Master Page:
Code:
<%@ Master Language="C#" AutoEventWireup="true" CodeBehind="Site.master.cs" Inherits="SBIPensionProcess.SiteMaster" %>
has contentPlace holder
Code:
<asp:ContentPlaceHolder ID="cplMain" runat="server">
                            </asp:ContentPlaceHolder>
Nested Master
Code:
<%@ Master Language="C#" MasterPageFile="~/MasterPage/Site.Master" AutoEventWireup="true" CodeBehind="NewEntryMaster.master.cs"
    Inherits="SBIPensionProcess.NewEntryMaster" %>
has conent place holder
Code:
<asp:Content ID="MainContent" ContentPlaceHolderID="cplMain" runat="server">
             <asp:ContentPlaceHolder ID="cplEntryScreen" runat="server">
    </asp:ContentPlaceHolder>
</asp:Content>

Content Page:
Code:
<%@ Page Title="" Language="C#" MasterPageFile="~/MasterPage/NewEntryMaster.master" AutoEventWireup="true"
    CodeBehind="PPOPersonal.aspx.cs" Inherits="SBIPensionProcess.PPOPersonal" %>

<asp:Content ID="Personal" ContentPlaceHolderID="cplEntryScreen" runat="server">
//controls

</asp:Content>

now i want to make all text boxes as readonly in PPOPersonal.aspx.cs
My code

Code:
foreach (System.Web.UI.Control c in Master.FindControl("cplMain").FindControl("cplEntryScreen").Controls)
            {
                if (c is System.Web.UI.WebControls.TextBox)
                {
                    ((System.Web.UI.WebControls.TextBox)(c)).ReadOnly = true;

                }

              }
I'm getting exception for null reference