How to flatten tree via LINQ?

Question

So I have simple tree:

class MyNode
{
 public MyNode Parent;
 public IEnumerable Elements;
 int group = 1;
}

I have a I

Answer 1

void Main()
{
    var allNodes = GetTreeNodes().Flatten(x => x.Elements);

    allNodes.Dump();
}

public static class ExtensionMethods
{
    public static IEnumerable Flatten(this IEnumerable source, Func> childrenSelector = null)
    {
        if (source == null)
        {
            return new List();
        }

        var list = source;

        if (childrenSelector != null)
        {
            foreach (var item in source)
            {
                list = list.Concat(childrenSelector(item).Flatten(childrenSelector));
            }
        }

        return list;
    }
}

IEnumerable GetTreeNodes() {
    return new[] { 
        new MyNode { Elements = new[] { new MyNode() }},
        new MyNode { Elements = new[] { new MyNode(), new MyNode(), new MyNode() }}
    };
}

class MyNode
{
    public MyNode Parent;
    public IEnumerable Elements;
    int group = 1;
}