Converting List<T> using covariance

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

   1: public interface IMyClass { }

   2:

   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>();

   2:

   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();

because:-

  • 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>();

   4:

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

   6:

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

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

   9:

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

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

  12:

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

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

  15:

  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: }

References:

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

Enjoy what you’re reading? Subscribe to my newsletter and get more content on AWS and serverless technologies delivered straight to your inbox.


Yan Cui

I’m an AWS Serverless Hero and the author of Production-Ready Serverless. I have run production workload at scale in AWS for nearly 10 years and I have been an architect or principal engineer with a variety of industries ranging from banking, e-commerce, sports streaming to mobile gaming. I currently work as an independent consultant focused on AWS and serverless.

You can contact me via Email, Twitter and LinkedIn.

Hire me.


Check out my new 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. Including basic concepts, HTTP and event triggers, activities, design patterns and best practices.

Get Your Copy