Application Management Overview

This topic provides an overview of the Windows Presentation Foundation (WPF) services for creating and managing applications. The kernel of a WPF application is the Application class, which supports a variety of the core application services. This topic provides an introduction to the most important of these services.

This topic contains the following sections.

• The Application Class
• The Application Definition
• Getting the Current Application
• Application Lifetime
• Other Application Services
• Related Topics

The Application Class

An application consists of many application-specific elements, including user interface (UI), business logic, data access logic, controls, and data. These elements typically differ from one application to the next. However, all applications tend to share a common set of functionality that facilitates application implementation and management. In WPF, this common application-scoped functionality is encapsulated by the Application class, which provides the following services:

• Creating and managing common application infrastructure.

• Tracking and interacting with application lifetime.

• Retrieving and processing command-line parameters.

• Sharing application-scope properties and resources.

• Detecting and responding to unhandled exceptions.

• Returning exit codes.

• Managing windows in standalone applications (see WPF Windows Overview).

• Tracking and managing navigation (see Navigation Overview).

To use these services in your application, you need to use the Application class to implement an application definition.

The Application Definition

A WPF application definition is a class that derives from Application and is configured with a special Microsoft build engine (MSBuild) setting.

Implementing an Application Definition

A typical WPF application definition is implemented using both markup and code-behind. This allows you to use markup to declaratively set application properties, resources, and register events, while handling events and implementing application-specific behavior in code-behind.

The following example shows how to implement an application definition using both markup and code-behind:

<Application 
  xmlns="https://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="https://schemas.microsoft.com/winfx/2006/xaml" 
  x:Class="SDKSample.App" />

using System.Windows;  // Application 

namespace SDKSample
{
    public partial class App : Application { }
}

To allow a markup file and code-behind file to work together, the following needs to happen:

• In markup, the Application element must include the x:Class attribute. When the application is built, the existence of x:Class in the markup file causes MSBuild to create a partial class that derives from Application and has the name that is specified by the x:Class attribute. This requires the addition of an XML namespace declaration for the XAML schema (xmlns:x="https://schemas.microsoft.com/winfx/2006/xaml").

• In code-behind, the class must be a partial class with the same name that is specified by the x:Class attribute in markup and must derive from Application. This allows the code-behind file to be associated with the partial class that is generated for the markup file when the application is built (see Building a WPF Application (WPF)).

This code is the minimum that is required to implement an application definition. However, an additional MSBuild configuration needs to be made to the application definition before building and running the application.

Configuring the Application Definition for MSBuild

Standalone applications and XAML browser applications (XBAPs) require the implementation of a certain level of infrastructure before they can run. The most important part of this infrastructure is the entry point. When an application is launched by a user, the operating system calls the entry point, which is a well-known function for starting applications.

Traditionally, developers have needed to write some or all of this code for themselves, depending on the technology. However, WPF generates this code for you when the markup file of your application definition is configured as an MSBuild ApplicationDefinition item, as shown in the following MSBuild project file:

<Project 
  DefaultTargets="Build"
  xmlns="https://schemas.microsoft.com/developer/msbuild/2003">
  ...
  <ApplicationDefinition Include="App.xaml" />
  <Compile Include="App.xaml.cs" />
  ...
</Project>

Because the code-behind file contains code, it is marked as an MSBuild Compile item, as is normal.

The application of these MSBuild configurations to the markup and code-behind files of an application definition causes MSBuild to generate code like the following:

using System; // STAThread
using System.Windows; // Application

namespace SDKSample
{
    public class App : Application
    {
        public App() { }
        [STAThread]
        public static void Main()
        {
            // Create new instance of application subclass
            App app = new App();

            // Code to register events and set properties that were
            // defined in XAML in the application definition
            app.InitializeComponent();

            // Start running the application
            app.Run();
        }

        public void InitializeComponent()
        {


...


        }
    }
}

The resulting code augments your application definition with additional infrastructure code, which includes the entry-point method Main. The STAThreadAttribute attribute is applied to the Main method to indicate that the main UI thread for the WPF application is an STA thread, which is required for WPF applications. When called, Main creates a new instance of App before calling the InitializeComponent method to register the events and set the properties that are implemented in markup. Because InitializeComponent is generated for you, you don't need to explicitly call InitializeComponent from an application definition like you do for Page and Window implementations. Finally, the Run method is called to start the application.

