I am doing work on .NET window application.

private void btnAdd_Click(object sender, EventArgs e)
      {
          /*
           * add two controls in the form and rename the 'name' property as txtA and txtB
           * add a button control and rename it to btnAdd, then double click the button, visual studio IDE wll add this event code for you
           * and you add your logic in this code block as below.
           */
          int a = Convert.ToInt32(txtA.Text.Trim()); // convert to int .. explore int.TryParse for validation
          int b = Convert.ToInt32(txtB.Text.Trim()); // convert to int .. explore int.TryParse for validation
          int sum = a + b;
          MessageBox.Show("the sum is=" + sum.ToString());

      }