c# - c#フィールドは決して割り当てられず、常にデフォルト値がnullになりますか?

Question

C# 継承。クラス四角形をクラス四角形に継承しようとしています。これを理解できず、次のエラーが発生しましたが、それが何を意味するのかわかりません。フィールド square.p1 は割り当てられず、常にデフォルト値が null になりますか?

    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() { }

}