MembershipProvider in .NET 4.0
Interesting. However, build errors are your friend ;)
Attempting to build a class library built as you describe, I get the following build error:
The type name 'MembershipProvider' could not be found. This type has been forwarded to assembly 'System.Web.ApplicationServices, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35'. Consider adding a reference to that assembly.
Following the instructions (adding a reference to System.Web.ApplicationServices) allows me to carry on as expected.
The MembershipProvider abstract class is part of the System.Web.ApplicationServices namespace in the .net 4.0 Framework. Add a reference to that assembly and it should solve the issue.
There is an reference not set oder an Missing usingNamespace is System.Web.Security
for more info checkt out MSDN