Is it so difficult to look at the MSDN documentation page on a class and find the method
The problem is: why MDI? How much longer people are going to get into it? Who needs MDI, ever? Why torturing yourself and scaring off your users?
Do yourself a great favor: do not use MDI at all. You can do much easier to implement design without it, with much better quality. MDI is highly discouraged even by Microsoft, in fact, Microsoft dropped it out of WPF and will hardly support it. More importantly, you will scare off all your users if you use MDI. Just don't. I can explain what to do instead.
Question on using MDI windows in WPF
MDIContainer giving error
How to Create MDI Parent Window in WPF?