As managed DX was discontinued, I'd like to know if XNA can be used in Windows applications using C#. The main purpose is for creating the tools, like stages/objects editors and others.

So, instead create a XNA Game project, create a normal C3 project and insert XNA functionality for draw 3D objects and other functionality.

I have found these:

http://msdn.microsoft.com/en-us/library/bb197956.aspx
http://msdn.microsoft.com/en-us/library/bb203940.aspx

So I think it can be used, but I'd like to be sure before begin a project that can't reach an end for using XNA instead MDX1.1 (that is a bit deprecated).

Yes. This sample shows how.

Here is another Example of using XNA in a Windows Forms Application.

http://www.codeplex.com/XNACommunity/Wiki/View.aspx?title=XNAEditorEnglish&referringTitle=XNAEditor

Just a note about the codeplex example. It states that the Winform Series one sample (that i posted a link to) does not allow you to use the xna content manager. This is incorrect. The GraphicsDeviceControl has a ServiceContainer member called Services, so its a simple matter to instantiate a content manager.

// somewhere on GraphicsDeviceControl or a subclass
ContentManager = new ContentManager(Services, "Content");

OK