//contract.requires
public void Withdraw(Account account, decimal amount)
{
	Contract.Requires(account != null, "account cannot be null");
	Contract.Requires(amount > 0, "amount must be greater than zero");
	...
}

//contract.ensures
public double CalculateDiscount(int originalAmount, string discountCode)
{
	Contract.Ensures(Contract.Result<double>() >= 0);

	return -32;
}

Reference
http://visualstudiomagazine.com/articles/2010/06/23/code-contracts.aspx

http://msdn.microsoft.com/en-us/library/dd264808(v=vs.110).aspx