后置条件

public double GetPaymentsTotal(string name)
{     
    Contract.Ensures(Contract.Result<double>() >= 0);
 
    double total = 0.0;
 
    foreach (var payment in this._payments) {
        if (string.Equals(payment.Name, name)) {
            total += payment.Amount;
        }
    }
 
    return total;
}