[.net]瞭解implicit operator隱性轉型
參考資料中的作者寫得很清楚了，所以我就直接做簡單的翻譯：
如果有一個類別叫做Currency(現金)，他的member是符號(string,例如$)以及數字(decimal代表金額)，傳統的方式要new的話，大概一般都是這麼寫的：
Currency cur = new Currency(100.50, "$");
於是乎微軟就想出了一些又酷又炫（個人是覺得不酷不炫，只是搞的很難懂拉...）的寫法，叫做implicit operator，在new的時候，可以像是下面這樣：
Currency cur = 100; //Currency directly accepts value
//Instantiating currency using currency sign.
Currency cur1 = "€";
cur1.Value = 100; //Implicit typecasting from number to Currency
//new之後，要再修改符號或是金額也都可以直接改
cur1 = "☺";
string signSmile = cur1;
cur1 = 777;
decimal amt = cur1;
//What about this
long lCur = cur; //Implicit typecasting from Currency to long
decimal dCur = cur; //Implicit type casting from Currency to decimal
上面的寫法就是可以將一個Currency物件以符號或是金額的方式直接new
而且還可以把new出來的變數直接帶入到其他的符號或是金額
Currency Class內容如下，其中寫法的技巧可以直接看註解：
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
namespace TestMongoDBWebForm
{
using System;
using System.Globalization;
///
/// Summary description for Currency. ///
public class Currency { decimal _value; string _sign; public Currency(decimal value, string sign) { _value = value; _sign = sign; } ///
/// Gets or sets the currency value. ///
public decimal Value { get { return _value; } set { _value = value; } } ///
/// Gets or sets the Currency sign. ///
public string Sign { get { return _sign; } set { _sign = value; } } public override string ToString() { return string.Format("{0}{1}", Sign, Value); } ///
/// Creates Currency object from string supplied as currency sign. /// New物件的時候，可以直接用string帶入，例如Currency cur1 = "☏" /// 或是重新設定物件的符號時,直接把符號assign給物件也可以，cur1 = "☏" ///
public static implicit operator Currency(string rhs) { Currency c = new Currency(0, rhs); //Internally call Currency constructor return c; } ///
/// Creates a currency object from decimal value. ///
public static implicit operator Currency(decimal rhs) { //NumberFormatInfo.CurrentInfo.CurrencySymbol是當地國家預設的金錢符號 Currency c = new Currency(rhs, NumberFormatInfo.CurrentInfo.CurrencySymbol); //Internally call Currency constructor return c; } ///
/// Creates a decimal value from Currency object, used to assign currency to decimal. /// 這個就是可以把Currency物件assign給一個decimal,例如：decimal amt = cur1; ///
public static implicit operator decimal(Currency rhs) { return rhs.Value; } ///
/// Creates a long value from Currency object, used to assign currency to long. ///
public static implicit operator long(Currency rhs) { return (long)rhs.Value; } ///
/// 這個是自己加的 /// 這個就是可以把Currency物件assign給一個string,例如：string mySymbol = cur1; ///
public static implicit operator string(Currency rhs) { return (string)rhs.Sign; } } }
個人覺得implicit operator是為了讓寫法更簡潔拉，如果把這個技巧應用在類別庫，那麼caller可以快速的將簡單類別(ex:string,long,decimal...)直接assign給一個物件
而且物件自動就會知道這個簡單類別的value要放在自己內部的哪一個欄位。
參考文章：
Understanding Implicit Operator Overloading in C#
http://www.codeproject.com/Articles/15191/Understanding-Implicit-Operator-Overloading-in-C
