Passing value from popup window to parent form's TextBox

Passing values from a popup window back to the parent page is an often asked question. Especially when there is a GridView type control in the popup. In the following example we will be using two forms. The parent form will be parent.aspx and the popup will be popup.aspx. Also note that the parent.aspx form is derived from some MasterPage. Code is provided both in VB.Net and C#.Net.
--- .aspx of parent form ---

<script type="text/javascript">

function OpenPopup() {

    window.open("popup.aspx","List","scrollbars=no,resizable=no,width=400,height=280");

    return false;

}

</script>

.      

.

.      

<asp:TextBox ID="txtPopupValue" runat="server" Width="327px"></asp:TextBox>

<asp:Button ID="Button1" runat="server" Text="Show List" />

--- .vb of parent.aspx if vb.net is the language ---

If Not IsPostBack Then

    Me.Button1.Attributes.Add("onclick", "javascript:return OpenPopup()")

End If

--- .cs of parent.aspx if C#.net is the language ---

if (!IsPostBack) {

    this.Button1.Attributes.Add("onclick", "javascript:return OpenPopup()");

}

--- .aspx of popup form ---

<script language="javascript">

function GetRowValue(val)

{

    // hardcoded value used to minimize the code.

    // ControlID can instead be passed as query string to the popup window

    window.opener.document.getElementById("ctl00_ContentPlaceHolder1_TextBox2").value = val;

    window.close();

}

</script>

<asp:GridView ID="GridView1" runat="server" DataSourceID="SqlDataSource1">

    <Columns>

        <asp:TemplateField>

            <AlternatingItemTemplate>

                <asp:Button ID="btnSelect" runat="server" Text="Select" />

            </AlternatingItemTemplate>

            <ItemTemplate>

                <asp:Button ID="btnSelect" runat="server" Text="Select" />

            </ItemTemplate>

        </asp:TemplateField>

    </Columns>

</asp:GridView>

--- .vb file if vb.net is the language ---

Protected Sub GridView1_RowDataBound(ByVal sender As Object, ByVal e As System.Web.UI.WebControls.GridViewRowEventArgs) Handles GridView1.RowDataBound

    If (e.Row.RowType = DataControlRowType.DataRow) Then

        'assuming that the required value column is the second column in gridview

        DirectCast(e.Row.FindControl("btnSelect"), Button).Attributes.Add("onclick", "javascript:GetRowValue('" & e.Row.Cells(1).Text & "')")

    End If

End Sub

--- .cs file if C#.net is the language ---

protected void GridView1_RowDataBound(object sender, System.Web.UI.WebControls.GridViewRowEventArgs e) {

    if ((e.Row.RowType == DataControlRowType.DataRow)) {

        //assuming that the required value column is the second column in gridview

        ((Button)e.Row.FindControl("btnSelect")).Attributes.Add("onclick", "javascript:GetRowValue('" + e.Row.Cells[1].Text + "')");

    }

}