C# is a single-inheritance programming language.
- This means that a class cannot derive from two classes directly.
When we need multiple inheritance class structures, we can use aggregation.
- We can implement the second class by making a field an instance of the second class.

Overriding the Base Class

public and protected members of a base class are inherited in the derived class.

`protected` Members

These members can only accessed from derived classes instances and not from base class instances.
They are accessible inside the class implementation.
Declaring a protected member in a sealed class is the same as declaring it as private.

You can override instance methods and properties but not fields or any static members.
The base class must mark each member for which it allows overriding as virtual.
If public or protected members do not include the virtual modifier, then sub-classes will not be able to override those members.
It is required that the overriding methods to use the override keyword explicitly.
Whenever the runtime encounters a virtual method, it calls the most derived and overriding implementation of the virtual member.
Converting a method from a virtual method to a non-virtual method could break derived classes that override the method.

sealed classes cannot be derived.
- sealed classes cannot be abstract.
The string class is an example of a sealed class.
sealed members prevent an override of a virtual member to be overridden be another subclass.
- This negates the virtual aspect of the member for any further derived class.