How to open Unity3d API in an external editor

I wanted to work with scripts to use my favorite program SharpDevelop. In the settings, I set an external editor to SharpDevelop. Code file is loaded fine, but the project will not load Unity3d Api. That means I can not use anything that is part of the Unity engine (eg. Vector3 type, Rigidbody class ,gameobjekt class).

What can I do to make the project added Unity3d Api