Getting the Current Application

Because the services of the Application class are shared across an application, there can be only one instance of the Application class per AppDomain. To enforce this, the Application class is implemented as a singleton class (see Implementing Singleton in C#), which creates a single instance of itself and provides shared access to it with the static Current property.

The following code shows how to acquire a reference to the Application object for the current AppDomain.

// Get current application
Application current = App.Current;

Current returns a reference to an instance of the Application class. If you want a reference to your Application derived class you must cast the value of the Current property, as shown in the following example.

// Get strongly-typed current application
App app = (App)App.Current;

You can inspect the value of Current at any point in the lifetime of an Application object. However, you should be careful. After the Application class is instantiated, there is a period during which the state of the Application object is inconsistent. During this period, Application is performing the various initialization tasks that are required by your code to run, including establishing application infrastructure, setting properties, and registering events. If you try to use the Application object during this period, your code may have unexpected results, particularly if it depends on the various Application properties being set.

When Application completes its initialization work, its lifetime truly begins.

Application Lifetime

The lifetime of a WPF application is marked by several events that are raised by Application to let you know when your application has started, has been activated and deactivated, and has been shut down.

This section contains the following subsections.

• Splash Screen
• Starting an Application
• Showing a User Interface
• Processing Command-Line Arguments
• Application Activation and Deactivation
• Application Shutdown
• Unhandled Exceptions
• Application Lifetime Events

Splash Screen

Starting in the .NET Framework 3.5 SP1, you can specify an image to be used in a startup window, or splash screen. The SplashScreen class makes it easy to display a startup window while your application is loading. The SplashScreen window is created and shown before Run is called. For more information, see Application Startup Time and How to: Add a Splash Screen to a WPF Application.

Starting an Application

After Run is called and the application is initialized, the application is ready to run. This moment is signified when the Startup event is raised:

using System.Windows; // Application, StartupEventArgs, WindowState

namespace SDKSample
{
    public partial class App : Application
    {
        void App_Startup(object sender, StartupEventArgs e)
        {
            // Application is running


...


        }
    }
}

At this point in an application's lifetime, the most common thing to do is to show a UI.

Showing a User Interface

Most standalone Windows applications open a Window when they begin running. The Startup event handler is one location from which you can do this, as demonstrated by the following code.

<Application
  xmlns="https://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="https://schemas.microsoft.com/winfx/2006/xaml"
  x:Class="SDKSample.App" 
  Startup="App_Startup" />

using System.Windows; // Application, StartupEventArgs 

namespace SDKSample
{
    public partial class App : Application
    {
        void App_Startup(object sender, StartupEventArgs e)
        {
            // Open a window
            MainWindow window = new MainWindow();
            window.Show();
        }
    }
}

When an XBAP first starts, it will most likely navigate to a Page. This is shown in the following code.

<Application 
  x:Class="SDKSample.App"
  xmlns="https://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="https://schemas.microsoft.com/winfx/2006/xaml"
  Startup="App_Startup" />

using System; // Uri, UriKind, EventArgs, Console 
using System.Windows; // Application, StartupEventArgs 
using System.Windows.Navigation; // NavigationWindow 

namespace SDKSample
{
    public partial class App : Application
    {        
        void App_Startup(object sender, StartupEventArgs e)
        {
            ((NavigationWindow)this.MainWindow).Navigate(new Uri("HomePage.xaml", UriKind.Relative));
        }
    }
}

If you handle Startup to only open a Window or navigate to a Page, you can set the StartupUri attribute in markup instead.

The following example shows how to use the StartupUri from a standalone application to open a Window.

<Application
    xmlns="https://schemas.microsoft.com/winfx/2006/xaml/presentation"
    StartupUri="MainWindow.xaml" />

The following example shows how to use StartupUri from an XBAP to navigate to a Page.

<Application
    xmlns="https://schemas.microsoft.com/winfx/2006/xaml/presentation"
    StartupUri="HomePage.xaml" />

This markup has the same effect as the previous code for opening a window.

You need to handle the Startup event to open a Window if you need to instantiate it using a non-default constructor, or you need to set its properties or subscribe to its events before showing it, or you need to process any command-line arguments that were supplied when the application was launched.

Processing Command-Line Arguments

In Windows, standalone applications can be launched from either a command prompt or the desktop. In both cases, command-line arguments can be passed to the application. The following example shows an application that is launched with a single command-line argument, "/StartMinimized":

wpfapplication.exe /StartMinimized

During application initialization, WPF retrieves the command-line arguments from the operating system and passes them to the Startup event handler via the Args property of the StartupEventArgs parameter. You can retrieve and store the command-line arguments using code like the following.

