Display Cascading DropDownList inside ModalPopupExtender and send selected value to another page using C# and VB.Net in ASP.Net

Hi haider6.9,

I have created sample that full fill your requirement. Refer the below sample.

webform2.aspx

<div>
    <asp:ScriptManager ID="ScriptManager1" runat="server">
    </asp:ScriptManager>
    <asp:LinkButton Text="" ID="lnkFake" runat="server" />
    <cc1:ModalPopupExtender ID="mpe" runat="server" PopupControlID="pnlPopup" TargetControlID="lnkFake"
        CancelControlID="btnClose" BackgroundCssClass="modalBackground">
    </cc1:ModalPopupExtender>
    <asp:Panel ID="pnlPopup" runat="server" CssClass="modalPopup" Style="display: none">
        <div class="header">
            Cascading DropDownList
        </div>
        <div>
            <table>
                <tr>
                    <td>
                        Country
                    </td>
                    <td>
                        <asp:DropDownList ID="ddlCountry" runat="server" AutoPostBack="true" OnSelectedIndexChanged="ddlCountry_SelectedIndexChanged">
                            <asp:ListItem Text="--Select Country--" Value=""></asp:ListItem>
                        </asp:DropDownList>
                    </td>
                </tr>
                <tr>
                    <td>
                        State
                    </td>
                    <td>
                        <asp:DropDownList ID="ddlState" runat="server" AutoPostBack="true" Enabled="false"
                            OnSelectedIndexChanged="ddlState_SelectedIndexChanged">
                            <asp:ListItem Text="--Select State--" Value=""></asp:ListItem>
                        </asp:DropDownList>
                    </td>
                </tr>
                <tr>
                    <td>
                        City
                    </td>
                    <td>
                        <asp:DropDownList ID="ddlCity" runat="server" Enabled="false" OnSelectedIndexChanged="ddlCity_SelectedIndexChanged"
                            AutoPostBack="true">
                            <asp:ListItem Text="--Select City--" Value=""></asp:ListItem>
                        </asp:DropDownList>
                    </td>
                </tr>
            </table>
        </div>
        <div class="footer" align="right">
            <asp:Button ID="btnClose" runat="server" Text="Close" CssClass="button" />
        </div>
    </asp:Panel>
</div>

webform2.aspx.cs

static string choice = "";
protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        PopulateDropDownList(ddlCountry, "SELECT [CountryId],[CountryName] FROM [Countries]", "CountryName", "CountryId");
        mpe.Show();
    }
}

private void PopulateDropDownList(DropDownList ddl, string query, string dataTextField, string dataValueField)
{
    ddl.AppendDataBoundItems = true;
    String strConnString = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
    SqlConnection con = new SqlConnection(strConnString);
    SqlCommand cmd = new SqlCommand();
    cmd.CommandType = CommandType.Text;
    cmd.CommandText = query;
    cmd.Connection = con;
    try
    {
        con.Open();
        ddl.DataSource = cmd.ExecuteReader();
        ddl.DataTextField = dataTextField;
        ddl.DataValueField = dataValueField;
        ddl.DataBind();
    }
    catch (Exception ex)
    {
        throw ex;
    }
    finally
    {
        con.Close();
        con.Dispose();
    }
}

protected void ddlCountry_SelectedIndexChanged(object sender, EventArgs e)
{
    choice += "Country=" + ddlCountry.SelectedItem.Text;
    string query = "SELECT [StateId],[CountryId],[StateName] FROM [States] WHERE CountryId = " + ddlCountry.SelectedValue;
    PopulateDropDownList(ddlState, query, "StateName", "StateId");
    ddlState.Enabled = true;
    mpe.Show();
}

protected void ddlState_SelectedIndexChanged(object sender, EventArgs e)
{
    choice += "&State=" + ddlState.SelectedItem.Text;
    string query = "SELECT [CityId],[StateId],[CityName] FROM [Cities] WHERE StateId = " + ddlState.SelectedValue;
    PopulateDropDownList(ddlCity, query, "CityName", "CityId");
    ddlCity.Enabled = true;
    mpe.Show();
}

