ValidateRequest = 'false' not working in .Net 4.0 and 4.5 in ASP.Net

To make the ValidateRequest work for .Net 4.0 or higher frameworks you will need to add the followings setting in the web.config <system.web> section

<httpRuntime requestValidationMode="2.0"/>

Once you add it then you can set validateRequest = false in web.config or in the @Page Directive. It will work!.

For more details refer ASP.Net Error: A potentially dangerous Request.Form value was detected from the client