I am working now on a large web application, that need to be used by more than one websites (at least 5 of them, websites and web services), therefore I have needed to do some isolating here with my main core projects.

Some background...
I have a common assembly (web application) that holds only the user controls, server controls and custom controls, which need to serve the all other web applications that are using them. This assembly has a reference to the other web application in order to have some information about some properties, session variables and global members, by this information, it knows to gereate some actions on runtime (or even in design). BUT, in the other hand, this web application need to use the controls that the first assembly has published, here we have a problem, we got a circular references, which is now allowed in .NET framework, also it isn't allowed anywhere I think...

So, how we gonna solve this problem?

The solution is quite simple and is known as Seperate Interface Pattern. (Click here to get some more info).

The main steps to implement it are:
Let define a project that called ProjectA and holds the user controls (etc...) implementations along with InterfaceB. ProjectA would maintain a reference to InterfaceB, which will hold any properties such as members, methods, events etc...

Now, lets define ProjectB which will implement InterfaceA. Now, ProjectB would reference ProjectA and (BUT) ProjectA would not reference ProjectB of course.

The result, ProjectA can access to ProjectB's specific exposed members and ProjectB can use the controls of ProjectA.