Error when passing friend class as type from public class to friend base class

Question

Disclaimer: I am fairly new to working with generics so I am not entirely sure if what I am trying to do even makes sense or is possible.

I have a bunch of user controls in a project. All of these user controls share a similar property so I want to move it into a base class. The only difference is the return type of the property.

I have three classes interacting in this scenario. The first class is a base type, which inherits from CompositeControl and will be inherited by other classes in my project:

Friend Class MyBaseClass(Of T As {New})
    Inherits CompositeControl

    Private _someProperty As T = Nothing

    Protected ReadOnly Property SomeProperty As T
        Get
            // dumbed down for the sake of example
            If _someProperty Is Nothing Then
                _someProperty = New T()
            End If

            Return _someProperty 
        End Get
    End Property
End Class

Then I have this control class, which inherits from MyBaseClass:

Public Class MyControlClass
    Inherits MyBaseClass(Of MyReturnTypeClass)

    // snip...
End Class

And finally MyReturnTypeClass which is what the base's SomeProperty should return:

Friend Class MyReturnTypeClass
    Public Property AutoProperty1 As Boolean = False
    Public Property AutoProperty2 As String = String.Empty
    // etc
End Class

When I attempt to build the project, I get this error from MyControlClass:

Inconsistent accessibility: type argument 'MyReturnTypeClass' is less accessible than Class 'MyControlClass'.

I need MyControlClass to be Public so it can be consumed by other projects, and I also want the MyBaseClass and MyReturnTypeClass to be Friend so they cannot be seen/used by consumers. Am I just missing some special keyword somewhere or is this not possible?

score 2 · Answer 1 · answered Dec 17 '12 at 17:11

The base class must be at least as accessible as the derived class. This is a language restriction (see here).

If you intend to avoid MyBaseClass being instantiated by consumers, consider marking it Public MustInherit instead of Friend. Hope this helps.

score 2 · Accepted Answer · answered Dec 17 '12 at 17:15

You cannot inherit from a base class that is less accessible than the derived class. So for instance, this won't work:

Friend Class MyBase
End Class

Public Class MyDerived
    Inherits MyBase  ' Won't compile because MyBase is less accessible
End Class

Therefore, since in your example, MyBaseClass(T) has is a friend type, but you are trying to inherit from it into a public MyControlClass type. Therefore, even if you took generics and MyReturnTypeClass out of the "equation", it still wouldn't work.

However, with generics, even if no member of the public interface of the class actually uses the generic type, the type must still be at least as accessible as the derived type. For instance:

Public Class MyBase(Of T)
    ' T not actually used at all
End Class

Friend Class MyOtherType
End Class

Public Class MyDerived
    Inherits MyBase(MyOtherType)  ' Won't compile because MyOtherType is less accessible
End Class

this explains why the code I have will not compile, but does not provide any alternatives? do I have no other choice than marking `MyReturnTypeClass` as `Public`? — jbabey, Dec 17 '12 at 17:58
Yes, if it is going to be a base class for a public derived class, then it must also be public. As channs pointed out, you could add the MustInherit modifier to the base class so that other projects couldn't use it directly. — Steven Doggart, Dec 17 '12 at 18:21

Error when passing friend class as type from public class to friend base class

2 Answers2