Save / Restore the TreeView nodes expansion state before and after a Refresh.

TreeViewExtensions.cs

using System.Collections.Generic;
using System.Linq;

namespace System.Windows.Forms
{
    internal static class TreeViewExtensions
    {
        internal static List<string> GetExpansionState(this TreeNodeCollection nodes)
        {
            return nodes.Descendants()
                        .Where(n => n.IsExpanded)
                        .Select(n => n.GetNamesFullPath())
                        .ToList();
        }

        internal static void SetExpansionState(this TreeNodeCollection nodes, List<string> savedExpansionState)
        {
            foreach (var node in nodes.Descendants()
                                      .Where(n => savedExpansionState.Contains(n.GetNamesFullPath())))
                node.Expand();
        }

        /// <summary>
        /// Returns a node path composed by the node names
        /// </summary>
        /// <param name="node"></param>
        /// <param name="separator"></param>
        /// <returns></returns>
        internal static string GetNamesFullPath(this TreeNode node, string separator = null)
        {
            return string.Join(separator ?? node.TreeView.PathSeparator, node.Ancestors(n => n.Parent).Select(n => n.Name).Reverse().ToArray());
        }

        /// <summary>
        /// Find a node from its full path
        /// </summary>
        /// <param name="nodes"></param>
        /// <param name="path"></param>
        /// <param name="comparison"></param>
        /// <returns></returns>
        internal static TreeNode FindByNamesFullPath(this TreeNodeCollection nodes, string path, StringComparison comparison = StringComparison.OrdinalIgnoreCase)
        {
            if (path == null)
                return null;

            return nodes.Descendants().FirstOrDefault(n => n.GetNamesFullPath().Equals(path, comparison));
        }

        internal static IEnumerable<TreeNode> Descendants(this TreeNodeCollection c)
        {
            foreach (var node in c.OfType<TreeNode>())
            {
                yield return node;

                foreach (var child in node.Nodes.Descendants())
                {
                    yield return child;
                }
            }
        }

        public static IEnumerable<T> Ancestors<T>(this T o, Func<T, T> action)
        {
            while (o != null)
            {
                yield return o;
                o = action(o);
            }
        }
    }
}