Design by Contract in C#

C# does in fact support 'Design by Contract' in the form of 'Code Contracts' though you need to install a Visual Studio extension to use it  Check out these links:

https://msdn.microsoft.com/en-us/library/dd264808%28v=vs.110%29.aspx

https://visualstudiogallery.msdn.microsoft.com/1ec7db13-3363-46c9-851f-1ce455f66970

Note that this can't be used with the Express versions.

As keywwords such as 'requires' and 'ensures' are not actually integrated into the C# language you have to use static library methods instead.

This means that the implementation is less than ideal and I don't think it is used much in practice.