Question

I have a interface that inherits from IList:

public interface IBase {}
public class Derived : IBase {}
public interface IMyList : IList<IBase> {}

I want to cast a variable of type IMyList to type IList<Derived> or List<Derived>, whichever is easier or makes the most sense. What’s the best way to do this?

Note that I’m using .NET 3.5

Answer 1

A direct cast isn’t going to be suitable since there are numerous problems that could occur (IMyList may contain types other than Derived, etc.)

As long as you’re not modifying the list (in which case, IEnumerable<Derived> would work) you could simply:

var validItems = myList.Cast<Derived>();

And then iterate over the result.

Edit

Based on the OP’s comment below, there are two other things to mention:

1) If you need an IList<Derived>, you could simply add to the above and call myList.Cast<Derived>().ToList();

2) If those are truly the requirements, then your code doesn’t make any sense. If IMyList should only ever contain Derived objects then IMyList should derive from IList<Derived>. Remember that while you know that there is only one type implementing the IBase interface, the compiler doesn’t so be specific.

Using interfaces everywhere just for the sake of using interfaces doesn’t help anybody!