🧾 ASP.NET – Directives – Configure Page Behavior at Compile-Time

🧲 Introduction – What Are ASP.NET Directives?

ASP.NET directives are special instructions used in .aspx and other ASP.NET files to control how pages and controls are compiled, processed, and rendered. They appear at the top of a page and are enclosed within <%@ ... %> tags.

Directives define important settings such as:

• Programming language
• Code-behind files
• Master pages
• Namespaces to import
• Output caching behavior

🎯 In this guide, you’ll learn:

• Different types of ASP.NET directives
• How to use the @Page, @Control, @Master, @Import, @OutputCache, and others
• Real examples with explanation of how each affects page behavior

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🧱 Common ASP.NET Directives Overview

🧾 Directive	📌 Description
@Page	Defines settings for .aspx pages (language, code file, etc.)
@Control	Used in user controls (.ascx)
@Master	Used in master pages (.master)
@Import	Imports namespaces
@Register	Links custom controls and user controls with a tag prefix
@OutputCache	Enables output caching for the page
@Reference	References other pages or user controls
@Assembly	Links to external assemblies

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✅ @Page Directive – For .aspx Pages

Placed at the top of every .aspx page to configure its compilation and execution.

<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="Default.aspx.cs" Inherits="MyApp.Default" %>

🔍 Explanation:

• Language="C#": Sets the programming language
• AutoEventWireup="true": Automatically binds events like Page_Load
• CodeBehind: Points to the backend file
• Inherits: Specifies the class that inherits from System.Web.UI.Page

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✅ @Control Directive – For User Controls

Used in .ascx files to define control settings.

<%@ Control Language="C#" AutoEventWireup="true" CodeBehind="MyControl.ascx.cs" Inherits="MyApp.MyControl" %>

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✅ @Master Directive – For Master Pages

Specifies the class and language for master pages (.master).

<%@ Master Language="C#" AutoEventWireup="true" CodeBehind="Site.master.cs" Inherits="MyApp.SiteMaster" %>

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✅ @Import Directive – Import Namespaces

Allows the page to access external .NET namespaces.

<%@ Import Namespace="System.Data" %>
<%@ Import Namespace="System.Text" %>

🔍 Used to access ADO.NET, string builders, collections, etc.

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✅ @Register Directive – Use Custom or User Controls

<%@ Register TagPrefix="uc" TagName="Header" Src="~/Controls/Header.ascx" %>

🔍 Usage in page body:

<uc:Header runat="server" ID="ucHeader" />

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✅ @OutputCache Directive – Enable Page Caching

<%@ OutputCache Duration="60" VaryByParam="None" %>

🔍 Explanation:

• Duration="60": Cache output for 60 seconds
• VaryByParam: Can vary output based on query string or form input

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✅ @Reference Directive – Reference Other Pages/Controls

Used for strongly-typed access to previous pages.

<%@ Reference Page="~/PreviousPage.aspx" %>

🔍 Useful for cross-page posting and accessing properties of the previous page.

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✅ @Assembly Directive – Load External DLLs

<%@ Assembly Name="MyLibrary" %>

🔍 Allows use of external assemblies without adding reference in project file.

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🧪 Example – Page with Multiple Directives

<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="Default.aspx.cs" Inherits="SampleApp._Default" %>
<%@ Import Namespace="System.Data" %>
<%@ Register TagPrefix="uc" TagName="Footer" Src="~/Controls/Footer.ascx" %>
<%@ OutputCache Duration="30" VaryByParam="None" %>

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📘 Best Practices for ASP.NET Directives

✅ Do:

• Always use @Page with proper CodeBehind and Inherits
• Use @OutputCache for performance boost
• Keep directives at the very top of the file

❌ Avoid:

• Mixing directive types (e.g., @Page in .ascx)
• Forgetting @Register when using user controls
• Overusing @Import—import only necessary namespaces

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📌 Summary – Recap & Next Steps

ASP.NET directives are essential for configuring how pages and controls behave, compile, and cache. They are the building blocks for how the server interprets and processes your ASP.NET files.

🔍 Key Takeaways:

• @Page is mandatory for .aspx pages
• Use @Register to integrate user/custom controls
• @OutputCache can improve page load speed

⚙️ Real-world Use Cases:

• Control partial caching on frequently hit pages
• Load third-party libraries via @Assembly
• Build modular websites with registered controls

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❓ FAQs – ASP.NET Directives

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❓ Where should I place directives in ASP.NET pages?
✅ Always at the very top of the page, before any HTML or server-side script.

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❓ Can I use multiple @Import directives in one page?
✅ Yes. You can use as many as needed for different namespaces.

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❓ What is the purpose of AutoEventWireup?
✅ If true, ASP.NET automatically connects event methods (like Page_Load) based on naming convention.

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❓ Is @Register required for every user control?
✅ Yes. Every custom or user control must be registered with a unique tag prefix and tag name.

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