private void Test()
{
    DataTable dt = new DataTable();
    BindingSource bindingSource = new BindingSource();
    bindingSource.DataSource = dt;

    dt.Columns.Add("Id", typeof(int));
    dt.Columns.Add("Value", typeof(string));
    dt.Columns.Add("Parent_Id", typeof(int));

    dt.Rows.Add(1, "value1", 0);
    dt.Rows.Add(2, "value2", 0);
    dt.Rows.Add(3, "value3", 0);
    dt.Rows.Add(4, "value4", 1);
    dt.Rows.Add(5, "value5", 1);
    dt.Rows.Add(6, "value6", 1);
    dt.Rows.Add(7, "value7", 2);
    dt.Rows.Add(8, "value8", 3);
    dt.Rows.Add(9, "value9", 3);
    dt.Rows.Add(10, "value10", 7);

    // 1 способ - с рекурсией (красивее)
    FillTreeView(treeView1, dt);

    // 2 способ - без рекурсии (чуточку шустрее)
    FillTreeView2(treeView1, bindingSource);

}

// ***************** 1 способ ***********************************************************

/// <summary>
///  Заполнение дерева из БД
/// </summary>
/// <param name="treeView"></param>
/// <param name="dt"></param>
private void FillTreeView(TreeView treeView, DataTable dt)
{
    treeView.Nodes.Clear();
    DataRow[] rows = dt.Select("Parent_Id = 0");
    foreach (DataRow row in rows)
    {
        TreeNode node = new TreeNode(row["Value"].ToString());
        node.Tag = row["Id"];
        treeView.Nodes.Add(node);
        FillNode(node, dt);
    }
}

/// <summary>
///  Добавление к ноде чилдов
/// </summary>
/// <param name="parentNode"></param>
/// <param name="dt"></param>
private void FillNode(TreeNode parentNode, DataTable dt)
{
    DataRow[] rows = dt.Select(string.Format("Parent_Id = {0}", parentNode.Tag));
    foreach (DataRow row in rows)
    {
        TreeNode treeNode = new TreeNode(row["Value"].ToString());
        treeNode.Tag = row["Id"];
        parentNode.Nodes.Add(treeNode);
        FillNode(treeNode, dt);
    }
}


// ***************** 2 способ ***********************************************************

/// <summary>
///  Заполнение дерева из БД
/// </summary>
/// <param name="treeView"></param>
/// <param name="bindingSource"></param>

private void FillTreeView2(TreeView treeView, BindingSource bindingSource)
{
    treeView.Nodes.Clear();
    bindingSource.Position = 0;
    for (int i = 0; i < bindingSource.Count; i++)
    {
        //Application.DoEvents();
        DataRow dr = (bindingSource.Current as DataRowView).Row;

        int id = Convert.ToInt32(dr["id"]);
        int parent_id = Convert.ToInt32(dr["parent_id"]);
        string name = dr["value"].ToString();

        if (parent_id == 0)
            treeView.Nodes.Add(FillNode(id, name));
        else
        {
            TreeNode[] treeNodes = treeView.Nodes.Find(parent_id.ToString(), true);
            if (treeNodes.Length != 0)
                (treeNodes.GetValue(0) as TreeNode).Nodes.Add(FillNode(id, name));
        }
        bindingSource.MoveNext();
    }
}

/// <summary>
/// Создание новой ноды
/// </summary>
/// <param name="id"></param>
/// <param name="name"></param>
/// <returns>TreeNode</returns>
private TreeNode FillNode(int id, string name)
{
    TreeNode treeNode = new TreeNode();
    treeNode.Tag = id;
    treeNode.Name = id.ToString();
    treeNode.Text = name;
    return treeNode;
}