C# 中的委托

C#特有的属性“委托”的本质就是像C/C++中的函数指针，回调函数那样。不过对于面对对象的C#语言来说委托的函数不是单独存在的，而是和其类实例化的对象相关联的。
1.创建C#的委托：
在C#中创建委托的格式： public delegate void OpDelegate(int First, int Second);
此时能与该委托匹配的方法为：static public void Subtact(int First, int Second){....}
public void Add(int First, int Second){...}
就是说委托是可以与静态方法或非静态方法共用的。

2.一个银行例子，透支的处理类就是用到委托

STEP1.先写一个用于透支处理的委托方法相关的透支处理类：


using System;


namespace MSPress.CSharpCoreRef.Bank




...{




/**//// <summary>


/// 下面的 withdrawal 方法, 提供有透支情况发生


/// 的委托方法


/// </summary>


public class OverdraftProtection




...{


public OverdraftProtection(decimal initialLoan)




...{


_availableLoan = initialLoan;


_currentLoan = 0;


}


public decimal AvailableLoan




...{




set ...{ _availableLoan = value; }




get ...{ return _availableLoan; }


}


public decimal CurrentLoan




...{




set ...{ _currentLoan = value; }




get ...{ return _currentLoan; }


}




/**//// <summary>


/// 与Withdrawal的实例共用的方法，用于管理帐户贷款情况


/// 透支会根据构造函数指定的固定数量给付


/// </summary>


/// <param name="debit">Attempted debit.</param>


/// <returns>True if successful, false if overdraft


/// loan amount exceeded.</returns>


public bool HandleDebit(decimal debit)




...{


if(debit > 0)




...{


if(debit <= _availableLoan)




...{


_availableLoan -= debit;


_currentLoan += debit;


}


else


return false;


}


return true;


}




/**//// <summary>


/// 可以透支的金额


/// </summary>


protected decimal _availableLoan;




/**//// <summary>


/// 当前透支的金额


/// </summary>


protected decimal _currentLoan;


}


}



STEP2.然后是创建应行的帐户类：




using System;


namespace MSPress.CSharpCoreRef.Bank




...{


public class Account //银行的帐户类




...{


public delegate bool DebitPolicy(decimal aWithdrawal); //定义透支委托方法


//帐户类的构造函数，第一个参数是开始帐户的数额


//第二个参数则为定义委托方法的委托签名，Account创建后


//将传递一个与该委托签名匹配的委托方法


public Account(decimal anInitialBalance, DebitPolicy aDebitPolicy)




...{


_balance = anInitialBalance;


_debitPolicy = aDebitPolicy;


}


public decimal Balance




...{


get




...{


return _balance;


}


}


public void Deposit(decimal aDeposit)




...{


if(aDeposit < 0)


throw new ArgumentOutOfRangeException();


_balance += aDeposit;


}


public bool Withdrawal(decimal aWithdrawal) //取款方法




...{


if(aWithdrawal < 0)


throw new ArgumentOutOfRangeException();


if(aWithdrawal < _balance)




...{


_balance -= aWithdrawal;


return true;


}


else




...{


// 如果没有借贷政策，就不允许透支.


if(_debitPolicy == null)


return false;


aWithdrawal -= _balance;


if(_debitPolicy(aWithdrawal))




...{


_balance = 0;


return true;


}


else




...{


return false;


}


}


}


protected DebitPolicy _debitPolicy;


protected decimal _balance;


}


}



STEP3.最后定义主函数类把他们都实例化：


using System;


namespace MSPress.CSharpCoreRef.Bank




...{


class BankApp




...{


static void Main(string[] args)




...{




//实例化用于透支处理类，透支金额为500


OverdraftProtection op = new OverdraftProtection(500);


//定义Account中的委托方法


Account.DebitPolicy policy;


//将实例化的对象op的HandleDebit与Account中的委托方法




//匹配




policy = new Account.DebitPolicy(op.HandleDebit);


//实例化我的帐户


Account myAccount = new Account(100, policy);


bool done = false;


do




...{


DisplayMenu();


string choice = Console.ReadLine();


switch(choice.ToLower())




...{


case "w":


HandleWithdrawal(myAccount);


break;


case "d":


HandleDeposit(myAccount);


break;


case "q":


done = true;


break;


default:


InvalidChoice(choice);


break;


}


CurrentBalance(myAccount, op);


}while(!done);


}


static void DisplayMenu()




...{


Console.WriteLine(" W) Withdraw");


Console.WriteLine("D) Deposit");


Console.WriteLine("Q) Quit");


Console.Write("->");


}


static void InvalidChoice(string choice)




...{


Console.WriteLine("{0} is not a valid choice.", choice);


}


static void HandleWithdrawal(Account anAccount)




...{


Console.Write("Withdrawal amount: ");


string request = Console.ReadLine();


decimal withdrawal = Convert.ToDecimal(request);


bool result = anAccount.Withdrawal(withdrawal);


if(result == true)


Console.WriteLine("Withdrawal of {0} was successful.", withdrawal);


else


Console.WriteLine("Withdrawal of {0} was not successful.", withdrawal);


}


static void HandleDeposit(Account anAccount)




...{


Console.Write("Deposit amount: ");


string request = Console.ReadLine();


decimal deposit = Convert.ToDecimal(request);


anAccount.Deposit(deposit);


}


static void CurrentBalance(Account anAccount, OverdraftProtection op)




...{


Console.WriteLine("Balance is {0}", anAccount.Balance);


Console.WriteLine("Available overdraft is {0}", op.AvailableLoan);


Console.WriteLine("Current overdraft loan is {0}", op.CurrentLoan);


}


}