Interface and List in c#

Question

im having issues with the following interface and a class:

public interface IRelated
{

}

public class BaseItem:IRelated
{
      public string Name{get;set;}
      public List<IRelated> RelatedItems{get;set;}
}

Now when i try to do in other classes the following it gives me a compilation error:

List<IRelated> listofrelateditems=new List<BaseItem>();

Cannot implicity convert type List<BaseItem> to List<IRelated>

The reason of the interface is that in the future maybe i will have another class that can be Related to this BaseItem.

Answer 1

You just can't do that - even the generic covariance in .NET 4 won't help you, because List<T> is a class and even IList<T> is invariant as it has T coming "in" as well as going out.

It's precisely because you might have a new implementation of IRelated in the future that you can't do that. Consider:

List<IRelated> listOfRelatedItems = new List<BaseItem>();
listOfRelatedItems.Add(new OtherRelatedItem());

where OtherRelatedItem implements IRelated but doesn't derive from BaseItem . Now you've got a List<BaseItem> which contains something other than a BaseItem ! In other words, it breaks type safety.

Basically you'll have to create a List<IRelated> instead of a List<BaseItem> .

For more on generic variance and why it's sometimes applicable and sometimes not, go to the NDC 2010 videos page and search for "variance" to find the video of a presentation I gave on the topic last year.

Answer 2

Like you yourself said: You may have another class that can be IRelated . How are you going to add an instance of such a class to a List<BaseItem> ?

You will have to write:

List<IRelated> listofrelateditems = new List<IRelated>();

Answer 3

改成：

List<IRelated> listofrelateditems=new List<IRelated>();