Converting List<T> using covariance

You can become a serverless blackbelt. Enrol to my 4-week online workshop Production-Ready Serverless and gain hands-on experience building something from scratch using serverless technologies. At the end of the workshop, you should have a broader view of the challenges you will face as your serverless architecture matures and expands. You should also have a firm grasp on when serverless is a good fit for your system as well as common pitfalls you need to avoid. Sign up now and get 15% discount with the code yanprs15!

I saw an interesting use of covariance today, consider an interface IMyClass and an implementing class MyClass:

   1: public interface IMyClass { }


   3: public class MyClass : IMyClass { }

If you want to convert an List<MyClass> to a List<IMyClass> you would normally use the Enumerable.Cast method but did you know that you can also use C# 4’s support for covariance in the type parameter and do this instead:

   1: var original = new List<MyClass>();


   3: var converted = original.ToList<IMyClass>()

Funky, eh? ;-)

Though I think it’s a party trick best avoided for any production code, for which you should still prefer:

   1: var converted = original.Cast<IMyClass>().ToList();


  • it achieves the same result
  • it is just as expressive
  • it is the standard way of doing this kind of conversions in LINQ
  • it is understood by most C# developers so unlikely to cause confusion

There’s another argument for using Cast, in the case of use-defined implicit/explicit operators. Imagine if you have another class which does not inherit from MyClass but defines an explicit operator which allows you to cast an instance of MyClass:

   1: public class MyOtherClass

   2: {

   3:     public static explicit operator MyClass(MyOtherClass other)

   4:     {

   5:         return new MyClass();

   6:     }

   7: }

In cases like this, you won’t be able to use the covariance trick:

   1: void Main()

   2: {

   3:     var original = new List<MyClass>();


   5:     Console.WriteLine(original.GetType());                               // List<MyClass>


   7:     // cast here doesn't actually do anything

   8:     Console.WriteLine(original.Cast<IMyClass>().ToList().GetType());     // List<IMyClass>


  10:     // changes the compile type, works because of covariance

  11:     Console.WriteLine(original.ToList<IMyClass>().GetType());            // List<IMyClass>


  13:     // casts the objs to MyOtherClass using the defined convertor

  14:     Console.WriteLine(original.Cast<MyOtherClass>().ToList().GetType()); // List<MyOtherClass>


  16:     // this line won't compile.

  17:     // it doesn't work because this is not covariance, there's no inheritance

  18:     // relationship between MyClass and MyOtherClass

  19:     // Console.WriteLine(objs.ToList<MyOtherClass>().GetType());

  20: }


StackOverflow question – Casting List<T> – covariance/contravariance problem

Liked this article? Support me on Patreon and get direct help from me via a private Slack channel or 1-2-1 mentoring.
Subscribe to my newsletter

Hi, I’m Yan. I’m an AWS Serverless Hero and I help companies go faster for less by adopting serverless technologies successfully.

Are you struggling with serverless or need guidance on best practices? Do you want someone to review your architecture and help you avoid costly mistakes down the line? Whatever the case, I’m here to help.

Hire me.

Skill up your serverless game with this hands-on workshop.

My 4-week Production-Ready Serverless online workshop is back!

This course takes you through building a production-ready serverless web application from testing, deployment, security, all the way through to observability. The motivation for this course is to give you hands-on experience building something with serverless technologies while giving you a broader view of the challenges you will face as the architecture matures and expands.

We will start at the basics and give you a firm introduction to Lambda and all the relevant concepts and service features (including the latest announcements in 2020). And then gradually ramping up and cover a wide array of topics such as API security, testing strategies, CI/CD, secret management, and operational best practices for monitoring and troubleshooting.

If you enrol now you can also get 15% OFF with the promo code “yanprs15”.

Enrol now and SAVE 15%.

Check out my new podcast Real-World Serverless where I talk with engineers who are building amazing things with serverless technologies and discuss the real-world use cases and challenges they face. If you’re interested in what people are actually doing with serverless and what it’s really like to be working with serverless day-to-day, then this is the podcast for you.

Check out my new course, Learn you some Lambda best practice for great good! In this course, you will learn best practices for working with AWS Lambda in terms of performance, cost, security, scalability, resilience and observability. We will also cover latest features from re:Invent 2019 such as Provisioned Concurrency and Lambda Destinations. Enrol now and start learning!

Check out my video course, Complete Guide to AWS Step Functions. In this course, we’ll cover everything you need to know to use AWS Step Functions service effectively. There is something for everyone from beginners to more advanced users looking for design patterns and best practices. Enrol now and start learning!