Форма:

namespace WinFormsApp1

{

   public partial class Form1 : Form

   {

       private readonly A _a;

       public Form1()

       {

           InitializeComponent();

           _a = new A();

       }

       private void C1Btn_Click(object sender, EventArgs e)

       {

           InfoListBox.Items.Add($"C1: {_a.C1}");

       }

       private void C2Btn_Click(object sender, EventArgs e)

       {

           InfoListBox.Items.Add($"C2: {_a.C2}");

       }

       private void D1Btn_Click(object sender, EventArgs e)

       {

           InfoListBox.Items.Add($"D1: {_a.D1()}");

       }

       private void D2Btn_Click(object sender, EventArgs e)

       {

           InfoListBox.Items.Add($"D2: {_a.D2()}");

       }

   }

}

Класс A:

using System;

namespace WinFormsApp1

{

   internal class A

   {

       private decimal a = 3;

       private decimal b = 5;

       public decimal C1 => a++;

       public decimal C2 => b++;

       public A() { }

       public decimal D1()

       {

           return C1 * C1;

       }

       public decimal D2()

       {

           return C2 * C2;

       }

   }

}