将接口列表转换为实现列表，反之亦然

Question

I am just not getting how to convert or cast List<ISomeImplementation> to List<SomeImplementation> .

// lstOfImplemantation is of type `List<SomeImplementation>`
    List<ISomeImplementation> lstOfInterfaces = lstOfImplemantation; // how to convert it.

any help is appreciated.

Answer 1

You can use LINQ in one way:

List<ISomeImplementation> lstOfInterfaces = lstOfImplemantation.Cast<IISomeImplementation>().ToList();

Casting interface to implementation is not good pratice. What if there some other implementations of that interface? You cannot assume that all implemtations are the of type SomeImplementation . If you are sure, use List<SomeImplementation> .

Answer 2

Just because A derives from B , doesn't necessarily mean X<A> derives from X<B> .

In fact, List<T> is invariant in its generic type paramater T . This means List<string> isn't a subtype nor a supertype of List<object> , even though string derives from object .

Imagine if your list of interfaces contained instances of type SomeOtherImplementation . How would the cast work?

Instead, you should create a new list and cast each item in your list of interfaces to the concrete type.

List<SomeImplementation> lstOfImplementation =
             lstOfInterfaces.Cast<SomeImplementation>()
                            .ToList();

If you're not sure whether all instances are of type SomeImplementation , you can filter out those who aren't, using OfType<T> .

List<SomeImplementation> lstOfImplementation =
             lstOfInterfaces.OfType<SomeImplementation>()
                            .ToList();

Read also:

• Covariance and Contravariance
• Covariance and Contravariance FAQ