Try to use texboxchanged event
Givina a example following below
design
<asp:TextBox ID="TextBox1" runat="server" AutoPostBack="True"
ontextchanged="TextBox1_TextChanged"></asp:TextBox>
COde
protected void txtProduct_TextChanged(object sender, EventArgs e)
{
if (txtProduct.Text != "")
{
string strSQL = "make your sql query ;
DataTable dt =Commanclass.returnData(strSQL);//return (DataTable) with yourdata
DropdownList1.DataSource = dt;
DropdownList1.DataTextField = "Name";
DropdownList1.DataValueField = "Name";
DropdownList1.DataBind();
}
}