Question

I have two classes, CheckboxItemsList which extends a generic list, and CheckboxItems, which contains a list of objects of type CheckboxItem.

I want to use LINQ to be able to filter CheckboxItemsList based on properties of its CheckboxItems objects. The return type is always a generic list, though, but I want it to be a CheckboxItemsList.

So I guess the basic question is, can linq be made to return a list of the same type that it starts with? Since I can’t cast a base class to a derived class, do I have any option other than iterating through the results of the linq query and rebuilding the derived list object row by row? Not that this is the end of the world, but I’m relatively new to linq and was wondering it there is a better way to do it.

What I want:

CheckboxItemsList newList = MyCheckboxItemsList.Where(item=>item.Changed);

(obviously doesn’t work since the query will return List<CheckboxItems>, not CheckboxItemsList)

The objects, generally:

public class CheckboxItemsList: List<CheckboxItems>
{
   // does not add any fields, just access methods
}

public class CheckboxItems : IEnumerable<CheckboxItem>
{
    public long PrimaryKey=0;
    protected CheckboxItem[] InnerList;
    public bool Changed
    {
        get {
            return (InnerList.Any(item => item.Changed));
        }
    }
    ....
}

Answer 1

No, this is not possible out of the box. You’ll need to add code to do this.

For example, you can add a constructor like so:

public CheckboxItemsList(IEnumerable<CheckboxItems> checkboxItems) {
    // something happens
}

Then you can say

CheckboxItemsList newList = new CheckboxItemsList(
    MyCheckboxItemsList.Where(item => item.Changed)
);

Additionally, you could add an extension method like so

static class IEnumerableCheckboxItemsExtensions {
    public static ToCheckboxItemsList(
        this IEnumerable<CheckboxItems> checkboxItems
    ) {
        return new CheckboxItemsList(checkboxItems);
    }
}

and then

CheckboxItemsList newList =
    MyCheckboxItemsList.Where(item => item.Changed)
                       .ToCheckboxItemsList();