Open a Single Instance of Window in WPF
We often need to limit some Windows in a system to allow only one open each time. If you want to open another instance of the Window, you must first close the opened one; on the other hand, if the opened Window instance is not closed, you cannot open a new instance of the same Window.
Singleton vs Static Class
Singleton pattern, is actually a design pattern that was from Microsoft patterns & practices. And, when we say Singleton, we should know it is a class. About more details, we can read msdn.