 namespace MartinOnDotNet.MasterPageBodyClass
 {
     public class BasePage : System.Web.UI.Page
     {
         public string BodyCssClass { get; set; }  
     }
 }
 
 using System;
  
 namespace MartinOnDotNet.MasterPageBodyClass
 {
     public partial class Global : System.Web.UI.MasterPage
     {
         protected override void OnPreRender(EventArgs e)
         {
             base.OnPreRender(e);
             BasePage current = Page as BasePage;
             if (current != null && !string.IsNullOrEmpty(current.BodyCssClass))
             {
                 Body.Attributes["class"] = current.BodyCssClass;
             }
         }
     }
 }
 
  <%@ Master Language="C#" AutoEventWireup="true" CodeBehind="Global.master.cs" 
	Inherits="MartinOnDotNet.MasterPageBodyClass.Global" %>
 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" 
	"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
 <html xmlns="http://www.w3.org/1999/xhtml" >
 <head runat="server">
     <asp:ContentPlaceHolder ID="head" runat="server">
     </asp:ContentPlaceHolder>
 </head>
 <body runat="server" id="Body">
     <form id="form1" runat="server">
     <div>
         <asp:ScriptManager ID="ScriptManager1" runat="server" />
         <asp:ContentPlaceHolder ID="ContentPlaceHolder1" runat="server">
         
         </asp:ContentPlaceHolder>
     </div>
     </form>
 </body>
 </html>
 
  <%@ Page Language="C#" 
     MasterPageFile="~/Global.Master" 
     AutoEventWireup="true" 
     Title="Content Page" 
     Inherits="MartinOnDotNet.MasterPageBodyClass.BasePage"
     BodyCssClass="content" %>
  
 <asp:Content ID="Content1" ContentPlaceHolderID="head" runat="server">
 </asp:Content>
 <asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" runat="server">
 </asp:Content>