.ascx file in asp.net application

If you have any common functionality that should be used in multiple pages, then that is the right candidate for user control (ascx). Here's the sample code that explains how to register a user control and how to access it. Hope I answered your question.

Here's the parent page with user control registration and accessing it.

<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="Page1.aspx.cs"
Inherits="Page1" %>

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Refer the Following Links : https://msdn.microsoft.com/en-us/library/wt3k2fyw.aspx

https://msdn.microsoft.com/en-us/library/sbz9etab.aspx