How can I copy a section of one WriteableBitmap to another WriteableBitmap

I need to copy a section (x,y,width,height) from one writeablebitmap to a section (same size) of another writeablebitmap.

In fact, the x,y,width,height are identical of the FROM writeablebitmap and the TO writeablebitmap.

How would I do this in C# (WPF)?

Thanks!