永远不会分配c＃字段，并且其默认值始终为null？

Question

C# Inhertance. I's trying to inherit class quadrilateral to class sqaure. I am unable to figure this out and I got the following error and I dont know what it means. Field square.p1 is never assigned to and will always have its default value null?

    using System;

    class testquadrilaterals 
    {
        static void Main() 
{
    quadrilateral obj1 = new quadrilateral(1.1, 1.2, 6.6, 2.8, 6.2, 9.9, 2.2, 7.4);
    obj1.printcoordinates();
    Console.WriteLine("\n");
    square obj2 = new square( 4.0, 0.0, 8.0, 0.0, 8.0, 4.0, 4.0, 4.0 );
    obj2.printsquare();

    Console.ReadLine();
}
    }

    class point 
    {
        private double x;
private double y;

public point(double x_coord, double y_coord)
{
    x = x_coord;
    y = y_coord;
}

public double X { get { return x; } set { x = value; } }
public double Y { get { return y; } set { y = value; } }

public override string ToString()
{
    return ("(" + X + ", " + Y + ")");
}
    }

    class quadrilateral
    {
point p1, p2, p3, p4;

public quadrilateral(double x1, double y1, double x2, double y2,
    double x3, double y3, double x4, double y4)
{
    p1 = new point(x1, y1);
    p2 = new point(x2, y2);
    p3 = new point(x3, y3);
    p4 = new point(x4, y4);
}

public point P1 { get { return p1; } set { p1 = value; } }
public point P2 { get { return p2; } set { p2 = value; } }
public point P3 { get { return p3; } set { p3 = value; } }
public point P4 { get { return p4; } set { p4 = value; } }

public void printcoordinates()
{
    Console.WriteLine("Coordinates of Quadrilateral: \n" + p1.ToString() + " " + p2.ToString() + " " + p3.ToString() + " " + p4.ToString() + " ");
}
    }


    class square : quadrilateral
    {
point p1, p2, p3, p4;

public square(double x1, double y1, double x2, double y2,
    double x3, double y3, double x4, double y4) : base (x1, y1, x2, y2, x3, y3, x4, y4)
{ }

public void printsquare()
{
    Console.WriteLine("Coordinates of Square: \n" + p1.ToString() + " " + p2.ToString() + " " + p3.ToString() + " " + p4.ToString() + " ");
    Console.WriteLine("Area: " + area());
   // Console.WriteLine("Sides: " + sides());
}

public double area() 
{

    return Math.Abs(p1.X - p2.X) * Math.Abs(p1.Y - p3.Y); 
}
// public double sides() { }

}

Answer 1

In this case the class square defines 4 fields which it never sets: p1, p2, p3 and p4

class square : quadrilateral {
  point p1, p2, p3, p4;

  ...
}

These fields are private and never set hence they are essentially useless as is. The C# compiler is alerting you to this because this is usually an indication of a bug.

In this case it looks like you may have intended for square to simply use the values in quadrilateral . The fields have the same name and type and there is an inheritance relationship between the two types. If that's the case you should delete the fields in square and change the accessibility to protected in quadrilateral

class quadrilateral {
  protected  point p1, p2, p3, p4;
  ...
}

This will allow square to access the values

Answer 2

如果从四面体继承，则不需要在square中再次定义变量p1 / p2 / p3 / p4，您将从基类继承它们。

Answer 3

In Square::area(), the p1s and p2s are referring to those defined within the square class, not those in the quadrilateral class. You have both square::p1, square::p2, etc. and quadrilateral ::p1, quadrilateral::p2, etc. The square p1s and p2s are never assigned to. To have your square class use the quadrilateral class' p1s and p2s, don't redefine them in square.