<Application
  xmlns="https://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="https://schemas.microsoft.com/winfx/2006/xaml"
  x:Class="SDKSample.App"
  Startup="App_Startup" />

using System.Windows; // Application, StartupEventArgs, WindowState 

namespace SDKSample
{
    public partial class App : Application
    {
        void App_Startup(object sender, StartupEventArgs e)
        {
            // Application is running 
            // Process command line args 
            bool startMinimized = false;
            for (int i = 0; i != e.Args.Length; ++i)
            {
                if (e.Args[i] == "/StartMinimized")
                {
                    startMinimized = true;
                }
            }

            // Create main application window, starting minimized if specified
            MainWindow mainWindow = new MainWindow();
            if (startMinimized)
            {
                mainWindow.WindowState = WindowState.Minimized;
            }
            mainWindow.Show();
        }
    }
}

The code handles Startup to check whether the /StartMinimized command-line argument was provided; if so, it opens the main window with a WindowState of Minimized. Note that because the WindowState property must be set programmatically, the main Window must be opened explicitly in code.

For a sample that demonstrates a more robust command-line parsing technique that uses regular expressions, see Processing Command Line Arguments Sample.

XBAPs cannot retrieve and process command-line arguments because they are launched using ClickOnce deployment (see Deploying a WPF Application (WPF)). However, they can retrieve and process query string parameters from the URLs that are used to launch them. For an example, see URI Query String Parameters Sample.

Application Activation and Deactivation

Windows allows users to switch between applications. The most common way is to use the ALT+TAB key combination. An application can only be switched to if it has a visible Window that a user can select. The currently selected Window is the active window (also known as the foreground window) and is the Window that receives user input. The application with the active window is the active application (or foreground application). An application becomes the active application in the following circumstances:

• It is launched and shows a Window.

• A user switches from another application by selecting a Window in the application.

You can detect when an application becomes active by handling the Application.Activated event.

Likewise, an application can become inactive in the following circumstances:

• A user switches to another application from the current one.

• When the application shuts down.

You can detect when an application becomes inactive by handling the Application.Deactivated event.

The following code shows how to handle the Activated and Deactivated events to determine whether an application is active.

<Application 
  xmlns="https://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="https://schemas.microsoft.com/winfx/2006/xaml"
  x:Class="SDKSample.App"
  StartupUri="MainWindow.xaml"
  Activated="App_Activated" 
  Deactivated="App_Deactivated" />

using System; // EventArgs 
using System.Windows; // Application 

namespace SDKSample
{
    public partial class App : Application
    {
        bool isApplicationActive;

        void App_Activated(object sender, EventArgs e)
        {
            // Application activated 
            this.isApplicationActive = true;
        }

        void App_Deactivated(object sender, EventArgs e)
        {
            // Application deactivated 
            this.isApplicationActive = false;
        }
    }
}

A Window can also be activated and deactivated. See Window.Activated and Window.Deactivated for more information.

Application Shutdown

The life of an application ends when it is shut down, which can occur for the following reasons:

• A user closes every Window.

• A user closes the main Window.

• A user ends the Windows session by logging off or shutting down.

• An application-specific condition has been met.

To help you manage application shutdown, Application provides the Shutdown method, the ShutdownMode property, and the SessionEnding and Exit events.

Shutdown Mode

Most applications shut down either when all the windows are closed or when the main window is closed. Sometimes, however, other application-specific conditions may determine when an application shuts down. You can specify the conditions under which your application will shut down by setting ShutdownMode with one of the following ShutdownMode enumeration values:

• OnLastWindowClose

• OnMainWindowClose

• OnExplicitShutdown

The default value of ShutdownMode is OnLastWindowClose, which means that an application automatically shuts down when the last window in the application is closed by the user. However, if your application should be shut down when the main window is closed, WPF automatically does that if you set ShutdownMode to OnMainWindowClose. This is shown in the following example.

<Application
    xmlns="https://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="https://schemas.microsoft.com/winfx/2006/xaml"
    x:Class="SDKSample.App"
    ShutdownMode="OnMainWindowClose" />

When you have application-specific shutdown conditions, you set ShutdownMode to OnExplicitShutdown. In this case, it is your responsibility to shut an application down by explicitly calling the Shutdown method; otherwise, your application will continue running even if all the windows are closed. Note that Shutdown is called implicitly when the ShutdownMode is either OnLastWindowClose or OnMainWindowClose.

Session Ending

The shutdown conditions that are described by the ShutdownMode property are specific to an application. In some cases, though, an application may shut down as a result of an external condition. The most common external condition occurs when a user ends the Windows session by the following actions:

• Logging off

• Shutting down

• Restarting

• Hibernating

To detect when a Windows session ends, you can handle the SessionEnding event, as illustrated in the following example.

<Application 
    xmlns="https://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="https://schemas.microsoft.com/winfx/2006/xaml"
    x:Class="SDKSample.App"
    StartupUri="MainWindow.xaml"
    SessionEnding="App_SessionEnding" />

using System.Windows; // Application, SessionEndingCancelEventArgs, MessageBox, MessageBoxResult, MessageBoxButton 

namespace SDKSample
{
    public partial class App : Application
    {
        void App_SessionEnding(object sender, SessionEndingCancelEventArgs e)
        {
            // Ask the user if they want to allow the session to end 
            string msg = string.Format("{0}. End session?", e.ReasonSessionEnding);
            MessageBoxResult result = MessageBox.Show(msg, "Session Ending", MessageBoxButton.YesNo);

            // End session, if specified 
            if (result == MessageBoxResult.No)
            {
                e.Cancel = true;
            }
        }
    }
}

In this example, the code inspects the ReasonSessionEnding property to determine how the Windows session is ending. It uses this value to display a confirmation message to the user. If the user does not want the session to end, the code sets Cancel to true to prevent the Windows session from ending.

Exit

When an application shuts down, it may need to perform some final processing, such as persisting application state. For these situations, you can handle the Exit event.

<Application
    xmlns="https://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="https://schemas.microsoft.com/winfx/2006/xaml"
    x:Class="SDKSample.App"
    StartupUri="MainWindow.xaml" 
    Startup="App_Startup" 
    Exit="App_Exit">


...


</Application>

using System.Windows; // Application, StartupEventArgs
using System.IO; // StreamReader, FileMode
using System.IO.IsolatedStorage; // IsolatedStorageFile, IsolatedStorageFileStream

namespace SDKSample
{
    public partial class App : Application
    {
        string filename = "App.txt";



...


        private void App_Exit(object sender, ExitEventArgs e)
        {
            // Persist application-scope property to isolated storage
            IsolatedStorageFile storage = IsolatedStorageFile.GetUserStoreForDomain();
            using (IsolatedStorageFileStream stream = new IsolatedStorageFileStream(filename, FileMode.Create, storage))
            using (StreamWriter writer = new StreamWriter(stream))
            {
                // Persist each application-scope property individually
                foreach (string key in this.Properties.Keys)
                {
                    writer.WriteLine("{0},{1}", key, this.Properties[key]);
                }
            }
        }
    }
}

For the complete example, see How to: Persist and Restore Application-Scope Properties Across Application Sessions.

Exit can be handled by both standalone applications and XBAPs. For XBAPs, Exit is raised when in the following circumstances:

• An XBAP is navigated away from.

• In Internet Explorer 7, when the tab that is hosting the XBAP is closed.

• When the browser is closed.

Exit Code

Applications are mostly launched by the operating system in response to a user request. However, an application can be launched by another application to perform some specific task. When the launched application shuts down, the launching application may want to know the condition under which the launched application shut down. In these situations, Windows allows applications to return an application exit code on shutdown. By default, WPF applications return an exit code value of 0.

To change the exit code, you can call the Shutdown(Int32) overload, which accepts an integer argument to be the exit code:

// Shutdown and return a non-default exit code
Application.Current.Shutdown(-1);

You can detect the value of the exit code, and change it, by handling the Exit event. The Exit event handler is passed an ExitEventArgs which provides access to the exit code with the ApplicationExitCode property. For more information, see Exit.

Unhandled Exceptions

Sometimes an application may shut down under abnormal conditions, such as when an unanticipated exception is thrown. In this case, the application may not have the code to detect and process the exception. This type of exception is an unhandled exception; a notification similar to that shown in the following figure is displayed before the application is closed.

From the user experience perspective, it is better for an application to avoid this default behavior by doing some or all of the following:

• Displaying user-friendly information.

• Attempting to keep an application running.

• Recording detailed, developer-friendly, exception information in the Windows event log.

Implementing this support depends on being able to detect unhandled exceptions, which is what the DispatcherUnhandledException event is raised for.

