A new way to perform parameter validation in C# 3.0

首先看用法:    class Program
    {
        static void Main(string[] args)
        {
            string param = null;
            try
            {
                DoSomething(param);
            }
            catch (ContractException ex)
            {
                Console.WriteLine("Exception: {0}", ex.Message);
            }
            Console.ReadKey();
        }

        private static void DoSomething(string param)
        {
            Contract.Requires(x => param != null);
            Console.WriteLine("after validating parameters..");
        }
    }

運行結果是:Exception: Contract broken: x => (value(CSharpByContract.Program+<>c__DisplayCla
ss0).param != null)

加粗的部分顯示了參數驗證失敗的原因.需要注意的是,我們在驗證時並不用手寫"param!=null"這樣的字串,節省了時間,也減少了不一致.

代碼:    public static class Contract
    {
        /**//// <summary>
        /// lazy programer's assert that gives a meaningful contract broken message
        /// </summary>
        /// <param name="condition">a bool expression to assert to be true</param>
        public static void Requires(Expression<Func<object, bool>> condition)
        {
            var x = condition.Compile();
            if (!x(null))
                throw new ContractException(condition.ToString());
        }
    }
    public class ContractException : ApplicationException
    {
        public ContractException(string message)
            : base(string.Format("Contract broken: {0}", message))
        {
        }
    }