How to pass List of class to List of Interface?

Question

I have a function like this:

DoSomething(List<IMyInterface>) 

IMyInterface is an interface and MyClass is a class implementing this interface Class MyClass:IMyInterface

I call DoSomething(List<MyClass>) and it looks it doesn't work. How could I pass the list of a class to a list of the interface of the class as function's parameter? Thanks!

Answer 1

If your code is simply iterating over the sequence inside the method (not adding, removing, or accessing by index), change your method to one of the following

DoSomething(IEnumerable<IMyInterface> sequence) DoSomething<T>(IEnumerable<T> sequence) where T : IMyInterface 

The IEnumerable<> interface is covariant (as of .NET 4) (first option). Or you could use the latter signature if using C# 3.

Otherwise, if you need indexed operations, convert the list prior to passing it. In the invocation, you might have

// invocation using existing method signature  DoSomething(yourList.Cast<IMyInterface>().ToList());  // or updating method signature to make it generic DoSomething<T>(IList<T> list) where T : IMyInterface 

What the latter signature would allow you to do is to also support adds or removes to the list (visible at the callsite), and it would also let you use the list without first copying it.

Even still, if all you do is iterate over the list in a loop, I would favor a method acceping IEnumerable<>.