asp checkbox state is not updated on postback

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ASP.NET

Javascript

C#3.5

I am stuck at very silly place..

what is happening on page load event i am trying to set a checkbox checked state =true;


auto post back of checkbox is not set .. and when i submit button again on load i am trying to set value=true after uncheck the checkbox

but its state is still unchecked

Load event of page is

protected void Page_Load(object sender, EventArgs e)
   {
       this.chkMain.Attributes.Add("onclick", "EnableDisableButton(this)");
       this.chkMain.Checked = true;

   }

and

aspx page is this

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

<script type="text/javascript">
    function EnableDisableButton(obj) {

        var btnSaveID=<%=btnSave.ClientID %>;

        if(obj.checked)
        {

        document.getElementById(btnSaveID.id).value='raman';

        }
        else
         document.getElementById(btnSaveID.id).value='Save';

    }
</script>
<asp:CheckBox runat="server" ID="chkMain" Text="CheckFirst" />
<asp:Button Text="Save" ID="btnSave" runat="server" />
</asp:Content>

please suggest..