You made it! References aren’t always easy, but learning how to use them unlocks a whole new set of superpowers in C#.

In this lesson you learned that:

• Classes and interfaces are reference types. A variable of this type holds a reference to the data, not the data itself. This is different from value types like int and bool
• The equality operator (==) uses a referential comparison for reference types and a value comparison for value types
• Multiple references can be created for a single object
• A reference and its corresponding object do not have to be the same type. For example, we can refer to a subclass object by an inherited superclass or implemented interface reference
• The functionality available to an object reference is determined by the reference’s type, not the object’s type
• Polymorphism is the ability in programming to present the same interface for differing data types
• Referencing an object by an inherited type or implemented interface is called upcasting. It can be done implicitly
• Referencing an object by a derived class is called downcasting, which must be made explicit by adding the type name in parentheses. It may cause an InvalidCastException error when the code is run
• To signify that a reference is “empty” or refers to no object, we set it equal to null
• If a reference is not set to any value it is unassigned and cannot perform any operations