Could you please specify your Sitefinity version and what error do you receive? Please, ensure you have also added a reference to the Microsoft.CSharp assembly in your project.
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Sitefinity CMS Ideas&Feedback Portal
and vote to affect the priority of the items