[Solved] Implement Cascading DropDownList inside Repeater control in ASP.Net

Here I have created sample for you that populates repeater containing Cascading dropdownlist Country and state.

I hope this will help you out.

HTML

<asp:Repeater ID="rptAddress" runat="server" OnItemDataBound="Address_OnItemDataBound">
<HeaderTemplate>
    <table>
        <tr>
            <th>
                Name
            </th>
            <th>
                Countries
            </th>
            <th>
                States
            </th>
        </tr>
</HeaderTemplate>
<ItemTemplate>
    <tr>
        <td>
            <asp:Label Text='<%#Eval("Name") %>' runat="server" />
        </td>
        <td style="width: 40%; text-align: left">
            <asp:DropDownList ID="ddlCountries" runat="server" Width="150px" AutoPostBack="true"
                OnSelectedIndexChanged="Countries_OnSelectedIndexChanged">
            </asp:DropDownList>
        </td>
        <td style="width: 40%; text-align: left">
            <asp:DropDownList ID="ddlStates" runat="server" Width="150px" Enabled="false">
            </asp:DropDownList>
        </td>
    </tr>
</ItemTemplate>
<FooterTemplate>
    </table>
</FooterTemplate>
</asp:Repeater>

Namespaces

using System.Data;
using System.Data.SqlClient;
using System.Configuration;

Code

string constr = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        DataTable names = new DataTable();
        names.Columns.AddRange(new DataColumn[1] { 
                        new DataColumn("Name", typeof(string))
                        });
        names.Rows.Add("David");
        names.Rows.Add("Muddasar");
        this.rptAddress.DataSource = names;
        this.rptAddress.DataBind();
    }
}

protected void Address_OnItemDataBound(object sender, RepeaterItemEventArgs e)
{
    if (e.Item.ItemType == ListItemType.Item || e.Item.ItemType == ListItemType.AlternatingItem)
    {
        DropDownList ddlCountries = (DropDownList)e.Item.FindControl("ddlCountries");
        ddlCountries.DataSource = this.GetCountries();
        ddlCountries.DataTextField = "CountryName";
        ddlCountries.DataValueField = "CountryId";
        ddlCountries.DataBind();
        ddlCountries.Items.Insert(0, new ListItem("", "0"));
    }
}

protected void Countries_OnSelectedIndexChanged(object sender, EventArgs e)
{
    RepeaterItem row = (RepeaterItem)(sender as DropDownList).NamingContainer;
    DropDownList country = (DropDownList)row.FindControl("ddlCountries");
    int countryId = int.Parse(country.SelectedItem.Value);
    DropDownList ddlStates = (DropDownList)row.FindControl("ddlStates");
    DataTable dtStates = this.GetStates(countryId);
    if (dtStates.Rows.Count > 0)
    {
        ddlStates.DataSource = dtStates;
        ddlStates.DataTextField = "StateName";
        ddlStates.DataValueField = "StateId";
        ddlStates.DataBind();
        ddlStates.Enabled = true;
    }
    else
    {
        ddlStates.Enabled = false;
    }
    ddlStates.Items.Insert(0, new ListItem("", ""));
}

private DataTable GetCountries()
{
    using (SqlConnection con = new SqlConnection(constr))
    {
        using (SqlCommand cmd = new SqlCommand("SELECT CountryId,CountryName FROM Countries", con))
        {
            using (SqlDataAdapter sda = new SqlDataAdapter(cmd))
            {
                DataTable dtCountries = new DataTable();
                sda.Fill(dtCountries);
                return dtCountries;
            }
        }
    }
}

private DataTable GetStates(int countryId)
{
    using (SqlConnection con = new SqlConnection(constr))
    {
        using (SqlCommand cmd = new SqlCommand("SELECT StateId,StateName FROM States WHERE CountryId = @CountryId", con))
        {
            cmd.Parameters.AddWithValue("@CountryId", countryId);
            using (SqlDataAdapter sda = new SqlDataAdapter(cmd))
            {
                DataTable dtStates = new DataTable();
                sda.Fill(dtStates);
                return dtStates;
            }
        }
    }
}

Screenshot