virtual 关键字[C#]

virtual（C# 参考）

例如，此方法可被任何继承它的类重写。

public virtual double Area()
{
    return x * y;
}

public virtual double Area() 
{
    return x * y;
}

virtual 关键字的更多信息，

请参见 使用 Override 和 New 关键字进行版本控制（C# 编程指南）和 了解何时使用 Override 和 New 关键字（C# 编程指南）。

备注

将调用大部分派生类中的该重写成员，如果没有派生类重写该成员，则它可能是原始成员。

不能重写非虚方法。

下面的示例演示一个虚拟属性：

class MyBaseClass
{
    // virtual auto-implemented property. Overrides can only
    // provide specialized behavior if they implement get and set accessors.
    public virtual string Name { get; set; }

    // ordinary virtual property with backing field
    private int num;
    public virtual int Number
    {
        get { return num; }
        set { num = value; }
    }
}


class MyDerivedClass : MyBaseClass
{
    private string name;

   // Override auto-implemented property with ordinary property
   // to provide specialized accessor behavior.
    public override string Name
    {
        get
        {
            return name;
        }
        set
        {
            if (value != String.Empty)
            {
                name = value;
            }
            else
            {
                name = "Unknown";
            }
        }
    }

}

class MyBaseClass
{
    // virtual auto-implemented property. Overrides can only
    // provide specialized behavior if they implement get and set accessors.
    public virtual string Name { get; set; }

    // ordinary virtual property with backing field
    private int num;
    public virtual int Number
    {
        get { return num; }
        set { num = value; }
    }
}


class MyDerivedClass : MyBaseClass
{
    private string name;

   // Override auto-implemented property with ordinary property
   // to provide specialized accessor behavior.
    public override string Name
    {
        get
        {
            return name;
        }
        set
        {
            if (value != String.Empty)
            {
                name = value;
            }
            else
            {
                name = "Unknown";
            }
        }
    }

}

除了声明和调用语法不同外，虚拟属性的行为与抽象方法一样。

• virtual 修饰符是错误的。

• override 修饰符的属性声明，可在派生类中重写虚拟继承属性。

示例

Area() 实现，程序为每个图形计算并显示适当的面积。

Cylinder 都使用了初始化基类的构造函数，例如：

public Cylinder(double r, double h): base(r, h) {}

public Cylinder(double r, double h): base(r, h) {}

这类似于 C++ 的初始化列表。

class TestClass
{
    public class Dimensions
    {
        public const double PI = Math.PI;
        protected double x, y;
        public Dimensions()
        {
        }
        public Dimensions(double x, double y)
        {
            this.x = x;
            this.y = y;
        }

        public virtual double Area()
        {
            return x * y;
        }
    }

    public class Circle : Dimensions
    {
        public Circle(double r) : base(r, 0)
        {
        }

        public override double Area()
        {
            return PI * x * x;
        }
    }

    class Sphere : Dimensions
    {
        public Sphere(double r) : base(r, 0)
        {
        }

        public override double Area()
        {
            return 4 * PI * x * x;
        }
    }

    class Cylinder : Dimensions
    {
        public Cylinder(double r, double h) : base(r, h)
        {
        }

        public override double Area()
        {
            return 2 * PI * x * x + 2 * PI * x * y;
        }
    }

    static void Main()
    {
        double r = 3.0, h = 5.0;
        Dimensions c = new Circle(r);
        Dimensions s = new Sphere(r);
        Dimensions l = new Cylinder(r, h);
        // Display results:
        Console.WriteLine("Area of Circle   = {0:F2}", c.Area());
        Console.WriteLine("Area of Sphere   = {0:F2}", s.Area());
        Console.WriteLine("Area of Cylinder = {0:F2}", l.Area());
        }
    }
    /*
        Output:
        Area of Circle   = 28.27
        Area of Sphere   = 113.10
        Area of Cylinder = 150.80
    */