# Overview
The **`private protected` access modifier** is introduced in C# 7.0 and specifies the following rules:
- Only the derived class in the same assemble can access the `private protected` members.
- Base class in the same or different assembly cannot access the `private protected` members.
- The `private protected` members of an assembly cannot be accessed by a derived class of another assembly. However, if, in the former assembly, `InternalsVisibleToAttribute` contains the name of the latter assembly, then the `private protected` members can be accessed in the latter assembly.

# Examples
The following code demonstrates the usage of the `private protected` access modifier:

class Base {
    private protected x;
}

class Derived_1 : Base {
}

class Program {
    static void Main() {
        Derived_1 d1 = new Derived_1();
        d1.x = 10;

        Base b = new Base()
        b.x = 100; // Error on this line
    }
}

In the example above, the `Base` class has a `private protected` member `x`. The `Derived_1` class inherits the `Base` class. In the `Main` method, `d1` is initialized with a `Derived_1` instance and `b` is initialized with a `Base` instance. Line 11 won't give an error; however, line 14 will give an error because a base class is trying to access its own `private protected` member.

The following code provides an example that contains two assembly files:

class Derived_2 : Base {
}

class Program {
    static void Main() {
        Derived_1 d1 = new Derived_1();
        d1.x = 10;

        Derived_2 d2 = new Derived_2();
        d2.x = 10; // Error on this line
    }
}

class Base {
    private protected x;
}

class Derived_1 : Base {
}

In the example above, `assembly2.cs` uses the `assembly1.cs` classes. In `assembly1.cs`, the `Base` and `Derived_1` classes are defined as in the previous example. In the `assembly2.cs` file, the `Derived_2` class is defined to inherit the `Base` class. 

In the `Main` method, `d1` stores an instance of the `Derived_1` class, and `d2` stores an instance of the `Derived_2` class. 

Line 7 doesn't give an error, but line 10 does give an error because a derived class in another assembly file is trying to access a `private protected` member of a different assembly file.

What is the private protected access modifier in C#?

Private protected restricts member access to derived classes within the same assembly, blocking access from base classes or different assemblies.