What is Asp:Content?

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What the hell is he? What does he do? 'Cause I can’t get one <body> inside him? I can change something in it via CSS?

Type of project I did, vs default does not let the page run without this tag, but I wanted to change my form via CSS and I can’t put the html tag inside it, either the one described in the third question or the <form>

asp.net aspx

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To understand how the tag works <asp:Content...> it is necessary to understand the concepts of master page and content page.

The Asp.Net allows a page to be segmented into small parts, being a "fixed" part that is called master page, which may be a layout.

Imagine that all or several pages of your site will have one layout similar, like header and menu at the top and a footer at the end. To avoid having to "duplicate" this code on each page, you create a "master page" with this layout, and create other pages only with content that will change.

Take this example:

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This page, in addition to the html content, will have a structure similar to this one, and we will call it "Master1.master":

<%@ Master Language="CS" %>
<body>
    <form id="form1" runat="server">
       <div><asp:contentplaceholder id="Header" runat="server" /></div>
       <div><asp:contentplaceholder id="PageSpecific" runat="server" /></div>
       <div><asp:contentplaceholder id="Advertisement" runat="server" /></div>
       <div><asp:contentplaceholder id="Recomended" runat="server" /></div>
       <div><asp:contentplaceholder id="Footer" runat="server" /></div>
    </form>
</body>

Note two things:

  • the tag <%@ Master at the beginning, indicating that it is a master page
  • the tag <asp:contentplaceholder, indicating that some page will "implement" that part

Now comes the second part, the content page, which is the content page that will "compose" a part of the master page, and this is where the tag of your question comes in, <asp:Content>.

Let’s see a page that will compose the "header" for example:

<%@ Page Language="CS" MasterPageFile="~/MasterPages/Master1.master" Title="Content Page" %>

<asp:Content ID="Header1" ContentPlaceHolderID="Header" Runat="Server">
    <header>Aqui vai o header </header>
</asp:Content>

Note here two important things:

  • the attribute MasterPageFile="~/MasterPages/Master1.master", saying that this page will compose a part of the master page "Master1.master", the one created above.
  • the tag <asp:Content ID="Header1" ContentPlaceHolderID="Header", indicating that the contents of this page will compose the "Header" of master page.

That is, it is a partial page, hence the limitation on the tags. As it is part of a master, tags like <html>, <head> and <body> should not exist here, but in master page, because at the end both will be rendered as a single page, and we cannot have these duplicated tags in the same document, but you can add css to it and modify the style normally.

If you want to know more, here’s a reference from the Microsoft documentation: ASP.NET Master Pages Overview

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