Senior Software Engineer

A base class, in the context of C#, is a class that is used to create, or derive, other classes. Classes derived from a base class are called child classes, subclasses or derived classes. A base class does not inherit from any other class and is considered parent of a derived class.
The base class forms the means by which inheritance is accomplished through derivation. A class derived from a base class inherits both data and behavior

Singleton design pattern in C# is one of the most popular design patterns. In this pattern, a class has only one instance in the program that provides a global point of access to it. In other words, a singleton is a class that allows only a single instance of itself to be created and usually gives simple access to that instance.
The following are the common characteristics of a singleton pattern.
Private and parameter less single constructor
Sealed class.
Static variable to hold a reference to the single created instance
A public and static way of getting the reference to the created instance.

C# has the following access modifiers:
Modifier Description
public The code is accessible for all classes
private The code is only accessible within the same class
protected The code is accessible within the same class, or in a class that is inherited from that class. You will learn more about inheritance in a later chapter
internal The code is only accessible within its own assembly, but not from another assembly. You will learn more about this in a later chapter

By Default, access modifier of Class is 'Internal' and 'Private' for Data Member and Member Function of Class.