ESharper is an Excel add-in that lets you write user defined functions and commands using C# interactively in a live Excel session.
v1.7.0 lets you add references to external assemblies for your code. For assemblies in the GAC you can specify a short or a full name. For other assemblies you need to specify the full path. References are separated by new lines:
https://tabsstudio.com/
Making Visual Studio perfect 