using System;
using System.Collections.Generic;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.HtmlControls;

public partial class test : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        HtmlGenericControl x = (HtmlGenericControl)Master.FindControl("body");
        x.Attributes.Add("class", "test");
    }
}