<p>I got a quick question about running a directX application (C++) in a managed environment. I'm attempting to write a MDI tool (in C#) where the background of the parent window would be an embedded render window of directX (C++). </p>

<p>I've read ways that involved writing the C++ into a native dll, however it would be prefered to be able to have the C++ projects included within the solution (I dont even know if that's possible though). Eitherway, if you know of some helpful steps, hints, or if this is a bad idea in general, please let me know. Thanks!</p>

First of all writing the C++ part in a different dll file doesn't mean that it couldn't be at the same solution as the C# project.
In order to use native DX to render on a managed window you need to pass the HWND window id (use form.WindowId.ToInt32) to the C++ D3Ddevice c'tor. after that each time you'll render using that device it would render on the .NET window.
To do this you probably need two saparate projects - a C++ dll & .NET project. use COM wrapper or p-invoke to pass the HWND to the C++ dll.