How to get all parents (up to root) nodes for selected in TreeView control?

I'd recomended you to create a set of your own tree helpers, for example, next one is for your problem:

    public static class TreeHelpers
    {
        public static IEnumerable<TItem> GetAncestors<TItem>(TItem item, Func<TItem, TItem> getParentFunc)
        {
            if (getParentFunc == null)
            {
                throw new ArgumentNullException("getParentFunc");
            }
            if (ReferenceEquals(item, null)) yield break;
            for (TItem curItem = getParentFunc(item); !ReferenceEquals(curItem, null); curItem = getParentFunc(curItem))
            {
                yield return curItem;
            }
        }

        //TODO: Add other methods, for example for 'prefix' children recurence enumeration
    }

And example of usage (in your context):

        IList<TreeNode> ancestorList = TreeHelpers.GetAncestors(node, x => x.Parent).ToList();

Why is this better than using list<>.Add()? - because we can use lazy LINQ functions, such as .FirstOrDefault(x => ...)

P.S. to include 'current' item into result enumerable, use TItem curItem = item, instead of TItem curItem = getParentFunc(item)

If you want the actual objects, use the TreeNode.Parent property recursively until you reach the root. Something like:

private void GetPathToRoot(TreeNode node, List<TreeNode> path)
{
    if(node == null) return; // previous node was the root.
    else
    {
        path.add(node);
        GetPathToRoot(node.Parent, path);
    }
}