SDK development in multiple programming languages

I am working to develop an SDK that will allow the users to use it and write code in C# or Java.

To achieve this most optimally, do I have to write the SDK in both languages? That would be a lot of code repetitions albeit in 2 different languages, and the maintenance overhead would be very high.

Are there efficient ways to solve this problem? What will happen if someday we think of adding a 3rd programming language support for the SDK?

Please note that I don't mind having an API instead of SDK. The idea is simple -- it's basically an interface or kit that would provide users to make simple calls to it to perform certain complex actions, all complexity being abstracted into the API or SDK.

Solution

To avoid code duplication, there are different options.

you can create the base library in portable C++. C++ can then be interfaced in both Java (e.g. with JNI) and C# (e.g. with C++/CLI). That means that you can create a wrapper for both target languages using C++. This option is more difficult if you want to target more different operating systems.
you can create the software as a service (e.g. SOAP or a simple json service). You can do that in the language you prefer and then create client applications in Java and C#.

It depends on your project, the people who will implement it and on your future goals what the best solution is.