Answer.

Background of the Question

Encapsulation is one of the pillars of object-oriented programming, achieved through access modifiers like Private and Protected. Classic versions of Visual Basic supported only basic visibility levels, but with the transition to VB.NET, modern mechanisms similar to C# emerged.

The Problem

The main task is to isolate internal implementation details from external code, including other parts of the program. Typical mistakes are related to incorrect access levels, attempts to access fields and methods outside allowed visibility areas, or misunderstandings of Protected behavior and its combinations with other modifiers.

The Solution

The following modifiers are supported:

Private — access only within the current class/module
Protected — access within the current class and all descendants (even from other assemblies)
Friend — access within a single assembly
Protected Friend — access within descendants or within the assembly

Example code:

Public Class BaseClass
    Private Sub PrivateMethod()
        Console.WriteLine("PrivateMethod")
    End Sub
    Protected Sub ProtectedMethod()
        Console.WriteLine("ProtectedMethod")
    End Sub
    Friend Sub FriendMethod()
        Console.WriteLine("FriendMethod")
    End Sub
    Protected Friend Sub ProtectedFriendMethod()
        Console.WriteLine("ProtectedFriendMethod")
    End Sub
End Class

Public Class DerivedClass
    Inherits BaseClass
    Public Sub AccessMethods()
        'PrivateMethod() 'Compilation error
        ProtectedMethod() 'Ok
        FriendMethod() 'Ok, if in the same assembly
        ProtectedFriendMethod() 'Ok
    End Sub
End Class

Key features:

The prefix Private restricts the visibility to the boundaries of the class
Protected works in all inherited classes across any assemblies
Protected Friend combines both rules

Trick Questions.

Can you access a private field from a derived class?

No, private members are always accessible only in the class where they are declared. A derived class does not have access to private members even through reflection (unless using non-standard methods).

What is the difference between Protected and Protected Friend?

Protected — accessible only from the class and its descendants, even in other assemblies; Protected Friend — accessible either from derived classes or any code within the same assembly.

Can a protected method of a class be accessed through an instance of the base class?

No, even if the protected method is publicly visible within the descendant, it cannot be invoked on an instance of the base class from external code. Protected methods are accessible only within the body of the class or its descendant.

Typical Errors and Anti-patterns

Exposing all methods as public or friend for speed
Using protected fields where private properties should be
Violating encapsulation due to excessively broad access

Real-Life Example

Negative Case

A developer for ease of testing makes internal fields public to access them directly from external code or unit tests. Over time, other calls begin to use these fields, relying on their internal structure.

Pros:

Quick testing
Less code for access

Cons:

Weak encapsulation
Increased number of bugs when the internal implementation changes
Difficulty in modifying the class

Positive Case

Access modifiers are applied clearly, all fields are private by default, and access from outside is only through properties and public methods. For testing needs, interfaces or friend classes within the same assembly are used.