I saw an interesting use of covariance today, consider an interface IMyClass and an implementing class MyClass:
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:
Funky, eh? ;-)
Though I think it’s a party trick best avoided for any production code, for which you should still prefer:
- 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:
In cases like this, you won’t be able to use the covariance trick:
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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.
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