c# circular generic type parameters

Question

I have 2 generic classes: a ManagerBase class and a ChildBase class. They're both abstract and are intended to be made concrete. ManagerBase has a list of ChildBase, which is why I want to make it generic, so a CatalogManager : ManagerBase would have a list of Catalogs. Also, each Catalog would have reference to its' Manager - CatalogManager.

public class ManagerBase<T1> : ChildBase<???>
{
    public ManagerBase()
    {
        ChildObjects = new List<T1>();
    }

    public List<T1> ChildObjects { get; set; }
}

public class ChildBase<T1> : ManagerBase<???>
{
    public ChildBase(T1 parentMgr)
    {
        ParentMgr = parentMgr;
        ParentMgr.ChildObjects.Add(this);
    }

    public T1 ParentMgr { get; set; }
}

How can I resolve this object model?

Thanks.

public class ManagerBase<TManager, TChild> where TManager : ManagerBase<TManager, TChild> where TChild : ChildBase<TManager, TChild>, public class ChildBase<TManager, TChild> where TManager : ManagerBase<TManager, TChild> where TChild : ChildBase<TManager, TChild>, class CatalogManager : ManagerBase<CatalogManager, Catalog>, class Catalog : ChildBase<CatalogManager, Catalog>. — user4003407
– user4003407, Commented Apr 9, 2017 at 20:04

Enigmativity · Accepted Answer · 2017-04-10 00:21:08Z

You have to define the base classes using the "curiously recursive" pattern:

public class ManagerBase<M, T1>
    where M : ManagerBase<M, T1>
    where T1 : ChildBase<M, T1>
{
    public ManagerBase()
    {
        ChildObjects = new List<T1>();
    }

    public List<T1> ChildObjects { get; set; }
}

public class ChildBase<T1, C>
    where T1 : ManagerBase<T1, C>
    where C : ChildBase<T1, C>
{
    public ChildBase(T1 parentMgr)
    {
        ParentMgr = parentMgr;
        ParentMgr.ChildObjects.Add((C)(object)this);
    }

    public T1 ParentMgr { get; set; }
}

Please note I've stuck with your usage of T1, but I think that's a bit confusing. I would have preferred to have used M and C for each.

The major downside with this is that you have to use the nasty double cast of (C)(object)this to make this work. C# doesn't allow full type safety on this pattern. A nefarious developer can create child classes that can break the pattern.

Then the concrete classes can be this:

public class CatalogManager : ManagerBase<CatalogManager, Catalog>
{
}

public class Catalog : ChildBase<CatalogManager, Catalog>
{
    public Catalog(CatalogManager parentMgr) : base(parentMgr)
    {
    }
}

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