private void newShapeButton_Click(object sender, EventArgs e)
 {
     shape = null;
     shapeTextBox.Text = "";
     createButton.Enabled = true;
     newShapeButton.Enabled = false;
 }
 private void createButton_Click(object sender, EventArgs e)
 {
     if (Convert.ToInt32(heightTextBox.Text) <= 0)
     {
         MessageBox.Show("Height must be a positive number.", "Error",
             MessageBoxButtons.OK, MessageBoxIcon.Error);
     } // end if
     else if (Convert.ToInt32(widthTextBox.Text) <= 0)
     {
         MessageBox.Show("Width must be a positive number.", "Error",
             MessageBoxButtons.OK, MessageBoxIcon.Error);
     } // end else if
     else
     {
         if (shapeComboBox.SelectedIndex == 0)
         {
             shape = new Rectangle(Convert.ToInt32(heightTextBox.Text),
                 Convert.ToInt32(widthTextBox.Text));
             output = "Rectangle\r\nHeight: ";
             output += shape.Height;
             output += "\r\nWidth: ";
             output += shape.Width;
             output += "\r\nArea: " + shape.ComputeArea();
             shapeTextBox.Text = output;
             createButton.Enabled = false;
             newShapeButton.Enabled = true;
         } // end if
         else if (shapeComboBox.SelectedIndex == 1)
         {
             if (Convert.ToInt32(heightTextBox.Text) !=
                 Convert.ToInt32(widthTextBox.Text))
             {
                 MessageBox.Show("Height and Width must be equal.", "Error",
             MessageBoxButtons.OK, MessageBoxIcon.Error);
             } // end if
             else
             {
                 shape = new Square(Convert.ToInt32(heightTextBox.Text));
                 output = "Square\r\nHeight: ";
                 output += shape.Height;
                 output += "\r\nWidth: ";
                 output += shape.Width;
                 output += "\r\nArea: " + shape.ComputeArea();
                 shapeTextBox.Text = output;
                 createButton.Enabled = false;
                 newShapeButton.Enabled = true;
             } // end else
         } // end else if
         else if (shapeComboBox.SelectedIndex == 2)
         {
             shape = new Triangle(Convert.ToInt32(heightTextBox.Text),
                 Convert.ToInt32(widthTextBox.Text));
             output = "Triangle\r\nHeight: ";
             output += shape.Height;
             output += "\r\nWidth: ";
             output += shape.Width;
             output += "\r\nArea: " + shape.ComputeArea();
             shapeTextBox.Text = output;
             createButton.Enabled = false;
             newShapeButton.Enabled = true;
         } // end else if
     } // end else
 }