Question

I want to create an extendable nested structure and it seems like I should be able to do this using generics, though I may not be using them “properly”.

I want to be able to create child classes from GroupType and/or OptionType. The problem is that I can’t perform the new operation on the generic types even though I specified they could only be of a certain base type.

Is there any way to do what I’m trying to do?

public class AllInfo<GroupType, OptionType> 
    where GroupType: GroupBase<OptionType>
    where OptionType: OptionBase
{
    public List<string> Names { set; get; }
    public List<GroupType> Groups { set; get; }

    public AllInfo()
    {
        DataSet ds = DatabaseRetreival();
        this.Groups.add(new GroupType(ds["Name"], ds["Type"]));
    }

}

public class GroupBase<OptionType> 
    where OptionType: OptionBase
{
    public string Name { set; get; }
    public string Type { set; get; }
    public List<OptionType> Options { set; get; }

    public GroupBase(string name, string type)
    {
         this.Name = name;
         this.Type = type;

         DataSet ds = DatabaseRetreival(this.Type);
         this.Options.Add(new OptionType(ds["Name"]));
    }
}

public class OptionBase
{
    public string Name { set; get; }

    public OptionBase(string name)
    {
        this.Name = name;
    }
}

Answer 1

You can’t specify which constructors a generic class should have. The constructors are not inherited, so even if the base class that you specified has that constructor, a class that derives from it doesn’t have to have that constructor.

The only constructor that you can require is the parameterless constructor:

where GroupType: GroupBase<OptionType>, new()

As that only let’s you use the parameterless constructor, you would also use a virtual method for putting the data in the object, for example:

GroupType group = new GroupType();
group.Init(ds["Name"], ds["Type"]);
this.Groups.add(group);