While I would not be certain without asking the developer, I would suspect that C# was chosen because Paint.NET was originally intended on being a simple replacement for Microsoft's Paint program. C# is very easy to use to write programs using GUI's and, using the .NET libraries, C# can do a lot of things very easily. Because the program was going to be extremely simple and the creation of an GUI (which is important for image editing) was so easy, C# was a good choice.
However, Paint.NET grew into a larger and more complex program. I haven't used it, so I am not sure how complex though. Usually in large programs or programs that require a lot of processing (some image filters in photoshop can use a lot of CPU power while running) you can get a little better performance using a language such as C++. Also, C# and .NET languages sometimes use more memory than an equivalent C++ program. (This is not to say C# performs poorly, though) However, standard C++ libraries do not include a way to create a GUI. You can use MS's API's to create windows and GUI's in C++, but those libraries are not easy to use. So, as more features were added to the original/simple version of Paint.NET, it became more complex. However, as the program was already written in C#, converting the program to another programming language would be a complex task and the benefits would probably not be worth it because most modern computers can make up for any performance or memory usage differences.
The only other problem I can easily see in using a .NET language, is you must use an operating system capable of using the .NET version your program uses. However, this isn't really a problem, as long as you know it while your are planning/designing the projects. Then you can just list it as a system requirement and not a problem in program portability. Why was Paint.net coded in C# and why was it a good choice?
I think they mentioned it was a college course project. C# is easier than C++ and similar (there is more C++ graphic handling source code than for any other language), and the .NET framework has a lot of functionality - it took less time to develop it.
C# was developed as a programming platform for .NET applications. It was a good choice because it was planned to work seamlessly with the .NET environment.
Which company does .net come from?
Which company does C# come from?
