Question

This is probably me just remembering things completely backwards, but I’d like to know more about what I’m doing wrong…

I have declared a class to be nothing more than a direct inheritance from a generic list (done to simplify naming), something like this:

public class FooList : List<Foo> {}

now in another method completely separate from this class, I am trying to return an instance of this class, however I want to filter the class based on a criterion, so I’m using a lambda expression:

var list = new FooList(); // imagine this fills it with different items
var filtered = list.FindAll(c => c.Something == "filter criteria");

now according to the FindAll method, this SHOULD return a List[Foo]. However, I want to return this object as a FooList, not a List[Foo]. Do I have to create a new instance of FooList and copy the items from the List[Foo]?

If so, why? why can’t I convert a List to a FooList directly, since they are the same object?

If this CAN be done, how do I do it?

many thanks!

Answer 1

They are not the same thing. A FooList is a List<foo> but a List<foo>, which is being returned by the FindAll() function inherited from List<foo>, is not a FooList. You will need to construct a new FooList.

You could do something like create a Constructor for FooList that takes a IEnumerable<foo> object like this:

class FooList : List<foo>
{
   public FooList(IEnumerable<foo> l)
        : base(l)
   {  }
}

Or you could also do something like this:

 var list = new FooList();
 var filtered = list.FindAll(c => c.Something == "filter criteria");
 var newList = new FooList();
 newList.AddRange(filtered);
 return newList;

However as mentioned in some of the other answers, if you are not adding any functionality then you should just create an alias with the using keyword.