You can write addin for Visual Studio on it addins model directly: http://www.c-sharpcorner.com/UploadFile/mgold/AddIns11292005015631AM/AddIns.aspx
This requires some hard work as the extension model is not very intuitive.
You can also do this with DXCore: http://blogs.microsoft.co.il/blogs/dhelper/archive/2009/06/17/how-to-write-a-simple-dxcore-plug-in.aspx
This is a bit simpler and the plugin module is free.
The last option is to write plugins using Resharper, I have never done this, but it has plugins between the Style-Cop plugin.
I had experience with both of the first options. Firstly, the model of VS-extensions, it is very difficult to work with it. This is an old API, coming from previous versions of VS, based on classic COM and skipping a lot of documentation. The second option, the DXCore plugins, which is much more intuitive, has a reacher API, but it also lacks documentation. Another problem is that it requires installation of ~ 30 MB. Good luck :)
Elisha
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