Second drop down does not populate

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C#

ASP.NET

The code snipnets are given below.

<asp:DropDownList ID="ddlState" runat="server" Height="28px" Width="354px" 
                            BorderColor="#C9CACA" BorderStyle="Solid" BorderWidth="1px" BackColor="White" 
                                change="UpdateDistrict()" >
                            
<script language="javascript" type="text/javascript">
    var xmlHttp;
    function UpdateDistrict()
    {
    xmlHttp = null;
    debugger;
    if(window.XMLHttpRequest)
    {
        xmlHttp = new XMLHttpRequest();
    }
    else if(window.ActiveXObject)
    {
        xmlHttp = new ActiveXObject("Microsoft.XMLHTTP"); 
    }
    
    if(xmlHttp != null)
    {
        var contName = document.getElementById('<%=ddlState.ClientID %>').value; 
        
        xmlHttp.onreadystatechange=state_Change;
        
        xmlHttp.open("GET","frmForAjaxCalls.aspx?cont="+contName,true);
        xmlHttp.send(null); 
    }
    }
</script>

public partial class frmForAjaxCalls : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        string Statename = Request.QueryString["cont"] as string;

        if (Statename != null)
        {
            DataTable table = Class1.getDistrictname(Statename);

            string result = string.Empty;

            foreach (DataRow r in table.Rows)
            {
                result += r["DistrictName"].ToString() + ";";
            }

            Response.Clear();
            Response.Write(result);
            Response.End();
        }
    }
}

this is within the class
public static DataTable getDistrictname(String StateSelected)
    {
        SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["con"].ConnectionString);
        con.Open();
        SqlDataAdapter da2 = new SqlDataAdapter("select '--Not in List--' as DistrictName from tblMSTStateDistrict UNION select distinct DistrictName from tblMSTStateDistrict where Stateid ='" + Convert.ToString(StateSelected) + "' order by DistrictName", con);
        DataSet ds2 = new DataSet();
        da2.Fill(ds2);

        return ds2.Tables[0];
        con.Close();

    }

this is within the code file
onselectedindexchanged="ddlState_SelectedIndexChanged" onchange="UpdateDistrict()"

if (!PostBack)
{
SqlDataAdapter da1 = new SqlDataAdapter("select 0 as Stateid, '' as StateName from tblMSTStateMaster UNION select distinct Stateid, StateName from tblMSTStateMaster", con);
DataSet ds1 = new DataSet();
da1.Fill(ds1);
ddlState.DataSource = ds1.Tables[0];
        ddlState.DataTextField = "StateName";
        ddlState.DataValueField = "StateID";
        ddlState.DataBind();

ddlDistrict.DataSource = Class1.getDistrictname(ddlState.SelectedValue);
        ddlDistrict.DataTextField = "DistrictName";
        ddlDistrict.DataValueField = "DistrictName";
        ddlDistrict.DataBind();
}

The problem is that the state is populating. The control is also fatching "--No in list--" when !not postback. But when I am selecting the state the district ddl is not getting populated. Moreover it is not executing the javascript updateDistrict()
Kindly help me out.