the window (e.g., center owner).

The Code Breakdown

Here's the core of the OpenWindowBehavior:

    /// <summary>
    /// Behavior to open a new window when attached to a ButtonBase or MenuItem.
    /// </summary>
    /// <example>
    /// <code>
    /// <![CDATA[
    ///     <b:Interaction.Behaviors>
    ///         <i:OpenWindowBehavior IsDialog = "True" WindowType="{x:Type dialog:AboutDialog}" />
    ///     </b:Interaction.Behaviors>
    /// ]]>
    /// </code>
    /// </example>
    public class OpenWindowBehavior : Behavior<FrameworkElement>
    {
        /// <summary>
        /// Gets or sets the type of the window to open.
        /// </summary>
        public Type WindowType
        {
            get => (Type)GetValue(WindowTypeProperty);
            set => SetValue(WindowTypeProperty, value);
        }

        /// <summary>
        /// Gets or sets the type of the window to open.
        /// </summary>
        public static readonly DependencyProperty WindowTypeProperty =
            DependencyProperty.Register(nameof(WindowType), typeof(Type), typeof(OpenWindowBehavior), new PropertyMetadata(null));

        /// <summary>
        /// Gets or sets a value indicating whether the window should be opened as a dialog.
        /// </summary>
        /// <value>
        /// <c>true</c> if the window should be opened as a dialog; otherwise, <c>false</c>.
        /// </value>
        public bool IsDialog
        {
            get => (bool)GetValue(IsDialogProperty);
            set => SetValue(IsDialogProperty, value);
        }

        /// <summary>
        /// Gets or sets a value indicating whether the window should be opened as a dialog.
        /// </summary>
        /// <value>
        /// <c>true</c> if the window should be opened as a dialog; otherwise, <c>false</c>.
        /// </value>
        public static readonly DependencyProperty IsDialogProperty =
            DependencyProperty.Register(nameof(IsDialog), typeof(bool), typeof(OpenWindowBehavior), new PropertyMetadata(false));

        /// <summary>
        /// Gets or sets the startup location of the window.
        /// </summary>
        public static readonly DependencyProperty WindowStartupLocationProperty =
            DependencyProperty.Register(nameof(WindowStartupLocation), typeof(WindowStartupLocation), typeof(OpenWindowBehavior), new PropertyMetadata(System.Windows.WindowStartupLocation.CenterOwner));

        /// <summary>
        /// Gets or sets the startup location of the window.
        /// </summary>
        public WindowStartupLocation WindowStartupLocation
        {
            get => (WindowStartupLocation)GetValue(WindowStartupLocationProperty);
            set => SetValue(WindowStartupLocationProperty, value);
        }

        /// <summary>
        /// <summary>
        /// Called after the behavior is attached to an AssociatedObject.
        /// </summary>
        /// <exception cref="InvalidOperationException">
        /// Thrown if the behavior is attached to an object that is not a ButtonBase or MenuItem.
        /// </exception>
        /// </summary>
        /// <exception cref="InvalidOperationException"></exception>
        protected override void OnAttached()
        {
            base.OnAttached();

            if (AssociatedObject is ButtonBase buttonBase)
            {
                buttonBase.Click += OpenWindow;
            }
            else if (AssociatedObject is MenuItem menuItem)
            {
                menuItem.Click += OpenWindow;
            }
            else
            {
                throw new InvalidOperationException("OpenWindowBehavior can only be attached to ButtonBase or MenuItem.");
            }
        }

        /// <summary>
        /// Called when the behavior is being detached from its AssociatedObject.
        /// </summary>
        protected override void OnDetaching()
        {
            base.OnDetaching();

            if (AssociatedObject is ButtonBase buttonBase)
            {
                buttonBase.Click -= OpenWindow;
            }
            else if (AssociatedObject is MenuItem menuItem)
            {
                menuItem.Click -= OpenWindow;
            }
        }

