Foreach loop on a generic list in a class which inherits from a abstract class

Question

(The title is not very good I supposed... feel free to change; I hope I can express myself better with an example)

I have the following classes:

abstract class AbstractObject<T>
{
List<T> Children { get; protected set; }
}

class A : AbstractObject<A>
{
//I have access to a List<A>
}

class B : AbstractObject<B>
{
//I have access to a List<B>
}

Then, at some point I have the following method:

private TreeNode PupulateRecursively<T>(AbstractObject<T> _us)
    {
        if (_us.Children == null || _us.Children.Count == 0)
        {
            return new TreeNode(string.Format("{0} - {1}", _us.FormattedID, _us.Name)) { Tag = _us };
        }
        else
        {
            Debug.Assert(_us.Children.Count > 0);

            TreeNode node = new TreeNode(string.Format("{0} - {1}", _us.FormattedID, _us.Name)) { Tag = _us };
            foreach (var child in _us.Children)
            {
                TreeNode n = PupulateRecursively(child);
                node.Nodes.Add(n);
            }
            return node;
        }
    }

But I'm getting a: The type arguments for method PupulateRecursively<T> (AbstractRallyObject<T>) cannot be inferred from the usage. Try specifying the type arguments explicitly. The type arguments for method PupulateRecursively<T> (AbstractRallyObject<T>) cannot be inferred from the usage. Try specifying the type arguments explicitly. , because the child var inside the foreach is of type T.

How can I fix this? It is a design issue?

Answer 1

The parameter to PupulateRecursively<T> is an AbstractRallyObject<T> but you're passing in a T :

foreach (var child in _us.Children)           // Children is a List<T>
{
    TreeNode n = PupulateRecursively(child);  //child is a T

you could define Children as a List<AbstractObject<T>> whichi would compile but I don't know enough about your design to know if it's right .

Answer 2

似乎您没有这样的通用参数就调用了PupulateRecursively方法：

 TreeNode n = PupulateRecursively<T>(child);

Answer 3

It looks like you need to define your collection as List<AbstractObject<T>>

abstract class AbstractObject<T>
{
    List<AbstractObject<T>> Children { get; protected set; }
}