Pass selected row from GridView inside Modal Popup to page in ASP.Net

Here I have created sample that will help you out.

CS.aspx

<div>
    <cc1:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server">
    </cc1:ToolkitScriptManager>
    <asp:LinkButton ID="lnkDummy" runat="server"></asp:LinkButton>
    <cc1:ModalPopupExtender ID="ModalPopupExtender1" BehaviorID="mpe" runat="server"
        PopupControlID="pnlPopup" TargetControlID="lnkDummy" BackgroundCssClass="modalBackground"
        CancelControlID="btnHide">
    </cc1:ModalPopupExtender>
    <asp:Panel ID="pnlPopup" runat="server" CssClass="modalPopup" Style="display: none">
        <div class="header">
            Modal Popup
        </div>
        <div class="body">
            <asp:GridView ID="GridView1" runat="server" HeaderStyle-BackColor="green" AutoGenerateColumns="false"
                Font-Names="Arial" OnPageIndexChanging="OnPaging" Font-Size="11pt" AlternatingRowStyle-BackColor="#C2D69B"
                AllowPaging="true">
                <Columns>
                    <asp:TemplateField>
                        <ItemTemplate>
                            <asp:RadioButton ID="RadioButton1" runat="server" />
                        </ItemTemplate>
                    </asp:TemplateField>
                    <asp:BoundField ItemStyle-Width="150px" DataField="CustomerID" HeaderText="CustomerID" />
                    <asp:BoundField ItemStyle-Width="150px" DataField="City" HeaderText="City" />
                    <asp:BoundField ItemStyle-Width="150px" DataField="PostalCode" HeaderText="PostalCode" />
                </Columns>
            </asp:GridView>
            <br />
            <asp:Button ID="btnHide" runat="server" Text="Hide Modal Popup" />
            &nbsp;&nbsp;<asp:Button ID="Button1" Text="Send" runat="server" OnClick="SendRowData" />
        </div>
    </asp:Panel>
    <br />
</div>

Code

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {

        ModalPopupExtender1.Show();
        GetSelectedRecord();
        BindGrid();
        Session["SelectedData"] = null;
    }
}

private void BindGrid()
{
    string strQuery = "SELECT TOP 10 CustomerID,City,PostalCode FROM CUSTOMERS";
    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(strQuery);
    cmd.CommandType = CommandType.Text;
    cmd.Connection = con;
    try
    {
        con.Open();
        sda.SelectCommand = cmd;
        sda.Fill(dt);
        GridView1.DataSource = dt;
        GridView1.DataBind();
    }
    catch (Exception ex)
    {
        throw ex;
    }
    finally
    {
        con.Close();
        sda.Dispose();
        con.Dispose();
    }
}

private void GetSelectedRecord()
{
    for (int i = 0; i < GridView1.Rows.Count; i++)
    {
        RadioButton rb = (RadioButton)GridView1.Rows[i].Cells[0].FindControl("RadioButton1");
        if (rb != null)
        {
            if (rb.Checked)
            {
                HiddenField hf = (HiddenField)GridView1.Rows[i].Cells[0].FindControl("HiddenField1");
                if (hf != null)
                {
                    ViewState["SelectedContact"] = hf.Value;
                }
                break;
            }
        }
    }
}

private void SetSelectedRecord()
{
    for (int i = 0; i < GridView1.Rows.Count; i++)
    {
        RadioButton rb = (RadioButton)GridView1.Rows[i].Cells[0].FindControl("RadioButton1");
        if (rb != null)
        {
            HiddenField hf = (HiddenField)GridView1.Rows[i].Cells[0].FindControl("HiddenField1");
            if (hf != null && ViewState["SelectedContact"] != null)
            {
                if (hf.Value.Equals(ViewState["SelectedContact"].ToString()))
                {
                    rb.Checked = true;
                    break;
                }
            }
        }
    }
}

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

protected void SendRowData(object sender, EventArgs e)
{
    DataTable table = new DataTable();
    GridViewRow headerRow = GridView1.HeaderRow;
    foreach (TableCell item in headerRow.Cells)
    {
        DataColumn column = new DataColumn();
        column.ColumnName = item.Text;
        table.Columns.Add(column);
    }

    foreach (GridViewRow rows in GridView1.Rows)
    {
        if (((RadioButton)rows.FindControl("RadioButton1")).Checked)
        {
            DataRow row = table.NewRow();
            int i = 0;
            foreach (TableCell item in rows.Cells)
            {
                row[i] = item.Text;
                i++;
            }
            table.Rows.Add(row);
        }
    }
    Session["SelectedData"] = table;
    Response.Redirect("~/Default.aspx");
}

Default.aspx

<div>
    <asp:Button Text="Back" runat="server" OnClick="Back" />
</div>

Code

protected void Page_Load(object sender, EventArgs e)
{
    if (Session["SelectedData"] != null)
    {
        DataTable selectedData = (DataTable)Session["SelectedData"];
        HtmlTable table = new HtmlTable();
        HtmlTableRow tableHeaderRow = new HtmlTableRow();
        foreach (DataColumn item in selectedData.Columns)
        {
            HtmlTableCell cell = new HtmlTableCell();
            cell.InnerHtml = item.ColumnName;
            tableHeaderRow.Cells.Add(cell);
        }
        table.Rows.Add(tableHeaderRow);

        foreach (DataRow rows in selectedData.Rows)
        {
            HtmlTableRow row = new HtmlTableRow();
            foreach (var item in rows.ItemArray)
            {
                HtmlTableCell cell = new HtmlTableCell();
                cell.InnerHtml = item.ToString();
                row.Cells.Add(cell);
            }
            table.Rows.Add(row);
        }
        this.form1.Controls.Add(table);
    }
}

protected void Back(object sender, EventArgs e)
{
    Response.Redirect("~/CS.aspx");
}

Screenshot