What’s the problem?

One of our colleagues, Petri, had a problem with using extension methods in his Visual Web Parts user control (.ascx file). He had developed some extension methods and tried to use them in user control:

And all he got was this error:

…

There are several solutions in web that deal with this problem, but they all seem to point you to use SPWebConfigModification (like in here and here) or are just giving manual editing instructions (like in here). We feel that this is all too complex and repetitive process. There is fortunately “a better” way to…