can not allocate 2 dimension inherted class

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C#3.5

Hellow i want to allocate a 2 dimension for a class that inhert from another class

class class1
    {   private double a;
        public class1() { }
        public class1(double Xa)
        {   A    = Xa; }
        public double A
        { get { return a; } set { a = value; } }
	}
    
	class class2 : class1
    {   private double b;
        public class2() { }
        public class2(double Xa,   double Xb) : base(Xa)
        {   B    = Xb; }
        public double B
        { get { return b; } set { b = value; } }
	}

in Form1 my global declaration were

class2[,] vary;

in Form1Load i create a 2 dimension like this

class2[100,100] vary;

            for (i = 0; i < 100; i++)
                for (j = 0; j < 100; j++)
                vary[i, j] = new class2();
				vary[10,20].A=1;//<--------error
				vary[10,20].B=20;//<----   error

needs help