ASP.NET Inherit Page

Child.aspx

// MasterPage(aspx)
<%@ Page Title="" Language="C#" MasterPageFile="~/Site.Master" AutoEventWireup="true" CodeBehind="WebForm1.aspx.cs" Inherits="WebApplicationSample.WebForm1" %>
<asp:Content ID="Content1" ContentPlaceHolderID="head" runat="server">
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" runat="server">
    <asp:Button ID="Button1" runat="server" Text="Button" OnClick="Button1_Click" />
</asp:Content>

// MasterPage(.cs)
using System;

namespace WebApplicationSample
{
    public partial class WebForm1 : WebApplicationBase.WebFormBase
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            // "ab" will be render. "c" will not render.
            Response.Write(Session["a"]);
            Response.Write(Session["b"]);
            Response.Write(Session["c"]);
        }

        protected void Button1_Click(object sender, EventArgs e)
        {
            // "abcc" will be render.
            Response.Write(Session["c"]);
        }
    }
}

Child.Master

// MasterPage(aspx)
<%@ Master Language="C#" AutoEventWireup="true" CodeBehind="Site.master.cs" Inherits="WebApplicationSample.Site" %>

// MasterPage(.cs)
using System;

namespace WebApplicationSample
{
    public partial class Site : WebApplicationBase.WebApplicationBase
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            //Page_Load Event will happen after WebForm1.Page_Load Event.
            Session["C"] = "c";
        }
    }
}

Parent.aspx

// WebForm(aspx)
<%@ Page Title="" Language="C#" MasterPageFile="~/WebApplicationBase.Master" AutoEventWireup="true" CodeBehind="WebFormBase.aspx.cs" Inherits="WebApplicationBase.WebFormBase" %>

// WebForm(.cs)
using System;

namespace WebApplicationBase
{
    public partial class WebFormBase : System.Web.UI.Page
    {
        protected void Page_Init(object sender, EventArgs e)
        {
            Session["b"] = "b";
        }
    }
}

Parent.Master

// MasterPage(aspx)
<%@ Master Language="C#" AutoEventWireup="true" CodeBehind="WebApplicationBase.master.cs" Inherits="WebApplicationBase.WebApplicationBase" %>

// MasterPage(.cs)
using System;

namespace WebApplicationBase
{
    public partial class WebApplicationBase : System.Web.UI.MasterPage
    {
        protected void Page_Init(object sender, EventArgs e)
        {
            Session["a"] = "a";
        }
    }
}