Usually I do this one of two ways:
If the login window is a one-time thing required before starting the application, I will put it in the OnStartup() method of the Application object
protected override void OnStartup(StartupEventArgs e)
{
base.OnStartup(e);
// Login
var login = new LoginDialog();
var loginVm = new LoginViewModel();
login.DataContext = loginVm;
login.ShowDialog();
if (!login.DialogResult.GetValueOrDefault())
{
// Error is handled in login class, not here
Environment.Exit(0);
}
// If login is successful, show main application
var app = new ShellView();
var appModel = new ShellViewModel();
app.DataContext = viewModel;
app.Show();
}
The other way I usually do this is through a ShellViewModel or ApplicationViewModel which handles all my window management. This method uses DataTemplates to define each screen, and uses a ContentControl as a placeholder for the current screen in the ShellView or ApplicationView.
I usually combine this with an Event System of some kind, like Microsoft Prism's EventAggregator, so it can listen for messages of a specific type, such as OpenWindow or CloseWindow messages. If you're interested, I have a blog post about Communication between ViewModels that should give you a better idea of what an event system looks like.
For example, my ShellViewModel might start by displaying a LoginViewModel (a DataTemplate is used to tell WPF to draw the LoginViewModel with the LoginView), and it would subscribe to receive messages of type SuccessfulLogin. Once the LoginViewModel broadcasts a SuccessfulLogin message, the ShellViewModel will close the LoginViewModel and replace it with the ApplicationViewModel. You can see an example of this in my article on Navigation with MVVM
