# C# 3.0 初體驗：Extension Method

昨天寫了一篇如何取得兩個 DateTime 之間的方法，今天將昨天的東西寫成一組 Extension Method，感覺十分的好用，分享出來給大家參考！

我基本上是參照在 SQL Server 中的 [DATEDIFF 函數](http://technet.microsoft.com/zh-tw/library/ms189794.aspx "DATEDIFF 函數")的用法，讓熟悉 SQL 的人很容易的就能夠上手使用。

\[code:c#\]

using System;

namespace ExtensionMethods  
{  
    static class DateTimeExtensionMethod  
    {  
        public static double DateDiff(this DateTime startdate, string datepart, DateTime enddate)  
        {  
            // 參考 SQL Server 2005 線上叢書：[http://technet.microsoft.com/zh-tw/library/ms189794.aspx](http://technet.microsoft.com/zh-tw/library/ms189794.aspx)

            double result = 0;

            TimeSpan tsDiff = new TimeSpan(enddate.Ticks - startdate.Ticks);

            switch (datepart.ToLower())  
            {  
                case "year":  
                case "yyyy":  
                case "yy":  
                    result = enddate.Year - startdate.Year;  
                    break;

                case "quarter":  
                case "qq":  
                case "q":  
                    // 每一季的平均天數  
                    const double AvgQuarterDays = 365 / 4;  
                    result = Math.Floor(tsDiff.TotalDays / AvgQuarterDays);  
                    break;

                case "month":  
                case "mm":  
                case "m":  
                    // 每一個月的平均天數  
                    const double AvgMonthDays = 365 / 12;  
                    result = Math.Floor(tsDiff.TotalDays / AvgMonthDays);  
                    break;

                case "day":  
                case "dd":  
                case "d":  
                    result = tsDiff.TotalDays;  
                    break;

                case "week":  
                case "wk":  
                case "ww":  
                    result = Math.Floor(tsDiff.TotalDays / 7);  
                    break;

                case "hour":  
                case "hh":  
                    result = tsDiff.TotalHours;  
                    break;

                case "minute":  
                case "mi":  
                case "n":  
                    result = tsDiff.TotalMinutes;  
                    break;

                case "second":  
                case "ss":  
                case "s":  
                    result = tsDiff.TotalSeconds;  
                    break;

                case "millisecond":  
                case "ms":  
                    result = tsDiff.TotalMilliseconds;  
                    break;

                default:  
                    throw new ArgumentException("You input a invalid datepart parameter.");  
            }

            return result;  
        }  
    }  
}  
\[/code\]

至於使用上有 2 點要注意：

1.  必須在 .NET Framework 3.5 的環境下才能執行  
    
2.  必須先引用 ExtensionMethods 命名空間  
    

範例如下：

\[code:c#\]

using System;

**using ExtensionMethods;**

namespace ConsoleApplication1  
{  
    class Program  
    {  
        static void Main(string\[\] args)  
        {

            DateTime date1 = DateTime.Now.AddHours(0);

            DateTime date2 = DateTime.Now.AddHours(24).AddMonths(3);

            double DiffDays = date1.**DateDiff("day", date2);**

            Console.Write(DiffDays);

            Console.ReadKey();  
        }  
    }  
}  
\[/code\]