Tonight, while working on a new project, I encountered the following error when trying to use Enterprise Library (EL) Data Access Application Block from Microsoft's Patterns & Practices. This error caught me off guard because I've used EL a lot before.
Somehow, when I created the new project in Visual Studio 2010, I must've selected this project to compile based on the target framework ".NET Framework 4 Client Profile" instead of the typical ".NET Framework 4".
For those of you that run accross this error. Check the "advanced compile options" under your project properties.