Question

I have an abstract class which defines a number of classes which can be voted on and then sorted. Since these classes all share the properties by which they are sorted, I would like to include a method at the abstract level which will let me sort them by these properties, but I’m running in to trouble with ‘not assignable to parameter’ errors.

How should I handle the following:

internal abstract class ESCO
{
    public double HotScore { get; set; }
    public double VoteTotal { get; set; }
    public DateTime Created { get; set; }

    protected static List<ESCO> SortedItems(List<ESCO> escoList, ListSortType sortType)
    {
        switch (sortType)
        {
            case ListSortType.Hot:
                escoList.Sort(delegate(ESCO p1, ESCO p2) { return p2.HotScore.CompareTo(p1.HotScore); });
                return escoList;
            case ListSortType.Top:
                escoList.Sort(delegate(ESCO p1, ESCO p2) { return p2.VoteTotal.CompareTo(p1.VoteTotal); });
                return escoList;
            case ListSortType.Recent:
                escoList.Sort(delegate(ESCO p1, ESCO p2) { return p2.Created.CompareTo(p1.Created); });
                return escoList;
            default:
                throw new ArgumentOutOfRangeException("sortType");
        }
    }
    private SPUser GetCreatorFromListValue(SPListItem item)
    {
        var user = new SPFieldUserValue(SPContext.Current.Web, (string)item["Author"]);
        return user.User;
    }
    private static VoteMeta InformationForThisVote(List<Vote> votes, int itemId)
    {} // There are more methods not being shown with code to show why I used
       // abstract instead of something else
}

Trying to implement as such:

class Post : ESCO
{
    public string Summary { get; set; } // Properties in addition to abstract
    public Uri Link { get; set; } // Properties in addition to abstract 

    public static List<Post> Posts(SPListItemCollection items, ListSortType sortType, List<Vote> votes)
    {
        var returnlist = new List<Post>();
        for (int i = 0; i < items.Count; i++) { returnlist.Add(new Post(items[i], votes)); }
        return SortedItems(returnlist, sortType);
    }

I am totally open to “You’re doing it all wrong”.

Answer 1

I can’t reproduce the same error message, but I think the error you are getting is because

return SortedItems(returnlist, sortType);

is attempting to return a list of the abstract base class. Try changing that to

return SortedItems(returnlist, sortType).Cast<Post>().ToList();

You’ll need to include the System.Linq namespace if you did not already.

FYI, the error I get (in a simplified test case) is

Cannot implicitly convert type System.Collections.Generic.List<MyNamespace.ESCO>' to 'System.Collections.Generic.List<MyNamespace.Post>'