When the standard toolkit of ASP.Net server controls do not provide the required functionality, Web User controls can be built or used. Web User controls are built in the same way as Web Forms (.aspx), though it cannot contain the <HTML>, <BODY>, and <FORM> html elements, as these will be held in the Web Form where the control will be hosted.
Web User controls in ASP.NET end with the extension .ascx. They can be used in an ASP.NET page by registering them either on the page with the Register tag or in the web.config file. User Controls are very useful when reusing logic. Custom server controls are used in a situations where the code will be reused through a DLL.
Example
<%@ Register src="~/Controls/LatestPosts.ascx" TagName="LatestPosts" TagPrefix="pageElements" %>
<pageElements:LatestPosts ID="latest1" runat="server"></pageElements:LatestPosts>
Links
This article was originally posted at
http://wiki.asp.net/page.aspx/379/ascx
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