Chapter 7 – Implementing Interfaces and Inheriting Classes

1. How is a base class and a derived class related?

   A derived class (or subclass) is a class that inherits from a base class (or superclass).

2. What is the difference between the is and as operators?

   The is operator returns true if an object can be cast to the type. The as operator returns a reference if an object can be cast to the type; otherwise, it returns null.

3. Which keyword is used to prevent a class from being derived from, or a method from being overridden?

   sealed

   Find more information on the sealed keyword at https://msdn.microsoft.com/en-us/library/88c54tsw.aspx.

4. Which keyword is used to prevent a class from being instantiated with the new keyword or force a method to be overridden?

   abstract

   Find more information on the abstract keyword at https://msdn.microsoft.com/en-us/library/sf985hc5.aspx.

5. Which keyword is used to allow a member to be overridden?

   virtual

   Find more information on the virtual keyword at https://msdn.microsoft.com/en-us/library/9fkccyh4.aspx.

6. What's the deal with polymorphism?

   Polymorphism is a fancy academic OOP concept that rarely has an impact on real-world code.

7. What are the signatures of the constructors that all exceptions should have?

   The following are the signatures of the constructors that all exceptions should have:

   • A constructor with no parameters

   • A constructor with a string parameter usually named message

   • A constructor with a string parameter, usually named message, and an Exception parameter usually named innerException

8. What is an extension method and how do you define one?

   An extension method is a compiler trick that makes a static method of a static class appear to be one of the members of a type. You define which type you want to extend by prefixing the type with this.