Can WPF and WinForms be mixed within an application?
It is fairly straightforward to host WPF controls in a WinForms app with an ElementHost adapter or WinForms controls in a WPF app with a WindowsFormsHost adapter. There are not too many resources on the web showing how to do either of these, however. In the process of learning how to do this for myself I quickly discovered the inherent symmetries between the two pathways. I distilled all my notes into an article comparing and contrasting these symmetries using a unique approach: the article is really two side-by-side articles, comparing every step in detail, starting from creating a user control in one technology to hosting it in an application in the "opposite" technology. My article, published on SimpleTalk.com in August 2010 is available here: Mixing WPF and WinForms.
For completeness, here are a couple good MSDN references, one for each pathway. In fact, the demo solution accompanying my article started from both of these:
I believe there is a WindowsFormsHost control you can put in your WPF apps which will do interop back to WinForms code:
We hosted significantly complex WPF controls in an existing LOB WinForms app. It can be done, but we did have issues (some no doubt caused by the steep learning curve). These primarily had to do with loss-of-focus events not being fired when expected, and also keyboard navigation issues.