Btukfyl

i'm new to MVVM and IoC. I use ViewModel-first approach for this project. My project assemblies organization look like this:

View (has reference to ViewModel)
ViewModel (has reference to Model)
Model

Worked fine at the beginning, until i need a Login view. Login run on program startup. On successful login MainWindow will be shown and Login has to close itself. The reverse can be done with LogoutCommand from MainWindow.

The main obstacle i'm facing is instantiating a View (Window) for a given ViewModel type, while ViewModel has no knowledge of its View. On search for the answer, i've found:

1. Mediator/Messenger pattern. Which i found very convenient, i used this pattern for showing dialog MessageBox. But only for that purpose, i'm not comfortable using this pattern for handling object instantiation.


2. Dependency Injection. A new concept to me. But i can see a real benefit of decoupling service constructor knowledge out of consumer class. Hence i tried MVVM-Light SimpleIoc for starter by injecting View instance to ViewModel constructor as interface and proceed to register them on ViewModelLocator

SimpleIoc.Default.Register<IMainWindowView, MainWindow>();  

SimpleIoc.Default.Register<ILoginView, Login>();  

SimpleIoc.Default.Register<LoginVM>();  

SimpleIoc.Default.Register<MainWindowVM>();  

On successful login, the MainWindow shown and Login closed. But when i logout of MainWindow to show Login again, i encountered InvalidOperationException Cannot set Visibility or call Show, ShowDialog, or WindowInteropHelper.EnsureHandle after a Window has closed. So i thought i need to "clean up" the instance created by SimpleIoC by unregistering it on Window Closing event.

public class ViewModelBaseEx : ViewModelBase

{

    protected SimpleIoc Ioc

    {

        get { return SimpleIoc.Default; }

    }



    protected virtual void Unregister<T>() where T : class

    {

        Cleanup();



        if (Ioc.IsRegistered<T>())

        {

            // Attempt to remove instance

            Ioc.Unregister<T>(); 



            // If i still need to show this window again, 

            // shouldn't i need to have this instance available? (not sure)

            Ioc.Register<T>();   

        }                

    }        

}

Alas, the above workaround didn't work and now i don't know what to do.

1. What are the merit/demerit of my approach?


2. How to handle dynamic instance with IoC container?

UPDATE

Register after unregister seems necessary as the type cannot be found if left only unregistered. As for the problem, it seems i also need to unregister the View type too. So i add a bit of code-behind in the View (Window)

private void MainWindow_Closed(object sender, EventArgs e)

{

    SimpleIoc.Default.Unregister<IMainWindowView>();

    SimpleIoc.Default.Register<IMainWindowView, MainWindow>();

}

Works fine so far, is this approach acceptable?