I'm not to familiar with VB.net and C#.net yet( C++ for life!
). But, even with those languages in thier "Express editions", you would still need find a way to create your own DLL string resource table to allow DarkBasic Pro to know what commands your DLL has. As far as I remmember, the Express editions doesn't allow you to create a string resource like the Standard and higher versions do. I remmember seeing third party tools offered here on the forums, way back when, that would allow you to edit the dll's resource table directly to enter your own strings. Dunno what search terms to use to search for them on google though...
I would suggest if you know C procedural or C++ object orientated programming, try creating a simple dll for dbpro in C/C++ first. Go into the Dark Basic Professional Help File -> Technical Documents -> Third Party Commands( That's what I learned from ). If you can successfully create a dll for dbpro that way, then it should be trivial enough to find "like ways" with the other vb and C# langauges to create a dll for dbpro.
Again, basically my main concern here is with the express editions which wont allow you to create string resource tables. Unless something changed lately...