C#9–Covariant return types

C# 9 adds a whole list of new language features. Most people talk about the support for record types and improved pattern matching but one of the features I’m happy that is finally added to the language are covariant return types.

You are probably asking covariant return what?????

Let’s explain; with return type covariance, you can override a base class method that has a less-specific type with one that returns a more specific type.

An example, before C#9 you would have to return the base type from the inherited class:

	public class EmployeeBuilder:PersonBuilder
	{
	//Our return type remains Person
	public override Person Create()
	{
	return new Employee();
	}
	}

view raw EmployeeBuilder.cs hosted with ❤ by GitHub

	public class PersonBuilder
	{
	public virtual Person Create()
	{
	return new Person();
	}
	}

view raw PersonBuilder.cs hosted with ❤ by GitHub

In C# 9, you can do the following:

	public class EmployeeBuilder:PersonBuilder
	{
	public override Employee Create()
	{
	return new Employee();
	}
	}

view raw EmployeeBuilder.cs hosted with ❤ by GitHub

	public class PersonBuilder
	{
	public virtual Person Create()
	{
	return new Person();
	}
	}

view raw PersonBuilder.cs hosted with ❤ by GitHub