Introducing C# 9: Covariant returns
source link: https://anthonygiretti.com/2020/10/12/introducing-c-9-covariant-returns/
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Introducing C# 9: Covariant returns
Introduction
In May 20th 2020, Microsoft introduced C# 9, among the features announced, there is one that has not yet been talked about much: covariant returns. Usually in C# when we inherit from a class, it is possible to override a method if it is declared abstract or virtual but it is not possible to change the return type of this method. C# 9 allows this, In addition to overriding a virtual or abstract method, in C# 9 you can now return a covariant type of the initial type declared in the parent class. For example, you can now return a type that is inherited from the parent class. In this article I will show you how it works with a very simple example and compare with earlier versions of C# 9. This post was produced with input from Dave Brock. Dave recently posted some really interesting C # 9 articles which you can find here: https://daveabrock.com/2020/06/29/c-sharp-9-deep-dive-inits
Before C# 9
Here is a simple example of “covariant returns” before C# 9, you had to return the initial type like this:
public abstract class Product { protected string Name { get; } protected int Id { get; }
protected Product(string name, int id) { Name = name; Id = id; }
public abstract ProductOrder Order(int quantity); }
public class Book : Product { public string ISBN { get; }
public Book(string name, int categoryId, string Isbn) : base(name, categoryId) { ISBN = Isbn; }
public override ProductOrder Order(int quantity) => new BookOrder { Quantity = quantity, Product = this }; }
public class Music : Product { protected Format Format { get; }
public Music(string name, int categoryId, Format format) : base(name, categoryId) { Format = format; }
public override ProductOrder Order(int quantity) => new MusicOrder { Quantity = quantity, Product = this }; }
public class ProductOrder { public int Quantity { get; set; } }
public class BookOrder : ProductOrder { public Book Product { get; set; } }
public class MusicOrder : ProductOrder { public Music Product { get; set; } }
public enum Format { Mp3, Disc }
Then after instanciation and calling the Order method, you had to cast the initial return type to its derived type like this:
public class Program { public static void Main(string[] args) { var book = new Book { Name = "My book", Id = 1, ISBN = "978-3-16-148410-0" };
var music = new Music { Name = "My music", Id = 2, Format = Format.Disc };
BookOrder orderBook = (BookOrder)book.Order(1); MusicOrder orderMusic = (MusicOrder)music.Order(1); } }
As you can see there a cast to perform, and it can avoided with C# 9 
With C# 9
Now let’s take the same example, but now you can explicitely return a covariant like this:
public abstract class Product { protected string Name { get; } protected int Id { get; }
protected Product(string name, int id) { Name = name; Id = id; }
public abstract ProductOrder Order(int quantity); }
public class Book : Product { public string ISBN { get; }
public Book(string name, int categoryId, string Isbn) : base(name, categoryId) { ISBN = Isbn; }
public override BookOrder Order(int quantity) => new BookOrder { Quantity = quantity, Product = this }; }
public class Music : Product { protected Format Format { get; }
public Music(string name, int categoryId, Format format) : base(name, categoryId) { Format = format; }
public override MusicOrder Order(int quantity) => new MusicOrder { Quantity = quantity, Product = this }; }
public class ProductOrder { public int Quantity { get; set; } }
public class BookOrder : ProductOrder { public Book Product { get; set; } }
public class MusicOrder : ProductOrder { public Music Product { get; set; } }
public enum Format { Mp3, Disc }
Its makes the consumption simpler:
public class Program { public static void Main(string[] args) { var book = new Book { Name = "My book", Id = 1, ISBN = "978-3-16-148410-0" };
var music = new Music { Name = "My music", Id = 2, Format = Format.Disc };
BookOrder orderBook = book.Order(1); MusicOrder orderMusic = music.Order(1); } }
Conclusion
It’s definitely not the most famous functionality of C# 9 but it could be pretty useful, your code is being a bit simpler :).
I would like to thanks Dave Brock for his participation as well, it was very fun to get his insights. I strongly encourage you to visit his awesome blog (https://daveabrock.com/) !
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