Why classes cannot be declared as protected?

In C#, classes cannot be declared as protected because the protected access modifier is specifically designed to control the visibility of class members within a class's inheritance hierarchy. It's not used to control the accessibility of the class itself.

Here's why classes cannot be declared as protected:

1. Inheritance Hierarchy: The protected access modifier is used to grant access to class members (methods, properties, fields, etc.) for derived classes (subclasses) within the same inheritance hierarchy. It allows derived classes to access the protected members of their base class. This helps in achieving encapsulation and controlled access to a class's internals while promoting code reuse.
2. Classes Are Not Inherited: Unlike class members, classes themselves are not inherited. When you inherit from a base class, you create a new derived class that extends the behavior of the base class. However, you don't inherit the base class itself. The derived class is a distinct type on its own.
3. Defining a New Type: When you declare a new class, you're defining a new type that can be instantiated. The accessibility of this new type is determined by access modifiers like public, internal, etc., not by protected.

In summary, the protected access modifier is applicable to class members and is used to control the visibility of those members within an inheritance hierarchy. It's not meant to control the accessibility of classes themselves, which is determined by other access modifiers that are suitable for defining new types.

Only netsted classes are allowed to be declared as protected:

You can declare nested classes with the protected access modifier in C#. The protected modifier for nested classes is used to make the nested class accessible only within its containing class and any derived classes, regardless of the assembly they are in. This can be useful when you want to encapsulate functionality within a nested class that is specifically intended for use within the containing class and its subclasses.

Here's an example of declaring a nested class with the protected access modifier:

public class ParentClass
{
    // Protected nested class
    protected class ProtectedNestedClass
    {
        // Class members
    }
    
    public void UseProtectedNestedClass()
    {
        ProtectedNestedClass nestedInstance = new ProtectedNestedClass();
        // You can access and use the protected nested class here
    }
}

public class DerivedClass : ParentClass
{
    public void UseProtectedNestedClassInDerived()
    {
        ProtectedNestedClass nestedInstance = new ProtectedNestedClass();
        // You can also access and use the protected nested class here
    }
}

class Program
{
    static void Main(string[] args)
    {
        ParentClass parentInstance = new ParentClass();
        // You cannot access the protected nested class directly here
        // parentInstance.ProtectedNestedClass nestedInstance = new parentInstance.ProtectedNestedClass(); // This will result in a compile-time error

        DerivedClass derivedInstance = new DerivedClass();
        // You cannot access the protected nested class directly here either
        // derivedInstance.ProtectedNestedClass nestedInstance = new derivedInstance.ProtectedNestedClass(); // This will also result in a compile-time error
    }
}

In the example above, the 'ProtectedNestedClass' is accessible within 'ParentClass' and its subclass 'DerivedClass', but it is not accessible from outside these classes or their subclasses. This demonstrates how the protected modifier for nested classes controls their visibility and accessibility.