Hi Vouksh. I'm not a dev for the project; however, I will offer my thoughts on this.
In the grand scheme of things, I think right now the answer would be "No". I can say that with some certainty as the code for RC is really rapidly changing as things are added, others are removed, and bugs are fixed. So unless you want to download every tarball every night, go through your C# code and rework it to fit the next build, I'd say it's gonna be a no.
Now, don't let me shoot your hopes down. If there's a way you can export the C# libraries and update them, or even keep them separate like RC does it would make updating easier. You could just grab the diffs and work them out manually for whatever patch / feature they incorporate. Now, I know that can't exactly happen, but as long as you know where you put stuff, I guess it wouldn't be too difficult, it'd just be time consuming. I don't know if you can even develop something that would update the code in the C# libraries automatically.
So anyway, here's my suggestion:
Email Thomas at roundcube(at)gmail(dot)com and let him know of your interest, and the goals of your side-project. After that, let him mull it over and decide what he wants to do. Maybe he wants to wait for a more stable release before you release the application; maybe he doesn't want you to release the application at all.
Now, that being said, I'd also like to point out that RC is a Web 2.0 app and is made to mimic that of a desktop application like Thunderbird, Eudora, Outlook, Kmail, etc. I personally don't see why those programs wouldn't work for you. If you're developing in C#, you most likely are using Visual Studio on a Windows machine, in which case I ask why not use Thunderbird? It's basically just a mail client, and Lightning is a calendar add-on (if you need it).
I personally think your idea is neat, but not needed right now. We're here to mimic desktop apps, not really create a desktop application. I personally think this would undermine the whole point of the project. But that's my personal opinion. I suggest you seek out Thomas' opinion at the email provided above.