如何限制添加到派生类的项目类型？

Question

I have DerivedClass which inherits from BaseClass. I want to take advantage of the BaseCollection class which overrides the InsertItem method so I defined DerivedCollection and inherited from BaseCollection. The problem is, DerivedCollection allows both DerivedClass and BaseClass types to be added. I only want to permit DerivedClass types to be added to DerivedCollection. What am I missing?

public class BaseClass
{
    public static string MyString;
}

public class BaseCollection<T> : Collection<BaseClass> where T : BaseClass
{
    protected override void InsertItem(int index, BaseClass item)
    {
        throw new UnauthorizedAccessException("I'm sorry Dave, I'm afraid I can't do that.");
    }
}

public class DerivedClass : BaseClass
{
    public static int MyInteger;
}

public class DerivedCollection : BaseCollection<DerivedClass>
{
}

Answer 1

1. Change BaseCollection<T> to inherit from Collection<T> , not from Collection<BaseClass> .
2. Change arguments of InsertItem appropriately.

public class BaseCollection<T> : Collection<T> where T : BaseClass
{
    protected override void InsertItem (int index, T item)
    {
        throw new UnauthorizedAccessException("I'm sorry Dave, I'm afraid I can't do that.");
    }
}

Answer 2

You only want to allow T:

protected override void InsertItem(int index, T item)
{
    throw new UnauthorizedAccessException("I'm sorry Dave, I'm afraid I can't do that.");
}

You also have to inherit from Collection<T> instead of Collection<BaseClass>

Answer 3

You need to override InsertItem like so:

public class DerivedCollection : BaseCollection<DerivedClass>
{
protected override void InsertItem(int index, DerivedClass item)
    {
        base.InsertItem(index, item);
    }
}