C# - Grey out window

Question

Hi all,

When my WPF application starts up, an immediate second window is shown to fill in login credentials.
What I would like to do now, is  whenever this "login window" is active, I want to grey out and de-activate the first 'general' window (mainwindow). I also want it to always remain behind the login window (as if the mainwindow was locked until proper credentials were given).

Can anyone help me regarding this 'issue'?

Thanks in advance!

Kind regards.