In previous releases of ASP.NET AJAX, Microsoft had created a series of Validators that were compatible with the UpdatePanel and had included them in the distribution. These validators were implemented by mapping them in the web.config over the conventional validators included in the .NET Framework. In the final release, Microsoft removed these validators because they wanted to push them out as an update to the framework itself at a later time via Windows Update. What this means is that out of the box, you can not use any of the validators with an UpdatePanel.

Luckily, they made the source code for the validators available and they can be compiled and included in your project until the final update comes out. To get the validators, go to the link below.

ASP.NET AJAX Compatible Validators

To use the validators it will be required to map them over the existing validators like before. Copy these lines to the system.web/pages/tagMapping section of your web.config.

 <add tagType="System.Web.UI.WebControls.CompareValidator" 
mappedTagType="Sample.Web.UI.Compatibility.CompareValidator, Validators, 
         <add tagType="System.Web.UI.WebControls.CustomValidator" 
mappedTagType="Sample.Web.UI.Compatibility.CustomValidator, Validators, 
         <add tagType="System.Web.UI.WebControls.RangeValidator" 
mappedTagType="Sample.Web.UI.Compatibility.RangeValidator, Validators, 
         <add tagType="System.Web.UI.WebControls.RegularExpressionValidator" 
mappedTagType="Sample.Web.UI.Compatibility.RegularExpressionValidator, Validators, 
         <add tagType="System.Web.UI.WebControls.RequiredFieldValidator" 
mappedTagType="Sample.Web.UI.Compatibility.RequiredFieldValidator, Validators, 
         <add tagType="System.Web.UI.WebControls.ValidationSummary" 
mappedTagType="Sample.Web.UI.Compatibility.ValidationSummary, Validators, 

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