System.Diagnostics.Contracts is included within the .Net Framework. To use Code Contracts you must install the Visual Studio extensions.
Extensions and Updates search for
Code Contracts then install the
Code Contracts Tools
After the tools are installed you must enable
Code Contracts within your Project solution. At the minimum you probably want to enable the
Static Checking (check after build). If you are implementing a library that will be used by other solutions you may want to consider also enabling