C# - Finding Control An A Page With Master Page?

I have to find a Control(a Table) in an aspx page with master page :

the Master page have this :

<asp:ContentPlaceHolder ID="MainContent" runat="server"/>

and int the child page :

<asp:Content ID="Content1" ContentPlaceHolderID="HeadContent" Runat="Server">
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="MainContent" Runat="Server">
</asp:Content>

I insert my Table in the Content2.

I used this code to get it :

protected void Ok_Click(object sender, EventArgs e)
{
Table tblForm = this.FindControl("MainContent").FindControl("formtable") as Table;
}