private void saveButton_Click(object sender, EventArgs e)
        {
            Mobile mobile = new Mobile();

            if (String.IsNullOrEmpty(modelNameTextBox.Text))
            {
                MessageBox.Show("Enter Model name");
                return;
            }

            if (String.IsNullOrEmpty(imeiTextBox.Text))
            {
                MessageBox.Show("Enter IMEI");
                return;
            }
            if (imeiTextBox.TextLength != 15)
            {
                MessageBox.Show("IMEI Must be 15 char");
                return;
            }

            mobile.ModelName = modelNameTextBox.Text;
            mobile.IMEI      = imeiTextBox.Text;
            if (_mobileManager.IsImeiExist(mobile))
            {
                MessageBox.Show("This IMEI exist");
                return;
            }
            if (String.IsNullOrEmpty(priceTextBox.Text))
            {
                MessageBox.Show("Enter price");
                return;
            }
            if (!Regex.IsMatch(priceTextBox.Text, @"\d"))
            {
                MessageBox.Show("Price Must be number");
                return;
            }

            mobile.Price = Convert.ToDouble(priceTextBox.Text);
            if (_mobileManager.Add(mobile))
            {
                MessageBox.Show("Mobile added");
                return;
            }
            else
            {
                MessageBox.Show("Cannot Added");
                return;
            }
        }