Thanks for pointing out the libraries and stuff and I gave it already few tries. But then suddenly had to move and was sick until now. No excuse.
Just facts. Sometimes you have to deal with surprises in a foreign country. 
So now I’d like to know one thing more before I going on to write such a tool.
I could figure out at least three ways to integrate a control into Renoise using C#. All having different advantages and disadvantages.
So I will continue on one of this paths. 
Thanks for pointing that out.
But what I couldn’t figure out:
How do I play two notes at once using the approach given you ?
Do I need to set the rules twice for each note I want to play?
I think the better approach would be to play to notes in one. If you know what I mean.
I can understand if you’re comfortable with something else - but if you really want to take control of Renoise -_do more than what can be done by handing over MIDI message -_it’s going to be through the Lua API in one way or the other.
I think it’s important to point out that the Lua API isn’t necessarily “either or”. You could come up with some kind of hybrid approach too - for example, putting together a sound-prism-alike thing in C#, with logic/UI and everything, and then just use the Lua API to trigger notes*. Obviously, a side benefit of this would be that the software you wrote would not necessarily be bound to Renoise.
- For such remote control of Renoise we are spoiled with choices: there’s this very slick libraryand similarly, the lua HTTP library that comes bundled with Renoise.
And then we’ve got the internal OSC server in Renoise. Of special interest, how OSC can be used to evaluate snippets of lua code on the fly.