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.

On my case, I am using Visual Studio 2019, I need to add both of these:

System.Web.ApplicationServices

and

System.Configuration

In order to build it successfully upon inheriting MembershipProvider to create my own MyMembershipProvider class.

There is an reference not set oder an missing using namespace is System.Web.Security Check if assembly System.Web.ApplicationServices.dll is referenced

for more info checkt out MSDN