Using Tag Mapping in ASP.NET 2.0

Provided by: Dave Juth, Senior Systems Architect

Tag mapping allows you to swap compatible controls at compile time on every page in your web application. A useful example is if you have a stock ASP.NET control, such as a DropDownList, and you want to replace it with a customized control that is derived from DropDownList. This could be a control that has been customized to provide more optimized caching of lookup data. Instead of editing every web form and replacing the built in DropDownLists with your custom version, you can have ASP.NET in effect do it for you by modifying web.config:

<pages>

  <tagMapping>

    <clear />

    <add tagType="System.Web.UI.WebControls.DropDownList" mappedTagType="SmartDropDown"/>

  </tagMapping>

</pages>

When you run your application, ASP.NET will replace every instance of DropDownList with SmartDropDown, which is derived from DropDownList:

public class SmartDropDown : DropDownList

{

    public SmartDropDown()

    {

    }

    // ... etc.

}

ASP.NET will throw a compile error if you specify controls that are not compatible:

<pages>

  <tagMapping>

    <clear />

    <add tagType="System.Web.UI.WebControls.DropDownList" mappedTagType=" System.Web.UI.WebControls.Label"/>

  </tagMapping>

</pages>

Tag mapping is a very effective way of implementing custom controls in your ASP.NET application, or for quickly testing custom controls. Make sure your mapped type is compatible with the type it will replace and you can make your web application development much easier by using this new feature of ASP.NET 2.0.

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