Get ChekBox Component in Gridview Inside Content Template - asp.net

Question

I have this aspx snippet:
<asp:updatepanel runat="server" id="resultPanel">
<contenttemplate>
<app:exgridview id="referenceGridView" runat="server" allowpaging="True" allowsorting="True">
<Columns>
<asp:TemplateField>
<HeaderTemplate>
<asp:CheckBox ID="checkAllBox" runat="server" CausesValidation="false"></asp:CheckBox>
</HeaderTemplate>
<asp:TemplateField>
.....
How do I get checkAllBox component in .vb code behind?
I have tried this:
referenceGridView.HeaderRow _
.Cells(0).FindControl("checkAllBox")
And
referenceGridView.FindControl("checkAllBox")
but it doesn't work neither.

Answer 1

<asp:GridView OnRowDataBound="MyGridView_RowDataBound" ... />
Then define MyGridView_RowDataBound:
void CustomersGridView_RowDataBound(Object sender, GridViewRowEventArgs e)
{
Label l = (Label) e.Row.FindControl("lblName");
}

Answer 2

EDITED
You can find the control in GridView.HeaderRow:
NOTE : This code block should be executed after referenceGridView.DataBind(). I am running this in a button click:
Protected Sub btnTest_Click(sender As Object, e As EventArgs) Handles btnTest.Click
Dim checkAllBox As CheckBox = TryCast(referenceGridView.HeaderRow.FindControl("checkAllBox"), CheckBox)
If checkAllBox IsNot Nothing Then
'checkAllBox exists here.
'Place your code for checkAllBox
If checkAllBox.Checked Then
lblResult.Text = "All checked"
Else
lblResult.Text = "All not checked"
End If
End If
End Sub
Presuming that you already have this in page load:
Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs)
'clear xml on page load if it contain data
If Not Page.IsPostBack Then
'Getting datasource
'referenceGridView.DataSource = MyDataSource.GetTable()
referenceGridView.DataBind()
End If
End Sub
And my test button in markup:
<asp:button id="btnTest" runat="server" Text="TEST" />
And you can download the test project here.