CODEWITHSUNDEEP
c#asp.net-mvcinheritancecollectionspolymorphism
Toza
Toza

Reputation: 1358

A way to turn one List into another (via polymorphism, child and parent classes)

Alright, so I have the following method:

private List<Store> NextPrevious(List<Store> model, int numFields, int page, ref bool nextRef, ref bool previousRef)
{
    if (model.Count > numFields && page == 0)
    {
        model = model.Skip(page * (numFields - 1)).Take(numFields - 1).ToList();
        next = true;
        previous = false;
    }
    else if (page > 0 && model.Skip(page + (numFields - 2)).Count() <= (numFields - 1))
    {
        model = model.Skip(page + (numFields - 2)).Take(numFields - 1).ToList();
        next = false;
        previous = true;
    }
    else if (page > 0 && model.Count > (page + 1))
    {
        model = model.Skip(page + (numFields - 2)).Take(numFields - 2).ToList();
        next = true;
        previous = true;
    }
    return model.ToList();
}

I'd call the method like this:

model.Groups = NextPrevious(model.Groups.ToList<Store>(), 3, groupPage, ref next, ref previous);

model.Groups is of type List<Group>. Group is a child class of a class Store. Now, is there a way to reference the group when I'm sending it in the method (ref), or at least return it, like I tried, unsuccessfully, and make it work. Right now if I type this and try to run it, I get an error:

Cannot implicitly convert type 'System.Collections.Generic.List<(path)Store> to 'System.Collections.Generic.List<(path)Group>'

I know what the error means, but I just don't have the knowledge to go around it, so I'm asking here for help. By the way, I can't cast it as return model.ToList<Group>() neither, since the method is created so it could be used for other class objects that inherit the Store class.

Thanks!

Upvotes: 2

Views: 75

Answers (2)

ocuenca
ocuenca

Reputation: 39366

A solution could be changing the NextPrevious as a generic method and declaring the following constraint on the type parameter:

private List<T> NextPrevious<T>(List<T> model, int numFields, int page, ref bool nextRef, ref bool previousRef) where T:Store
{
  //...
}

This way you just need to do this:

model.Groups = NextPrevious(model.Groups, 3, groupPage, ref next, ref previous);//call model.Groups.ToList() in case the Groups property is not a List<Group>

You can omit the type argument at the time you call the NextPrevious<T> method because the compiler will infer it based on the first method argument you pass.

Upvotes: 1

jdphenix
jdphenix

Reputation: 15445

Well, one way you could do this is use Cast(), something like: 

model.Groups = NextPrevious(...).Cast<Group>().ToList();

This will blow up at runtime with an InvalidCastException if any of the objects returned from NextPrevious can't be casted to a Group, but...

You could also within NextPrevious() cast before you return, 

....
return model.Cast<Group>.ToList();

Upvotes: 1

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