ValidateRequestMode is .NET 4.5 project

In a .net 4.5 web project i set the property ValidateRequestMode="Disabled" in a TextBox and in WebConfig requestValidationMode=4.5

When I try to build I get the following error.

System.Web.UI.WebControls.TextBox' does not contain a definition for 'ValidateRequestMode' and no extension method 'ValidateRequestMode' accepting a first argument of type 'System.Web.UI.WebControls.TextBox' could be found (are you missing a using directive or an assembly reference?) 

But if I execute the WebSite on IIS it is OK!

Is there something missing in Visual Studio (2012)?