Boo has some support for Design by Contract with the Required attribute.

def doit([required(x >= 0)] x as int):
	print x

doit(-1) //raises a System.ArgumentException

Since boo has an extensible compiler architecture (unlike for example C# or VB.NET), it is possible to eventually add more complete Design By Contract functionality to boo, by creating custom compiler macros, attributes, and pipelines.

Here are some other custom languages for .NET and Mono that include DbC features that we might check out: