Non working dialogs when xhtmlConformance mode Legacy is set in the web.config

Updated on Dec 16, 2025

PROBLEM

Non working dialogs when xhtmlConformance mode="Legacy" is set in the web.config

SYMPTOMS

The following symptons or errrors could be caused by the xhtmlConformance mode="Legacy" web.config setting:

1. Blank managers

2. Non accessible textboxes, dropdowns or other elements in the dialogs

3. Error: Sys.WebForms is undefined

SOLUTION

The problem is caused by the following line in project's web.config file:

ASP.NET

<xhtmlConformance mode="Legacy"/>

This setting is incompatible with ASP.NET AJAX controls (like RadEditor) and it should be removed. This setting could be added when upgrading an old project with Visual Studio.