You may build a Custom ComboBox control which contains some additional data. for example something like below.
<asp:DropDownList ID="DropDownList1" runat="server"
onselectedindexchanged="DropDownList1_SelectedIndexChanged" >
<asp:ListItem valueForTextBox="0">Transaction</asp:ListItem>
<asp:ListItem valueForTextBox="100">Non Transaction</asp:ListItem>
</asp:DropDownList>
and in code-behind
protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
{
Textbox1.Text = DropDownList1.SelectedItem.valueForTextBox;
}
following link can help
Custom ComboBox server control for ASP.NET[
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