'My Application Page' is not allowed here because it does not extend class 'System.Web.UI.Page'

I have a custom SharePoint 2010 solution that includes an aspx page. The aspx page in is in the /layouts folder within the solution and I created it by just adding an application page to the solution. I am trying to create a parent-child relationship between two different lists in SharePoint. From the parent I have a custom button on the ribbon that creates a child item with the ID of the parent stamped on it.

The page is just a processing page that forwards on parameters from the parent to the new child item. (i.e. the ID value)

The code generated when I add the aspx page is below:

<%@ Assembly Name="$SharePoint.Project.AssemblyFullName$" %>
<%@ Import Namespace="Microsoft.SharePoint.ApplicationPages" %>
<%@ Register Tagprefix="SharePoint" Namespace="Microsoft.SharePoint.WebControls" Assembly="Microsoft.SharePoint, Version=14.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" %>
<%@ Register Tagprefix="Utilities" Namespace="Microsoft.SharePoint.Utilities" Assembly="Microsoft.SharePoint, Version=14.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" %>
<%@ Register Tagprefix="asp" Namespace="System.Web.UI" Assembly="System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" %>
<%@ Import Namespace="Microsoft.SharePoint" %>
<%@ Assembly Name="Microsoft.Web.CommandUI, Version=14.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" %>
<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="processingpage.aspx.cs" Inherits="MY.Solution.Layouts.MY.Solution.processingpage" MasterPageFile="~/_layouts/application.master" %>

<asp:Content ID="PageHead" ContentPlaceHolderID="PlaceHolderAdditionalPageHead" runat="server">

</asp:Content>

<asp:Content ID="Main" ContentPlaceHolderID="PlaceHolderMain" runat="server">

</asp:Content>

<asp:Content ID="PageTitle" ContentPlaceHolderID="PlaceHolderPageTitle" runat="server"> Processing Page </asp:Content>

<asp:Content ID="PageTitleInTitleArea" ContentPlaceHolderID="PlaceHolderPageTitleInTitleArea" runat="server" > Processing Page    </asp:Content>

The code behind is as follows:

using System;
using Microsoft.SharePoint;
using Microsoft.SharePoint.Administration;
using Microsoft.SharePoint.WebControls;
using Microsoft.SharePoint.Utilities;
using System.Reflection;

namespace MY.Solution.Layouts.MY.Solution
{
public partial class processingpage : LayoutsPageBase
{
    protected void Page_Load(object sender, EventArgs e)
    {           
        try
        {
            //Get a reference to the SPWeb object 
            SPWeb oWeb = SPContext.Current.Web;

            //Use the Parameters That Are Passed In
            SPList thisList = oWeb.Lists[new Guid(Request.QueryString["List"])];
            SPListItem thisItem = thisList.GetItemById(int.Parse(Request.QueryString["ID"]));

            sContentType = thisItem["ContentType"].ToString();
            sContentTypeID = thisItem.ContentTypeId.ToString();

            if (sContentType == "Some Content Type")
            {                   
                sContentTypeID = "";                    
                sAIID = thisItem["ID"].ToString();
                //Redirect to newform.aspx with the Appropriate parameters.
                Context.Response.Redirect(oWeb.Url + "/Lists/Blist" + "/NewForm.aspx?AIID=" + sAIAuditID.ToString() + "&ContentTypeId=" + sContentTypeID + "&ParentItemID" + Context.Request["ID"]);
            }
            else if (sContentType == "Some Content Type")
            {

                sContentTypeID = "";

                sAIID = thisItem["AIID"].ToString();
                //Redirect to newform.aspx with the Appropriate parameters.
                Context.Response.Redirect(oWeb.Url + "/Lists/AList" + "/NewForm.aspx?AIID=" + sAIID.ToString() + "&ContentTypeId=" + sContentTypeID + "&ParentItemID" + Context.Request["ID"]);
            }
            else if (sContentType == "Some Content Type")
            {
                sContentTypeID = "";                    
                sAICID = thisItem["AICID"].ToString();
                //Redirect to newform.aspx with the Appropriate parameters.
                Context.Response.Redirect(oWeb.Url + "/Lists/CList" + "/NewForm.aspx?AICID=" + sAICID.ToString() + "&ContentTypeId=" + sContentTypeID + "&ParentItemID" + Context.Request["ID"]);
            }
            else
            {                   
                LoggingService.LogError("MY.Solution - Processing Page", "No Applicable Content Type Found.");
            }
        }
        catch (Exception ex)
        {               
            LoggingService.LogError("My.Solution - Processing Page", ex.Message);
        }
        finally
        {
            //DO SOME FINAL THINGS HERE WHEN REQUIRED.    
        } 

    }
}
}

In the page I need to use Request.QueryString to get the values from the URL. But when I deploy the solution and load the page I get the error:

'MY.Solution.Layouts.MY.Solution.processingpage' is not allowed here because it does not extend class 'System.Web.UI.Page'.

When I change the line:

<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="processingpage.aspx.cs" Inherits="MY.Solution.Layouts.MY.Solution.processingpage" MasterPageFile="~/_layouts/application.master" %>

to inherit as follows:

<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="processingpage.aspx.cs" Inherits="Microsoft.SharePoint.WebControls.LayoutsPageBase" MasterPageFile="~/_layouts/application.master" %>

it does not work either.

If I change it to inherit like below:

<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="processingpage.aspx.cs" Inherits="System.Web.UI.Page" MasterPageFile="~/_layouts/application.master" %>

it also does not work.

Why does the code by Visual Studio for a SharePoint application page not work?