ServerHtmlEncode + ModalPopupExtender

See more:

ModalPopUpExtender

I have a modalpopupextender. after I selected the gridview and when the value of the textbox is installment, it will compute the values and show it to the txtInsAmount. Then the Value in the txtInsAmount must be the same as txtAmount. But it isn't working. Here are the codes.

protected void GridView1_SelectedIndexChanged(object sender, EventArgs e)
    {
        //txtInsAmount.Text = Server.HtmlEncode(txtAmount.Text);
        txtOID.Text = GridView1.SelectedRow.Cells[2].Text;
        txtCName.Text = GridView1.SelectedRow.Cells[1].Text;
        txtTAP.Text = GridView1.SelectedRow.Cells[4].Text;
        txtRBalance.Text = GridView1.SelectedRow.Cells[5].Text;
        txtMoP.Text = GridView1.SelectedRow.Cells[7].Text;
        if (txtMoP.Text == "Installment")
        {
            decimal a = Convert.ToDecimal(txtRBalance.Text);
            decimal b = Convert.ToDecimal(2);

            Label7.Visible = true;
            txtInsAmount.Visible = true;
            txtInsAmount.Text = (a / 2).ToString();
            txtInsAmount.Text = Server.HtmlEncode(txtAmount.Text);
    }

txtInsAmount.Text = Server.HtmlEncode(txtAmount.Text);

Maybe there is something wrong in this code? Thank you!