Hi Altaful,
The SelectedIndexChanged events of DropDownLists can be used here. I have tried modifying your code.
Please see the below code:
<asp:dropdownlist id="ddlSUser" runat="server" cssclass="input" width="145px" onselectedindexchanged="ddlSUser_SelectedIndexChanged" xmlns:asp="#unknown"></asp:dropdownlist>
And in the code behind write the event like this:
protected void ddlSUser_SelectedIndexChanged(object sender, EventArgs e)
{
BUID.Text = Convert.ToString(ddlSUser.SelectedValue);
}
I hope this helps :)