Cannot show up WPF application when setting MainWindow manually and composing application (MEF)

I got my hands om MEF for a week now and I am trying to build up a WPF application that loads imported controls from MEF.

I created a WPF application project and removed the default window and application start up URI. Then I handled the application startup event to compose the application:

public partial class App : Application, IPartImportsSatisfiedNotification
{
    {...}

    private void App_Startup(object sender, StartupEventArgs e)
    {
        this.Compose();
    }

    public void Compose()
    {
        try
        {
            globalCatalog.Catalogs.Add(new DirectoryCatalog(extensionsDirectoryPath));
            CompositionContainer container = new CompositionContainer(globalCatalog);
            container.ComposeParts(this);
        }
        catch (Exception ex)
        {
            // Do something
        }
    }

    {...}
}

Actually, when debugging and watching objects after imports are satisfied, everything has hierarchically composed fine like I wanted. But when I try to show up the MainWindow of the application an exception is thrown on MainWindow.Show() call:

"Specified element is already the logical child of another element. Disconnect it first."

Though my code in OnImportsSatisfied method seems fine as it is working when not using MEF mecanism:

public void OnImportsSatisfied()
{
    Window mainWindow = new Window();
    mainWindow.Content = this.importedControl;
    this.MainWindow = mainWindow;
    this.MainWindow.Show();
}

I insist on the fact that this works perfectly when not importing controls with MEF. What is surprising is that this code does not work too:

Window mainWindow = new Window();
//mainWindow.Content = this.importedControl;
this.MainWindow = mainWindow;
this.MainWindow.Show();

So I suspect that ComposeP开发者_Python百科arts is doing a bit more than what it says as it is the only member acting on my actual application instance.

Hope someone can help me (Glenn?). Thanks.

Edit:

I discovered that when I remove the IPartImportsSatisfiedNotification interface from my parts, no exception is thrown and the window shows up. But of course the window is empty as I need this OnImportsSatisfied method to set the DataContext of the window to its associated imported view model.

The sample applications of the WPF Application Framework (WAF) show how to use MEF within a WPF application.

I finally discovered that I was importing my WPF user controls by using the default ImportAttribute constructor, which in fact will make a shared instance of the class if the creation policy is not specified during export. And as many of my controls were implementing the same interface and I was binding them in my views, I was actually trying to add this shared user control instance to different visual elements, which is not permited by WPF (and so the exception).

I marked my imports using the RequiredCreationPolicy set to NonShared and everything got back in order! That was all about learning MEF...