asked on

Casting generic collection containing interface.

I have some objects that are cacheable. The interface looks like this:

public interface ICacheable {
string EntityTag { get; }
}

One of the classes that implement it look like this:

public class User : ICacheable {
public int Id { get; }
public int Version { get; }

public string EntityTag { get { return String.Format("{0}-{1}", Id, Version); } }
}

Now, I have a method that returns a readonly collection of users (System.Collections.ObjectModel.ReadOnlyCollection<User>) and I need to pass this to a method that only accepts a readonly collection of ICacheable objects. I would think that the compiler would handle this kind of cast, but it isn't working.

It boils down to why won't this work:

public System.Collections.ObjectModel.ReadOnlyCollection<ICacheable> GetUsers() {
ReadOnlyCollection<Users> userList = DataManager.GetUsers();

return userList;
}

Gururaj Badam

When you say it isn't working what exactly is happening?

ChetOS82

ASKER

Error: Cannot implicitly convert type ReadOnlyCollection<User> to ReadOnlyCollection<ICacheable>.

If I try to explictly cast, I get the same error without the word "implicitly".

Gururaj Badam

public System.Collections.ObjectModel.ReadOnlyCollection<ICacheable> GetUsers() {
ReadOnlyCollection<ICloneable> userList = DataManager.GetUsers();

return userList;
}

Observe the userList declaration

ChetOS82

ASKER

Not sure what you are getting at. 1) DataManager.GetUsers returns a ReadonlyCollection<User>, 2) User is not ICloneable (if I use ICacheable instead, I still get invalid cast error).

ASKER CERTIFIED SOLUTION

kris_per

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Gururaj Badam

Since you're a base type (interface) it's good to replace all references with ICloneable include your GetUsers unless it's not your code.

ChetOS82

ASKER

When you say ICloneable, do you mean ICacheable? I cannot replace the return of DataManager.GetUsers() because other code in the system doesn't care that the User object is ICacheable.

@kris_per: Is that essentially the same as this:
List<ICacheable> cacheableUsers = new List<ICacheable>();
foreach (ICacheable user in userList) {
cacheableUsers.Add(user);
}
return new ReadOnlyCollection<ICacheable>(cacheableUsers);

That was what I used as a workaround. I was hoping that C# could handle the cast implicily. If not, I'll replace this code with the Cast call you recommended.

SOLUTION

Gururaj Badam

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Gururaj Badam

http://blogs.msdn.com/b/ericlippert/archive/2007/10/16/covariance-and-contravariance-in-c-part-one.aspx

ChetOS82

ASKER

I realize that this is an issue of covariance, but I was hoping that someone knew if I was just looking at this wrong. Apparently covariance is supported everywhere except with Generics. So, in C# 4 I will be able to do this implicitly.

Some points to Novice for giving the problem a name, most points to kris_per for validation.

I ended up writting a method that can be replaced later. I prefer it over the Linq Cast<> for two reasons. 1) I don't have Linq referenced in this project, 2) Cast returns an IEnumerable, but I want a ReadOnlyCollection.

public ObjectModel.ReadOnlyCollection<ICacheable> MakeCollectionCacheable<T>(ObjectModel.ReadOnlyCollection<T> list) {
var cacheableList = new List<ICacheable>(list.Count);
foreach (ICacheable cacheableObject in list) {
cacheableList.Add(cacheableObject);
}
return new ObjectModel.ReadOnlyCollection<ICacheable>(cacheableList);
}