C# 关键字---override

要扩展或修改继承的方法、属性、索引器或事件的抽象实现或虚实现，必须使用 override修饰符。由 override 声明重写的方法称为重写基方法。

使用override时需要注意以下几点：

• override 方法提供从基类继承的成员的新实现。
• 重写的基方法必须与 override 方法具有相同的签名。
• 不能重写非虚方法或静态方法。重写的基方法必须是 virtual、abstract 或 override 的。
• override 声明不能更改 virtual 方法的可访问性。 override 方法和 virtual 方法必须具有相同的访问级别修饰符。
• 您不能使用 new、static 或 virtual 修饰符来修改 override 方法。

以下是MSDN举得例子：

class TestOverride
{
    public class Employee
    {
        public string name;

        // Basepay is defined as protected, so that it may be 
        // accessed only by this class and derrived classes.
        protected decimal basepay;

        // Constructor to set the name and basepay values.
        public Employee(string name, decimal basepay)
        {
            this.name = name;
            this.basepay = basepay;
        }

        // Declared virtual so it can be overridden.
        public virtual decimal CalculatePay()
        {
            return basepay;
        }
    }

    // Derive a new class from Employee.
    public class SalesEmployee : Employee
    {
        // New field that will affect the base pay.
        private decimal salesbonus;

        // The constructor calls the base-class version, and
        // initializes the salesbonus field.
        public SalesEmployee(string name, decimal basepay, 
                  decimal salesbonus) : base(name, basepay)
        {
            this.salesbonus = salesbonus;
        }

        // Override the CalculatePay method 
        // to take bonus into account.
        public override decimal CalculatePay()
        {
            return basepay + salesbonus;
        }
    }

    static void Main()
    {
        // Create some new employees.
        SalesEmployee employee1 = new SalesEmployee("Alice", 
                      1000, 500);
        Employee employee2 = new Employee("Bob", 1200);

        Console.WriteLine("Employee4 " + employee1.name + 
                  " earned: " + employee1.CalculatePay());
        Console.WriteLine("Employee4 " + employee2.name + 
                  " earned: " + employee2.CalculatePay());
    }
}

输出结果：

Employee4 Alice earned: 1500
Employee4 Bob earned: 1200