In C#, constructors can be declared as public, private, protected or internal. When a class declares only private constructors, it is not possible other classes to derive from this class or create an instance of this class. Private constructors are commonly used in classes that contain only static members.

When a class declares only private instance constructors, it is not possible for classes outside the program to derive from the class or to directly create instances of it. (Except Nested classes)
Make a constructor private if:
1. You want it to be available only to the class itself. For example, you might have a special constructor used only in the implementation of your class' Clone method.
2.You do not want instances of your component to be created. For example, you may have a class containing nothing but Shared utility functions, and no instance data. Creating instances of the class would waste memory.