I'd consider myself pretty well versed in C#. It's my language of choice at the moment, and it's where basically all my professional experience lies.

Still, I'm puzzled by the existence of the MonoDroid project. My understanding has always been that C# and Java are very close. Like, if you know one, you can learn the other really quickly. So, as I've considered developing my first Android app, I just assumed I would familiarize myself with Java enough to get started and then just sort of learn as I go.

Wouldn't this make more sense than using MonoDroid, which is likely to be less feature-rich than the Java Android SDK, and requires learning its own API (albeit a .NET API) anyway? I just feel like it would be better to learn a new language (and an extremely popular one at that) and get some experience in it—when it's so close to what you already know anyway—rather than stick with a technology you're experienced with, without gaining any more valuable skills.

Maybe I'm grossly misrepresenting the average potential MonoDroid user. Maybe it's more for people who are experienced in Java and .NET and just prefer .NET. Or maybe (in fact it's likely) there are other factors I just haven't considered. I'm just wondering, why would you use MonoDroid instead of just developing for Android using Java?

Any competent C# programmer should be able to quickly pick up enough Java to write an Android program, but that's not the point. It's a matter of code reuse.

Think about six months from now, when your Android program is popular and your users are asking for a version for iPhone and Windows Phone 7. If you had used MonoDroid, you can reuse most of your application logic with MonoTouch (Mono for iOS) and the Windows Phone SDK. Now they want a web-based version, so you include the same class libraries in an ASP.Net project. Desktop versions? No problem, that same class library works with .Net under Windows or Mono on Linux and OS X.

Other than C or C++, I can't think of any other languages that would let you reuse the same code on all of those targets.

Edit to address some concerns in the comments: .Net and Mono will not let you write a complete program and use that same program everywhere. They will let you share some code, and like all cross-platform programming the amount of shared code depends on the type of programs you're writing and how well you separate the UI and hardware code from the application logic.

However, if you write your Android app in Java, how much of that is reusable on iOS or Windows Phone? That's the point I was trying to make. I had existing C# libraries that were working on Mono for Android in much less time than it would have taken to reimplement them, even though I already knew Java. I have some code that is shared--unmodified--between a web site, desktop programs, and mobile apps on two different mobile platforms, thanks to Mono.

I didn't mean to imply, even indirectly, that Mono was the perfect tool for every mobile development situation. It's a tradeoff, but I firmly believe that there are situations when Mono is a much better choice.

Please see (and upvote!) Jason S's answer for another perspective.
