大家先看下面的代码：

using System;

public class AccountTest
{
    // Main method begins execution of C# application
    public static void Main(string[] args)
    {
        Account account1 = new Account( 50.00M ); // create Account object
        Account account2 = new Account( -7.53M ); // create Account object

        // display initial balance of each object using a property
        Console.WriteLine( "account1 balance: {0:C}",
            account1.Balance ); // display Balance property
        Console.WriteLine( "account2 balance: {0:C}\n",
            account2.Balance ); // display Balance property

        decimal depositAmount; // deposit amount read from user

        // prompt and obtain user input
        Console.Write( "Enter deposit amount for account1: " );
        depositAmount = Convert.ToDecimal( Console.ReadLine() );
        Console.WriteLine( "adding {0:C} to account1 balance\n",
            depositAmount );
        account1.Credit( depositAmount ); // add to account1 balance

        // display balances
        Console.WriteLine( "account1 balance: {0:C}",
            account1.Balance );
        Console.WriteLine( "account2 balance: {0:C}\n",
            account2.Balance );

        // prompt and obtain user input
        Console.Write( "Enter deposit amount for account2: " );
        depositAmount = Convert.ToDecimal( Console.ReadLine() );
        Console.WriteLine( "adding {0:C} to account2 balance\n",
            depositAmount );
        account2.Credit( depositAmount ); // add to account2 balance

        // display balances
        Console.WriteLine( "account1 balance: {0:C}", account1.Balance );
        Console.WriteLine( "account2 balance: {0:C}", account2.Balance );
    }
}

public class Account
{
    private decimal balance; // instance variable that stores the balance

    // constructor
    public Account(decimal initialBalance)
    {
        Balance = initialBalance; // set balance using property
    } // end Account constructor

    // credit (add) an amount to the account
    public void Credit(decimal amount)
    {
        Balance = Balance + amount; // add amount to balance
    } // end method Credit

    // a property to get and set the account balance
    public decimal Balance
    {
        get
        {
            return balance;
        } // end get
        set
        {
            // validate that value is greater than or equal to 0;
            // if it is not, balance is left unchanged
            if (value >= 0)
                balance = value;
        } // end set
    } // end property Balance
} // end class Account

最后运行结果是：

account1 balance: ￥50.00
account2 balance: ￥0.00

Enter deposit amount for account1: 10
adding ￥10.00 to account1 balance

account1 balance: ￥60.00
account2 balance: ￥0.00

Enter deposit amount for account2: 10
adding ￥10.00 to account2 balance

account1 balance: ￥60.00
account2 balance: ￥10.00

问题：
在Account类中的set和get只判定了value >= 0的情况就返回值，而如果value < 0的话，运行结果Balance则会是0. 为什么没有添加value < 0的情况，返0值呢？

这里其实有个默认的值，decimal默认为0，所以，当value<=0的情况下，自然为0，不需要程序中写。
当然你也可以写成这样的。
public decimal Balance
    {
        get
        {
            return balance;
        } // end get
        set
        {
            // validate that value is greater than or equal to 0;
            // if it is not, balance is left unchanged
            if (value >= 0)
            {
                balance = value;
            }
            else
            {
                balance=0;
             }
        } // end set
    } // end property Balance

谢谢帮助。我再研究下

很明显,这是一个借记卡帐号,不可以为负值.

顶2楼一下
其实说的再明白点，类在初始化的时候会调用构造函数，如果调用的是无参构造函数的话，类中所有的字段的初始化情况如下：
1.值类型初始化为0。
2.引用类型初始化为NULL。
3.bool类型初始化为FALSE。
4.枚举初始化为枚举的第一项值。（这个是凭印象的，楼主可以自己设个枚举字段调式）
5.结构字段，这个结构本身什么值不清楚，但是结构里面的字段类型同上。