WPF-Calculator is a project mainly written in C#, it's free.
Just a rewrite without XAML of the Microsoft sample
Calculator Demo
To get a better understanding of the .NET framework and C#, I refactored out all the XAML from the original sample into pure C#. It is a direct port so uses WPF and follows the same structure of the original program.
Original: ~28megs RAM (16KB exe) No XAML: ~27.5megs RAM (21KB exe)
The original program is copywrite Microsoft so I do not claim to hold the copywrite on this derived work. However, as this project was concieved for educational purposes, I assume to hold the right to distribute it.
Original: http://msdn.microsoft.com/en-us/library/ms771362.aspx