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FakeStandard
FakeStandard

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Virtual modifier for C#

什麼是 virtual ?

它的定義很簡單,virtual 關鍵字可以使用於屬性、方法、索引子或事件定義,繼承內部具有 virtual 關鍵字成員的類別之子類別可以被覆寫,例如使用 virtual 關鍵字的方法,可以再子類別中使用 override 來覆寫此方法

class Shape
{
    protected int size;

    // 定義為虛擬方法
    public virtual int GetArea()
    {
        return size * size;
    }
}

class Rectangle : Shape
{
    public Rectangle(int x, int y)
    {
        X = x;
        Y = y;
    }

    public int X { get; }
    public int Y { get; }

    // 覆寫方法內容
    public override int GetArea() => X * Y;
}
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反之,如果覆寫的方法不是虛擬方法,編譯器會回報錯誤,例如下列 B 類別覆寫 Foo 方法會失敗

class F
{
    public void Foo() { }
}

class B : F
{
    public override void Foo() { }
}
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編譯器回報被覆寫的方法至少必須是虛擬、抽象或覆寫的其中一種

pic-015

virtual 修飾詞不可與 staticabstractoverrideprivate 一起使用

嘗試將屬性定義為 virtual,並在子類別覆寫該屬性

public class Person
{
    // virtual property
    public virtual string Name { get; set; }
}

public class Employee : Person
{
    private string name;

    // override property
    public override string Name
    {
        get { return name; }
        set
        {
            if (!string.IsNullOrEmpty(value))
                name = value;
            else
                name = "Unknown";
        }
    }
}
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練習使用 virtual 關鍵字

public class Shape
{
    public const double PI = Math.PI;
    protected double r, h;

    public Shape() { }

    public Shape(double r)
    {
        this.r = r;
    }

    public Shape(double r, double h)
    {
        this.r = r;
        this.h = h;
    }

    // virtual method
    public virtual double GetArea() => r;
}

public class Circle : Shape
{
    public Circle(double r) : base(r) { }

    // override method
    public override double GetArea() => r * r * PI;
}

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

    public double GetVolume() => base.GetArea() * 2 * h;
}
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Reference

virtual


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