Excel UDF library in C# from scratch

To make it short I'm an IT trainer specialized in the .Net ecosystem (framework, C#, WPF, Excel addins...).
(I'm available in France and bordering countries, and I only teach in french.)

I like to learn new things, particularly to understand what happens under the hood, and I do my best to share my humble knowledge with others by direct teaching, by posting articles on my blog (pragmateek.com), or by answering questions on forums.