System.Contract.Requires C# (CSharp) Method

        public static void Requires(bool condition)
        {
            if (!condition) {
                throw new ArgumentException();
            }
        }

 public override TResult Switch <TResult>(Func <TLeft, TResult> left, Func <TRight, TResult> right)
 {
     Contract.Requires(left != null);
     Contract.Requires(right != null);
     return(default(TResult));
 }