泛型构造函数和继承

Question

I have a generic class with a class constraint on it.

public class MyContainer<T> where T : MyBaseRow

MyBaseRow is an abstract class which I also want to contain a member of some flavour of MyContainer.

public abstract class MyBaseRow
{
    public MyContainer<MyBaseRow> ParentContainer;

    public MyBaseRow(MyContainer<MyBaseRow> parentContainer)
    {
        ParentContainer = parentContainer;
    }
}

I am having problems with the constructors of classes inherited from MyBaseRow eg.

public class MyInheritedRowA : MyBaseRow
{
    public MyInheritedRowA(MyContainer<MyInheritedRowA> parentContainer)
    : base(parentContainer)
    { }
}

Won't allow MyInheritedRowA in the constructor, the compiler only expects and only allows MyBaseRow. I thought the generic class constraint allowed for inheritance? What am I doing wrong here and is there any way I can redesign these classes to get around this? Many thanks in advance for any responses.

Answer 1

Basically, you can't use generics that way, because the covariance system doesn't work that way with classes . See here: http://geekswithblogs.net/abhijeetp/archive/2010/01/10/covariance-and-contravariance-in-c-4.0.aspx

You can however use an interface like this:

public interface MyContainer<out T> where T : MyBaseRow {

    }

And that code will compile.

Answer 2

You can make a covariant generic interface (C#4.0):

  public interface IContainer<out T> where T : MyBaseRow
  {

  }

  public class MyContainer<T> : IContainer<T> where T : MyBaseRow
  {

  }

  public abstract class MyBaseRow
  {
    public IContainer<MyBaseRow> ParentContainer;

    public MyBaseRow(IContainer<MyBaseRow> parentContainer)
    {
      ParentContainer = parentContainer;
    }
  }

  public class MyInheritedRowA : MyBaseRow
  {
    public MyInheritedRowA(IContainer<MyInheritedRowA> parentContainer)
      : base(parentContainer)
    { }
  }