Using generics in abstract classes

Question

I'm working on an abstract class where the implementing class needs to implement a list of T. The problem is that this doesn't work:

public class AbstractClass
{
    public int Id { get; set; }
    public int Name { get; set; }

    public abstract List<T> Items { get; set; }
}

public class Container : AbstractClass
{
    public List<Widgets> Items { get; set; }
}

I'm sure that there is an obvious answer that I'm missing, and I know that I can build an abstract base type to put in the list, but when I use my Linq command to build the list, the abstract type (ItemBase) doesn't play nicely with the .ToList() method. Is what I'm trying to do so unique?

It turns out that I need to learn to research better. Apologies to all of the answers that were the same that didn't get the check mark. I chose the top one based off of it's completeness and the fact it was top. — thaBadDawg, Mar 01 '10 at 22:31
Do you actually want `Items` to be abstract? (ie to override the get or set properties?) If you extend from `AbstractClass` like most people are suggesting, `Container.Items` will implicitly be a `List`, and you won't even need to override it. — Tanzelax, Mar 01 '10 at 22:39
I want to leave the possibility of the getter and setter methods being overridden later on. In theory I don't need to, but in practice trying to undo that mess leads to a world of hurt. — thaBadDawg, Mar 01 '10 at 23:15

score 45 · Accepted Answer · edited Oct 20 '20 at 06:54

45

You need the declaration on the class as well, to know what type T is:

public abstract class AbstractClass<T>
{
    public int Id { get; set; }
    public int Name { get; set; }

    public abstract List<T> Items { get; set; }
}

public class Container : AbstractClass<Widgets>
{
    public override List<Widgets> Items { get; set; }
}

You can also restrict what T can be, like say it must implement IWidgets:

public class AbstractClass<T> where T : IWidgets

edited Oct 20 '20 at 06:54

Igor

3,573
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answered Mar 01 '10 at 22:25

Nick Craver

623,446
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3

Don't forget to add the `abstract` key word in the class declaration line itself, not just the property. – Nick Mar 01 '10 at 22:30

score 11 · Answer 2 · answered Mar 01 '10 at 22:28

You need to declare the type T.
You need to declare the class AbstractClass as abstract.
You need to use the override keyword.

Try this:

public class Widgets { }

public abstract class AbstractClass<T>
{
    public int Id { get; set; }
    public int Name { get; set; }

    public abstract List<T> Items { get; set; }
}

public class Container : AbstractClass<Widgets>
{
    public override List<Widgets> Items { get; set; }
}

score 6 · Answer 3 · answered Mar 01 '10 at 22:25

6

You need to make AbstractClass generic

public class AbstractClass<T> {
  ...
}

public class Container : AbstractClass<Widgets> { ...
}

answered Mar 01 '10 at 22:25

JaredPar

733,204
149
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1,454

score 3 · Answer 4 · answered Mar 01 '10 at 22:37

You need to mark AbstractClass abstract, because it contains abstract property
Specify the generic type in the AbstractClass declaration
Implement abstract property with override


public abstract class AbstractClass<T>
{
    public int Id { get; set; }
    public int Name { get; set; }

    public abstract List<T> Items { get; set; }
}

public class Container : AbstractClass<Widgets>
{
    public override List<Widgets> Items { get; set; }
}

score 2 · Answer 5 · answered Mar 01 '10 at 22:25

You need to define T like so

public class AbstractClass<T>
{
    public int Id { get; set; }
    public int Name { get; set; }

    public abstract List<T> Items { get; set; }
}

public class Container : AbstractClass<Widget>
{
    public List<Widgets> Items { get; set; }
}

score 1 · Answer 6 · answered Mar 01 '10 at 22:25

You need to specify the type in the abstract class:

public class AbstractClass<T>
{
    public int Id { get; set; }
    public int Name { get; set; }

    public abstract List<T> Items { get; set; }
}

public class Container : AbstractClass<Widgets>
{
    public List<Widgets> Items { get; set; }
}

score 0 · Answer 7 · answered Jan 11 '18 at 09:29

You are missing to define abstract keyword for your AbstractClass class and also you need to add override keyword for Container Class Items property. I am giving the example code which will run for sure.

namespace AbstractGeneric
{

public abstract class AbstractClass<T>
{
    public int Id { get; set; }
    public int Name { get; set; }

    public abstract List<T> Items { get; set; }
}

public class Widgets
{
    public int ID { get; set; }
    public string Name { get; set; }
}

public class Container : AbstractClass<Widgets>
{
    public override List<Widgets> Items { get; set; }
    public Container()
    {
        Items = new List<Widgets>();
    }

}


class Program
{
    static void Main(string[] args)
    {
        Container c = new Container();
        c.Items.Add(new Widgets() { ID = 1, Name = "Sample Widget 1" });
        c.Items.Add(new Widgets() { ID = 2, Name = "Sample Widget 2" });

        foreach (Widgets item in c.Items)
        {
            Console.WriteLine(item.ID + " : " + item.Name);
        }

        Console.ReadLine();
    }
}
}

Using generics in abstract classes

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