c# implicit explicit關鍵字(隱式和顯式數據類型轉換)

implicit關鍵字用於聲明隱式的用戶定義類型轉換運算符。(explicit反之)explicit則用於顯示轉換用戶自定義類型。
static implicit operator target_type ( source_type identifier ){......}
隱式轉換能夠經過消除沒必要要的類型轉換來提升源代碼的可讀性。可是，由於能夠在未指定的狀況下發生隱式轉換，所以必須注意防止使人不愉快的後果。

通常狀況下，隱式轉換運算符應當從不引起異常而且從不丟失信息，以即可以在不知曉的狀況下安全使用它們。若是轉換運算符不能知足那些條件，則應將其標記爲 explicit 做爲顯示轉換數據。 

下邊是在網上找的幾個簡單例子

例1

class Digit
    {
        public Digit(double d) { val = d; }
        public double val;
   
        // User-defined conversion from Digit to double
        public static implicit operator double(Digit d)
        {
            return d.val;
        }
        //  User-defined conversion from double to Digit
        public static implicit operator Digit(double d)
        {
            return new Digit(d);
        }
    }
    class Program
    {
        static void Main(string[] args)
        {
            Digit dig = new Digit(7);
            //This call invokes the implicit "double" operator
            double num = dig;
            //This call invokes the implicit "Digit" operator
            Digit dig2 = 12;
            Console.WriteLine("num = {0} dig2 = {1}", num, dig2.val);
            Console.ReadLine();
        }
    }

例2

//基本數據類型到用戶自定義類型 
class Distance
{
    private int feet;
    private double inches;

    //默認構造函數 
    public Distance()
    {
        feet = 0;
        inches = 0.0;
    }

    //帶有單參數的構造函數              
    public Distance(double metres)
    {
        double f;
        f = 3.28 * metres;
        this.feet = (int)f;
        this.inches = 12 * (f - feet);
    }

    //由一個double隱式構造一個Distance 
    public static implicit operator Distance(double metres)
    {
        return new Distance(metres);
    }

    //由一個Distance顯式返回一個double 
    public static explicit operator double(Distance d)
    {
        double metres;
        metres = d.inches / 12 + (double)d.feet;
        return (metres / 3.28);
    }

    public override string ToString()
    {
        return String.Format("{0}英尺{1}英寸 ", this.feet, this.inches);
    }

}

class DistanceDemo
{
    public static void Main()
    {
        Distance d1 = 1.25;
        Console.WriteLine(d1);

        double d = (double)d1;
        Console.WriteLine(d);
    }
}

例3

using System;

namespace Hunts.Keywords
{
    // 定義一我的民幣結構。數據類型轉換的語法對於結構和類是同樣的
    public struct RMB
    {
        // 注意:這些數的範圍可能不能知足實際中的使用
        public uint Yuan;
        public uint Jiao;
        public uint Fen;
        public RMB(uint yuan, uint jiao, uint fen)
        {
            if (fen > 9)
            {
                jiao += fen / 10;
                fen = fen % 10;
            }
            if (jiao > 9)
            {
                yuan += jiao / 10;
                jiao = jiao % 10;
            }
            this.Yuan = yuan;
            this.Jiao = jiao;
            this.Fen = fen;
        }

        public override string ToString()
        {
            return string.Format("￥{0}元{1}角{2}分", Yuan, Jiao, Fen);
        }

        // 一些操做
        public static RMB operator +(RMB rmb1, RMB rmb2)
        {
            return new RMB(rmb1.Yuan + rmb2.Yuan, rmb1.Jiao + rmb2.Jiao, rmb1.Fen + rmb2.Fen);
        }

        public static implicit operator float(RMB rmb)
        {
            return rmb.Yuan + (rmb.Jiao / 10.0f) + (rmb.Fen / 100.00f);
        }

        public static explicit operator RMB(float f)
        {
            uint yuan = (uint)f;
            uint jiao = (uint)((f - yuan) * 10);
            uint fen = (uint)(((f - yuan) * 100) % 10);
            return new RMB(yuan, jiao, fen);
        }

        // more
    }

    class App
    {
        static void Main()
        {
            RMB r1, r2, r3, r4;
            // 記得小學時的某次捐款，我把口袋裏藏好的一塊錢加6張一毛錢以及13個一分錢的硬幣都貢獻出去了：（
            r1 = new RMB(1, 6, 13);
            // 其實當時其餘人都已經交過了，他們總共交了：
            r2 = new RMB(46, 9, 3);
            // 那麼加上個人就是：
            r3 = r1 + r2;
            Console.WriteLine("r3 = {0}", r3.ToString());
            // 隱式轉換
            float f = r3;
            Console.WriteLine("float f= {0}", f);
            // 顯式轉換
            r4 = (RMB)f;
            Console.WriteLine("r4 = {0}", r4.ToString());
            //若是不進行顯示轉換,將出現錯誤 CS0266: 沒法將類型「float」隱式轉換爲「Hunts.Keywords.RMB」。存在一個顯式轉換(是否缺乏強制轉換?)
            Console.Read();
        }
    }
}

出處：http://www.cnblogs.com/chengxiaohui/articles/1914190.html