        /// <summary>
        /// <summary>
        /// Opens the window specified by the <see cref="WindowType"/> property.
        /// </summary>
        /// <param name="sender">The source of the event.</param>
        /// <param name="e">The event data.</param>
        /// <exception cref="InvalidOperationException">
        /// Thrown if <see cref="WindowType"/> is not set to a type that derives from <see cref="Window"/>.
        /// </exception>
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        /// <exception cref="InvalidOperationException"></exception>
        private void OpenWindow(object sender, RoutedEventArgs e)
        {
            if (WindowType == null || !typeof(Window).IsAssignableFrom(WindowType))
            {
                throw new InvalidOperationException("WindowType must be set to a type that derives from Window.");
            }

            var window = Activator.CreateInstance(WindowType) as Window;

            // No window was found of the type requested.
            if (window == null)
            {
                return;
            }

            window.WindowStartupLocation = this.WindowStartupLocation;

            // Get the parent window when the behavior is attached
            var parentWindow = Window.GetWindow(AssociatedObject);

            // Optionally set the owner to the ParentWindow if it exists
            if (parentWindow != null && IsDialog)
            {
                window.Owner = parentWindow;
            }

            if (IsDialog)
            {
                window.ShowDialog();
            }
            else
            {
                window.Show();
            }
        }
    }

Note: The full code includes dependency properties and event handling for attaching to the appropriate UI elements.

Adding Microsoft.Xaml.Behaviors to Your Project Before you can use behaviors in WPF, you need to add the Microsoft.Xaml.Behaviors library to your project. This library provides the necessary infrastructure for behaviors and triggers in XAML.

Steps to Add the Library:

Open Your Project in Visual Studio:
- Ensure your WPF project is open in Visual Studio.

Manage NuGet Packages: - Right-click on your project in the Solution Explorer. - Select Manage NuGet Packages....
Browse and Install:
- In the NuGet Package Manager, go to the Browse tab.
- Search for Microsoft.Xaml.Behaviors.Wpf.
- Select the package from the search results.
- Click Install and accept any licenses.
Add Namespace in XAML:

In your XAML files where you plan to use behaviors, add the following namespace:

xmlns:b="http://schemas.microsoft.com/xaml/behaviors"

What Is Microsoft.Xaml.Behaviors?

Microsoft.Xaml.Behaviors is a library that provides the implementation of behaviors, actions, and triggers in XAML. It was originally part of the Blend SDK but is now available as a standalone NuGet package, making it easier to use in WPF applications without additional SDKs.

Using OpenWindowBehavior in XAML

Once you have the Microsoft.Xaml.Behaviors library and the OpenWindowBehavior class in your project, you can easily attach the behavior to buttons or menu items in your XAML.

Example Usage:

Here's how you can use the OpenWindowBehavior in your XAML to open a window called AboutDialog as a dialog when a button is clicked:

<Window x:Class="YourNamespace.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:b="http://schemas.microsoft.com/xaml/behaviors"
        xmlns:i="clr-namespace:YourNamespace.UI.Behaviors"
        xmlns:dialog="clr-namespace:YourNamespace.Dialogs"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <Button Content="Open About Dialog">
            <b:Interaction.Behaviors>
                <i:OpenWindowBehavior IsDialog="True" WindowType="{x:Type dialog:AboutDialog}" />
            </b:Interaction.Behaviors>
        </Button>
    </Grid>
</Window>

Explanation:

Namespaces:
- b: Points to the Microsoft.Xaml.Behaviors namespace.
- i: Points to your custom behaviors namespace.
- dialog: Points to the namespace where your AboutDialog window is defined.
Behavior Attachment:
- The OpenWindowBehavior is attached within the <b:Interaction.Behaviors> tag inside the Button.
- The IsDialog property is set to True to open the window as a dialog.
- The WindowType is set using the {x:Type} markup extension to specify the type of the window to open.

Using with a MenuItem:

<MenuItem Header="About">
    <b:Interaction.Behaviors>
        <i:OpenWindowBehavior IsDialog="True" WindowType="{x:Type dialog:AboutDialog}" />
    </b:Interaction.Behaviors>
</MenuItem>

Conclusion

The OpenWindowBehavior provides a clean and reusable way to open windows in your WPF applications without cluttering your code-behind files. By leveraging the power of behaviors and the Microsoft.Xaml.Behaviors library, you can create more maintainable and modular applications.

WPF: Opening Windows with XAML Behaviors