protected void ddlCity_SelectedIndexChanged(object sender, EventArgs e)
{
    choice += "&City=" + ddlCity.SelectedItem.Text;
    Response.Redirect("webform1.aspx?" + choice);
}

webform2.aspx.vb

Protected Sub Page_Load(sender As Object, e As EventArgs) Handles Me.Load
    If Not IsPostBack Then
        PopulateDropDownList(ddlCountry, "SELECT [CountryId],[CountryName] FROM [Countries]", "CountryName", "CountryId")
        mpe.Show()
    End If
End Sub

Private Sub PopulateDropDownList(ddl As DropDownList, query As String, dataTextField As String, dataValueField As String)
    ddl.AppendDataBoundItems = True
    Dim strConnString As [String] = ConfigurationManager.ConnectionStrings("constr").ConnectionString
    Dim con As New SqlConnection(strConnString)
    Dim cmd As New SqlCommand()
    cmd.CommandType = CommandType.Text
    cmd.CommandText = query
    cmd.Connection = con
    Try
        con.Open()
        ddl.DataSource = cmd.ExecuteReader()
        ddl.DataTextField = dataTextField
        ddl.DataValueField = dataValueField
        ddl.DataBind()
    Catch ex As Exception
        Throw ex
    Finally
        con.Close()
        con.Dispose()
    End Try
End Sub

Protected Sub ddlCountry_SelectedIndexChanged(sender As Object, e As EventArgs)
    choice += "Country=" + ddlCountry.SelectedItem.Text
    Dim query As String = "SELECT [StateId],[CountryId],[StateName] FROM [States] WHERE CountryId = " + ddlCountry.SelectedValue
    PopulateDropDownList(ddlState, query, "StateName", "StateId")
    ddlState.Enabled = True
    mpe.Show()
End Sub

Protected Sub ddlState_SelectedIndexChanged(sender As Object, e As EventArgs)
    choice += "&State=" + ddlState.SelectedItem.Text
    Dim query As String = "SELECT [CityId],[StateId],[CityName] FROM [Cities] WHERE StateId = " + ddlState.SelectedValue
    PopulateDropDownList(ddlCity, query, "CityName", "CityId")
    ddlCity.Enabled = True
    mpe.Show()
End Sub

Protected Sub ddlCity_SelectedIndexChanged(sender As Object, e As EventArgs)
    choice += "&City=" + ddlCity.SelectedItem.Text
    Response.Redirect(Convert.ToString("webform1.aspx?") & choice)
End Sub

webform1.aspx

<asp:Label ID="lblChoices" runat="server" />

webform1.aspx.cs

protected void Page_Load(object sender, EventArgs e)
{
    if (!string.IsNullOrEmpty(Request.QueryString["Country"]) && !string.IsNullOrEmpty(Request.QueryString["State"]) && !string.IsNullOrEmpty(Request.QueryString["City"]))
    {
        string country = "Country = " + Request.QueryString["Country"];
        string state = "State = " + Request.QueryString["State"];
        string city = "City = " + Request.QueryString["City"];
        lblChoices.Text = country + "<br/>" + state + "<br/>" + city;
    }
}

webform1.aspx.vb

Protected Sub Page_Load(sender As Object, e As EventArgs)
	If Not String.IsNullOrEmpty(Request.QueryString("Country")) AndAlso Not String.IsNullOrEmpty(Request.QueryString("State")) AndAlso Not String.IsNullOrEmpty(Request.QueryString("City")) Then
		Dim country As String = "Country = " + Request.QueryString("Country")
		Dim state As String = "State = " + Request.QueryString("State")
		Dim city As String = "City = " + Request.QueryString("City")
		lblChoices.Text = Convert.ToString((Convert.ToString(country & Convert.ToString("<br/>")) & state) + "<br/>") & city
	End If
End Sub

Screenshot