什麼是里氏替換原則以及如何在 C# 中實現？

派生類型必須完全可取代其基底類型。

定義：

我們應該能夠將子類別視為父類別。本質上，這意味著所有衍生類別都應保留其父類別的功能，並且不能取代父類別提供的任何功能。

在 Liskov 替換之前

public class Ellipse {
   public double MajorAxis { get; set; }
   public double MinorAxis { get; set; }

   public virtual void SetMajorAxis(double majorAxis){
      this.MajorAxis = majorAxis;
   }
   public virtual void SetMinorAxis(double minorAxis){
      this.MajorAxis = minorAxis;
   }
   public virtual double Area() {
      return MajorAxis * MinorAxis * Math.PI;
   }
}
public class Circle : Ellipse {
   public override void SetMajorAxis(double majorAxis) {
      base.SetMajorAxis(majorAxis);
      this.MinorAxis = majorAxis; //In a cirle, each axis is identical
   }
}

public class Result {
   public void Method1() {
      Circle circle = new Circle();
      circle.SetMajorAxis(5);
      circle.SetMinorAxis(4);
      var area = circle.Area(); //5*4 = 20, but we expected 5*5 = 25
   }
}

利斯科夫替換後

internal class Program {
   private static void Main() {
   }
}
public class Ellipse {
   public double MajorAxis { get; set; }
   public double MinorAxis { get; set; }

   public virtual void SetMajorAxis(double majorAxis) {
      MajorAxis = majorAxis;
   }
   public virtual void SetMinorAxis(double minorAxis) {
      MajorAxis = minorAxis;
   }
   public virtual double Area() {
      return MajorAxis * MinorAxis * Math.PI;
   }
}
public class Circle : Ellipse {
   public override void SetMajorAxis(double majorAxis) {
      base.SetMajorAxis(majorAxis);
      this.MinorAxis = majorAxis; //In a cirle, each axis is identical
   }
   public override void SetMinorAxis(double minorAxis) {
      base.SetMinorAxis(minorAxis);
      this.MajorAxis = minorAxis;
   }
   public override double Area() {
      return base.Area();
   }
}

public class Circle1 {
   public double Radius { get; set; }
   public void SetRadius(double radius) {
      this.Radius = radius;
   }
   public double Area() {
      return this.Radius * this.Radius * Math.PI;
   }
}

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