You will have to separate your code into separate projects (in the same solution). Usually you would have the following setup:

-Engine Core       (DLL)
-Game + Game Logic (EXE)
-Content Pipeline  (DLL)
-Editor            (EXE)

For even larger projects you might want to separate the Engine Core over multiple projects like Core, Physics, Graphics, Audio, Input and Networking. All these projects can't reference each other, but they all reference Core where you can store some common base types.

If they would reference each other the separation would be useless.

Anyway this should help you reuse parts of your engine in other projects. For example for the editor you would need to reference the Graphics project which references Core itself.

This separation also means that you can now use Windows Only technologies, like WPF, in your editor while your game can still target Windows and XBLIG.

For more info on using XNA inside a WPF environment see this link: http://blogs.msdn.com/b/nicgrave/archive/2010/07/25/rendering-with-xna-framework-4-0-inside-of-a-wpf-application.aspx

You will have to separate your code into separate projects (in the same solution). Usually you would have the following setup:

-Engine Core       (DLL)
-Game + Game Logic (EXE)
-Content Pipeline  (DLL)
-Editor            (EXE)

For even larger projects you might want to separate the Engine Core over multiple projects like Core, Physics, Graphics, Audio, Input and Networking. All these projects can't reference each other, but they all reference Core where you can store some common base types.

If they would reference each other the separation would be useless.

Anyway this should help you reuse parts of your engine in other projects. For example for the editor you would need to reference the Graphics project which references Core itself.

This separation also means that you can now use Windows Only technologies, like WPF, in your editor while your game can still target Windows and XBLIG.

For more info on using XNA inside a WPF environment see this link.