I want to use nested master page so i create the following master page :
<%@ Master Language="C#" AutoEventWireup="true" CodeBehind="test.master.cs" Inherits="DocumentFlowUI.test" MasterPageFile="~/MasterPage2.master" %>
<!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">
<title></title>
<asp:ContentPlaceHolder ID="head" runat="server">
</asp:ContentPlaceHolder>
</head>
<body>
<form id="form1" runat="server">
<div>
<asp:ContentPlaceHolder ID="ContentPlaceHolder1" runat="server">
<asp:Button ID="Button1" runat="server" Text="Button" />
</asp:ContentPlaceHolder>
</div>
</form>
</body>
</html>
and i create the following page to use that master page :
<%@ Page Title="" Language="C#" MasterPageFile="~/test.Master" AutoEventWireup="true" CodeBehind="WebForm4.aspx.cs" Inherits="DocumentFlowUI.WebForm4" %>
<asp:Content ID="Content1" ContentPlaceHolderID="head" runat="server">
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" runat="server">
<asp:Label ID="Label1" runat="server" Text="Label"></asp:Label>
</asp:Content>
I get the following error :
Content controls have to be top-level controls in a content page or a nested master page that references a master page
The HTML-code in your nested masterpage must be wrapped with an asp:content-tag with a contentplaceholderid from the "master" masterpage.
Just to demonstrate Erik's point:
Parent masterpage:
<asp:ContentPlaceHolder ID="head" runat="server" />
Child masterpage:
<asp:Content ID="Content1" ContentPlaceHolderID="head" runat="server">
<asp:ContentPlaceHolder ID="head" runat="server" />
</asp:Content>
Page:
<asp:Content ID="Content1" ContentPlaceHolderID="head" runat="server">
<!-- content -->
</asp:Content>
来源：https://stackoverflow.com/questions/13745449/content-controls-have-to-be-top-level-controls-in-a-content-page-or-a-nested-mas