System.Windows.Point can't be found

[Miralkong]

Hi, I'm practicing the basic of AutoCAD develop. And based on the ".NET developer's Guide", you could set a point for the main window's top left corner using the following code:

But when I try this code in my VS2017, it shows the followings:

I've already referenced the "PresentationCore" to the project. What did I miss?

[Miralkong]

Problem solved. Only need to refer the WindowsBase to the project.