As a C++ developer I'm quite used to C++ header files, and find it beneficial to have some kind of forced "documentation" inside the code. I usually have a bad time when I have to read some C# code because of that: I don't have that sort of mental map of the class I'm working with.

Let's assume that as a software engineer I'm designing a program's framework. Would it be too crazy to define every class as an abstract unimplemented class, similarly to what we would do with C++ headers, and let developers implement it?

I'm guessing there may be some reasons why someone could find this to be a terrible solution but I'm not sure why. What would one have to consider for a solution like this?