Display GridView Row Details inside ASP.Net AJAX ModalPopupExtender Modal Popup in ASP.Net

Here I have created sample that will help you out.

HTML

<div>
    <cc1:ToolkitScriptManager runat="server">
    </cc1:ToolkitScriptManager>
    <asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="false" AllowPaging="true"
        PageSize="10" Font-Names="Arial" Font-Size="11pt" AlternatingRowStyle-BackColor="#C2D69B"
        HeaderStyle-BackColor="green" OnPageIndexChanging="OnPaging">
        <Columns>
            <asp:BoundField DataField="ContactName" HeaderText="Contact Name" />
            <asp:BoundField DataField="City" HeaderText="City" />
            <asp:TemplateField>
                <HeaderTemplate>
                    Country:
                    <asp:DropDownList ID="ddlCountry" runat="server" OnSelectedIndexChanged="CountryChanged"
                        AutoPostBack="true" AppendDataBoundItems="true">
                    </asp:DropDownList>
                </HeaderTemplate>
                <ItemTemplate>
                    <asp:Label ID="lblCountry" Text='<%# Eval("Country") %>' runat="server" />
                </ItemTemplate>
            </asp:TemplateField>
            <asp:BoundField DataField="PostalCode" HeaderText="Postal Code" />
            <asp:TemplateField>
                <ItemTemplate>
                    <asp:LinkButton Text="Review" runat="server" CommandArgument="CustomerId" OnClick="Review" />
                </ItemTemplate>
            </asp:TemplateField>
        </Columns>
    </asp:GridView>
    <cc1:ModalPopupExtender ID="mpe" BehaviorID="mpe" runat="server" PopupControlID="pnlPopup"
        TargetControlID="lnkDummy" BackgroundCssClass="modalBackground" CancelControlID="btnHide">
    </cc1:ModalPopupExtender>
    <asp:LinkButton ID="lnkDummy" runat="server"></asp:LinkButton>
    <asp:Panel ID="pnlPopup" runat="server" CssClass="modalPopup" Style="display: none">
        <div class="header">
            Details
        </div>
        <div class="body">
            <table border="0" cellpadding="0" cellspacing="0">
                <tr>
                    <td>
                        Contact Name :
                    </td>
                    <td>
                        <asp:Label ID="lblContactName" runat="server" />
                    </td>
                </tr>
                <tr>
                    <td>
                        City :
                    </td>
                    <td>
                        <asp:Label ID="lblCity" runat="server" />
                    </td>
                </tr>
                <tr>
                    <td>
                        Country :
                    </td>
                    <td>
                        <asp:Label ID="lblCountry" runat="server" />
                    </td>
                </tr>
                <tr>
                    <td>
                        Postal Code :
                    </td>
                    <td>
                        <asp:Label ID="lblPostalCode" runat="server" />
                    </td>
                </tr>
            </table>
            <br />
            <asp:Button ID="btnHide" runat="server" Text="Hide Modal Popup" />
        </div>
    </asp:Panel>
</div>

C#

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        ViewState["CountryName"] = "ALL";
        BindGrid();
    }
}

private void BindGrid()
{
    DataTable dt = new DataTable();
    String strConnString = System.Configuration.ConfigurationManager
                .ConnectionStrings["conString"].ConnectionString;
    SqlConnection con = new SqlConnection(strConnString);
    SqlDataAdapter sda = new SqlDataAdapter();
    SqlCommand cmd = new SqlCommand("[Customers_GetCustomersCountryWise]");
    cmd.CommandType = CommandType.StoredProcedure;
    if (ViewState["CountryName"] != null)
    {
        cmd.Parameters.AddWithValue("@CountryName", ViewState["CountryName"].ToString());
    }
    cmd.Connection = con;
    sda.SelectCommand = cmd;
    sda.Fill(dt);
    GridView1.DataSource = dt;
    GridView1.DataBind();
    DropDownList ddlCountry =
        (DropDownList)GridView1.HeaderRow.FindControl("ddlCountry");
    this.BindCountryList(ddlCountry);
}

protected void CountryChanged(object sender, EventArgs e)
{
    DropDownList ddlCountry = (DropDownList)sender;
    ViewState["CountryName"] = ddlCountry.SelectedValue;
    this.BindGrid();
}

private void BindCountryList(DropDownList ddlCountry)
{
    String strConnString = System.Configuration.ConfigurationManager
                .ConnectionStrings["conString"].ConnectionString;
    SqlConnection con = new SqlConnection(strConnString);
    SqlDataAdapter sda = new SqlDataAdapter();
    SqlCommand cmd = new SqlCommand("select distinct Country" +
                    " from customers");
    cmd.Connection = con;
    con.Open();
    ddlCountry.DataSource = cmd.ExecuteReader();
    ddlCountry.DataTextField = "Country";
    ddlCountry.DataValueField = "Country";
    ddlCountry.DataBind();
    con.Close();
    if (ViewState["CountryName"] != null)
    {
        ddlCountry.Items.FindByValue(ViewState["CountryName"].ToString())
                .Selected = true;
    }
}

protected void OnPaging(object sender, GridViewPageEventArgs e)
{
    GridView1.PageIndex = e.NewPageIndex;
    this.BindGrid();
}

protected void Review(object sender, EventArgs e)
{
    LinkButton lbtn = (sender as LinkButton);
    GridViewRow row = (GridViewRow)(lbtn.NamingContainer);
    lblContactName.Text = row.Cells[0].Text;
    lblCity.Text = row.Cells[1].Text;
    lblCountry.Text = ((Label)row.FindControl("lblCountry")).Text;
    lblPostalCode.Text = row.Cells[3].Text;
    this.mpe.Show();
}

Screenshot