How to implement input parameter sorted precondition enforcement?
A bit of code would help us all with helping you.
I'll assume you have a method along the lines of this..
public void MyMethod(params string[] list) {...}
and you want to use Code Contracts to ensure that this method will only ever be called with a sorted list. Did you try something like the following for the body?
Contract.Requires(list.OrderBy(s => s).SequenceEquals(list));