Question

I’m trying to run a window, close it, and then run a second window, in a similar way that seems to work with Windows Forms.

namespace WpfApplication1
{

public partial class App : Application
{
    [STAThread]
    public static void Main()
    {
        Application app = new Application();
        //windowMain.Show();
        app.ShutdownMode = ShutdownMode.OnExplicitShutdown;
        MainWindow windowMain = new MainWindow();
        app.Run(windowMain);
        Window1 window1 = new Window1();
        window1.Show();
        app.Run(window1);
    }
}
}

I’ve set the Build Action in the App.xaml properties from ApplicationDefinition to Page, but the programme throws an exception when window1 is initialised. What am I doing wrong?

Edit: I’ve modified the xaml in App.xaml as suggested by first answer and edited main as suggested by the comment.

<Application x:Class="WpfApplication1.App"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
StartupUri="MainWindow.xaml"
ShutdownMode="OnExplicitShutdown">
<Application.Resources>
</Application.Resources>
</Application>

but I’m still getting the exception.

Answer 1

OK this is what I’ve divined so far. The Solution Builder looks for a Main() function. Why its not a WinMain() function I’m still not a hundred per cent clear on. If there is no Main(), you get an error. You can have more than one Main() as long as the Project properties: “Application” page/tab: property: “StartUp Object” is set to point to one of the main()s. This is done from an automatically created drop down list.

When a “WPF Application” project is created, Visual Studio(VS) create an xaml file called “App.xaml”. This is a class declaration where “App” is derived from the “Application” Class. VS also automatically generates hidden files for an xaml file. It creates a “name.g.i.cs” file, when the xaml file is created. It creates a “name.g.cs” file the first time the project is built after the creation of the xaml file. In this case it creates “App.g.cs” and “App.g.i.cs”. These files are hidden by default. To view them, press the “Show all files” button at the top of the Solution Explorer, they can be found in “\ obj\86\Debug” folder. When you delete an xaml file the “name.g.i.cs” and the “name.g.cs” files remain and are not deleted.

The “App.xaml” file’s “build Action” property is set to “Application Definition” when created by VS. When this property is set to “Application Definition” a Main() function is automatically created in “name.g.i.cs”:

    [System.STAThreadAttribute()]
    [System.Diagnostics.DebuggerNonUserCodeAttribute()]
public static void Main()
{
    WpfApplication8.App app = new WpfApplication8.App();
    app.InitializeComponent();
    app.Run();
}

When this property is set to “Page”, the Main() function is automatically removed by VS. You can create new “Application” derived classes in code or in xaml. I haven’t found a neat way to do it in xaml. There doesn’t seem to be a template for an xaml “Application” derived class. I created a “.cs” code file and then renamed it to an .xaml file. For some reason VS won’t allow you to have more than one xaml “Application” declaration file set to “Application Build”, it doesn’t even give you the option of choosing one in the “Project: Properties: Application”: “Startup Object” property.

As you can see in the hidden Main(), an instance of “App” is instantiated and run. If using your own Main() function: an instance of, the base “Application” class, or an “Application” derived class (whether declared in code or in xaml), can be declared and run. The “Application” class should only be instantiated once and should only be run once. If the “Application” derived class is declared in xaml then a simple application can be run by using the StartUpUri property in the xaml file: StartupUri=”Windowname.xaml”. Alternatively the top level UI programme logic can be placed in a Startup event handler. If “Startup=”Application_Startup” is placed in the “App.xaml” file then an event handler can be written:

private void Application_Startup(object sender, StartupEventArgs e)
{
    MainWindow windowMain = new MainWindow();
    windowMain.ShowDialog();
    Window1 window1 = new Window1();
    window1.ShowDialog();
    Shutdown();
}

You have to use ShowDialog() here, because it blocks until the window is closed. If you used Show() instead, it would show one window, then immediately show the other one and shutdown the application. In this case there’s no need to call the Run() method yourself, that’s done automatically.
The “Application” class instance can be run in code whether its declared in code or in xaml. You can then perform initialisation code prior to calling Run(). This would be placed in the Application_ Startup() event handler using the other way. However, if the “Application.Run” call is ever made in the programme, then no windows should be opened (using show() or ShowDialog()) in Main() or anywhere outside of the Application Class or within events and functions called from those events, called during “Application.Run()”.

The Application class has a ShutdownMode property (Application.ShutdownMode). The default for this is: “OnMainWindowClose”. This property can also be set to “OnLastWindowClose” or “OnExplicitShutdown” in code or in the xaml. You will need to reset this if you don’t want the programme to close down when the MainWindow is closed.

I think for my purposes it is better not to use the Application class at all and just call the windows using Show() and “ShowDialog()”. This way I can use WPF pages but I could also call Windows Forms, or DirectX screens, as long as they are not open at the same time, or have no UI at all, if the programme is running remotely. Is there any reason for not doing it this way?