C# 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。

在此示例中，Square 类必须提供 Area 的重写实现，因为
Area 继承自抽象的 ShapesClass：

abstract class ShapesClass
{
    abstract public int Area();
}
class Square : ShapesClass
{
    int side = 0;

    public Square(int n)
    {
        side = n;
    }
    // Area method is required to avoid
    // a compile-time error.
    public override int Area()
    {
        return side * side;
    }

    static void Main() 
    {
        Square sq = new Square(12);
        Console.WriteLine("Area of the square = {0}", sq.Area());
    }

    interface I
    {
        void M();
    }
    abstract class C : I
    {
        public abstract void M();
    }

}
// Output: Area of the square = 144

　　

override 方法提供从基类继承的成员的新实现。由 override 声明重写的方法称为重写基方法。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。

不能使用 new、static、virtual 或 abstract 修饰符来修改 override 方法。

 

重写属性声明必须指定与继承属性完全相同的访问修饰符、类型和名称，并且被重写的属性必须是 virtual、abstract 或 override 的。

此示例定义了一个名为 Employee 的基类和一个名为 SalesEmployee 的派生类。SalesEmployee 类包括一个额外的属性 salesbonus，并重写方法 CalculatePay 以便将该属性考虑在内。

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());
    }
}
/*
    Output:
    Employee4 Alice earned: 1500
    Employee4 Bob earned: 1200
*/