<Application
  xmlns="https://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="https://schemas.microsoft.com/winfx/2006/xaml"
  x:Class="SDKSample.App"
  StartupUri="MainWindow.xaml"
  DispatcherUnhandledException="App_DispatcherUnhandledException" />

using System.Windows; // Application
using System.Windows.Threading; // DispatcherUnhandledExceptionEventArgs

namespace SDKSample
{
    public partial class App : Application
    {
        void App_DispatcherUnhandledException(object sender, DispatcherUnhandledExceptionEventArgs e)
        {
            // Process unhandled exception


...


            // Prevent default unhandled exception processing
            e.Handled = true;
        }
    }
}

The DispatcherUnhandledException event handler is passed a DispatcherUnhandledExceptionEventArgs parameter that contains contextual information regarding the unhandled exception, including the exception itself (DispatcherUnhandledExceptionEventArgs.Exception). You can use this information to determine how to handle the exception.

When you handle DispatcherUnhandledException, you should set the DispatcherUnhandledExceptionEventArgs.Handled property to true; otherwise, WPF still considers the exception to be unhandled and reverts to the default behavior described earlier. If an unhandled exception is raised and either the DispatcherUnhandledException event is not handled, or the event is handled and Handled is set to false, the application shuts down immediately. Furthermore, no other Application events are raised. Consequently, you need to handle DispatcherUnhandledException if your application has code that must run before the application shuts down.

Although an application may shut down as a result of an unhandled exception, an application usually shuts down in response to a user request, as discussed in the next section.

Application Lifetime Events

Standalone applications and XBAPs don't have exactly the same lifetimes. The following figure illustrates the key events in the lifetime of a standalone application and shows the sequence in which they are raised.

Likewise, the following figure illustrates the key events in the lifetime of an XBAP, and shows the sequence in which they are raised.

Other Application Services

In addition to managing the lifetime of an application, Application provides services that include the following:

• Shared application-scope properties.

• Shared application-scope resources.

• Application resource, content, and site-of-origin data files.

• Window management.

• Navigation management.

Shared Application-Scope Properties

Application provides the Properties property to expose state that can be shared across the breadth of an application. The following provides an example of using Properties:

// Set an application-scope property with a custom type
CustomType customType = new CustomType();
Application.Current.Properties["CustomType"] = customType;


...


// Get an application-scope property
// NOTE: Need to convert since Application.Properties is a dictionary of System.Object
CustomType customType = (CustomType)Application.Current.Properties["CustomType"];

See the following for more information:

• How to: Get and Set Application-Scope Properties

• How to: Persist and Restore Application-Scope Properties Across Application Sessions

Shared Application-Scope Resources

Application provides the Resources property to allow developers to share UI resources across an application. The following provides an example of using Resources:

// Set an application-scope resource
Application.Current.Resources["ApplicationScopeResource"] = Brushes.White;


...


// Get an application-scope resource
Brush whiteBrush = (Brush)Application.Current.Resources["ApplicationScopeResource"];

See the following for more information:

• TryFindResource

• FindResource

• How to: Get and Set Application-Scope Resources

• Resources Overview

Application Resource, Content, and Site-Of-Origin Data Files

WPF applications can manage several types of non-code data files, including resource files, content files, and site-of-origin files. The following helper methods can be used to load these types of data files:

• GetResourceStream

• GetContentStream

• GetRemoteStream

Window Management

Application and Window have a close relationship. As you've seen, the lifetime of an application can depend on the lifetime of its windows, as specified by the ShutdownMode property. Application records which window is designated the main application window (Application.MainWindow), and maintains a list of currently instantiated windows (Application.Windows).

For more information, see WPF Windows Overview.

Navigation Management

For standalone applications with navigation (using NavigationWindow and Frame) or XBAPs, Application detects any navigation within an application and raises the following events as appropriate:

• Navigating

• Navigated

• NavigationProgress

• NavigationFailed

• NavigationStopped

• LoadCompleted

• FragmentNavigation

Furthermore, Application provides the ability for applications of any type to create, persist, and retrieve cookies, using GetCookie and SetCookie

For more information, see Navigation Overview.

See Also

Concepts

WPF Windows Overview

Navigation Overview

Windows Presentation Foundation Application Resource, Content, and Data Files

Reference

Application

Change History

Date	History	Reason
December 2008	Made a correction in Implementing an Application Definition, first bullet: the existence of x:Class in the markup file causes MSBuild to create a partial class that derives from Application.	Customer feedback.
July 2008	Added section about using a splash screen.	SP1 feature change.