If you build .NET applications you can easily choose between 32 bit and 64 bit compilation modes, or even better you can have the application auto-detect which OS it's running under and use 64 bit or 32 bit depending on the OS bitness. .NET makes it easy to either create pure 64 bit applications or a mixed mode application that can run either in 64 bit or 32 bit mode depending on the OS Bitness that launched it. Most of the time I'd recommend going with the Any CPU build target which provides this auto-detect operation of the final executable.