error on clearing textbox

Dear do like this ,


            if (textBox2.TextLength > 0)
            {
                if (chkTax.Checked)
                {
                    taxAmnt = ((fAuctionProfit * Convert.ToDouble(txtTax.Text)) / 100);
                }
                else
                {
                    taxAmnt = Convert.ToDouble(txtTax.Text);
                }
                txtNetProfit.Text = (fAuctionProfit - taxAmnt - bonusAmnt - serviceChrg).ToString();
            }