We just saw how useful it is to have the same interface for multiple data types. This is a common concept across many programming languages, and it’s called polymorphism.

The concept really includes two related ideas. A programming language supports polymorphism if:

  1. Objects of different types have a common interface (interface in the general meaning, not just a C# interface), and
  2. The objects can maintain functionality unique to their data type

Let’s prove to ourselves that this is true in C#. We’ll use the example of Stringify: Dissertation and Book have different Stringify() methods but can both be referenced as Books.

Here are snippets from each class:

class Dissertation : Book { public override string Stringify() { return "This is a Dissertation object!"; } } class Book { public virtual string Stringify() { return "This is a Book object!"; } }

Given that information, what will the below program print?

Book bk = new Book(); Book bdiss = new Dissertation(); Console.WriteLine(bk.Stringify()); Console.WriteLine(bdiss.Stringify());

The answer is:

This is a Book object! This is a Dissertation object!

Even though bk and bdiss are both Book type references, their behavior is different. Dissertation overrides the Stringify() method, so all Dissertation objects (regardless of reference type) will use that method.

Therefore, C# support polymorphism!

You’ll never have to write polymorphism in your code, but this vocabulary is essential to communicating with other developers!

So remember: polymorphism is the ability in programming to present the same interface for differing data types.



In Program.cs, there are Book type references to one Book and one Diary object. First, call b1.Stringify() and print it to the console.


Below that call b2.Stringify() and print it to the console.

To check your understanding, find both Stringify() methods in Diary.cs and Book.cs.

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