CODEWITHSUNDEEP
c#genericsabstraction
thaBadDawg
thaBadDawg

Reputation: 5230

Using generics in abstract classes

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?

Upvotes: 29

Views: 68121

Answers (7)

Nick Craver
Nick Craver

Reputation: 630637

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

Upvotes: 48

Humayoun Kabir
Humayoun Kabir

Reputation: 11

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();
    }
}
}

Upvotes: 1

Stanislav Basovn&#237;k
Stanislav Basovn&#237;k

Reputation: 449

  1. You need to mark AbstractClass abstract, because it contains abstract property

  2. Specify the generic type in the AbstractClass declaration

  3. 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; }
}

Upvotes: 3

Mark Byers
Mark Byers

Reputation: 839234

  • 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; }
}

Upvotes: 11

JDMX
JDMX

Reputation: 1509

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; }
}

Upvotes: 2

Reed Copsey
Reed Copsey

Reputation: 564931

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; }
}

Upvotes: 1

JaredPar
JaredPar

Reputation: 755587

You need to make AbstractClass generic

public class AbstractClass<T> {
  ...
}

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

Upvotes: 6

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