private void mnuViewProduct_Click(object sender, EventArgs e)
 {
     if (productForm != null)
     {
         productForm.Show();
     }
     else
     {
         productForm           = new ProductForm();
         productForm.MdiParent = this;
         productForm.Dock      = DockStyle.Fill;
         productForm.Show();
         productForm.FormClosing += productForm_FormClosing;
     }
 }
 public void productForm_FormClosing(object sender, FormClosingEventArgs e)
 {
     productForm = null;
 }