public class Menus
{
    //菜单Id
    public int Id { get; set; }
    //菜单名
    public string MenuName { get; set; }
    //父节点Id
    public int ParentId { get; set; }
}

private static List<Menus> menus;

private void button1_Click(object sender, EventArgs e)
{
    menus = new List<Menus>();
    menus.Add(new Menus { Id = 1, MenuName = "基础设置", ParentId = 0 });
    menus.Add(new Menus { Id = 2, MenuName = "采购管理", ParentId = 0 });
    menus.Add(new Menus { Id = 3, MenuName = "系统设置", ParentId = 1 });
    menus.Add(new Menus { Id = 4, MenuName = "用户管理", ParentId = 1 });
    menus.Add(new Menus { Id = 5, MenuName = "采购订单", ParentId = 2 });
    menus.Add(new Menus { Id = 6, MenuName = "采购入库", ParentId = 2 });
    menus.Add(new Menus { Id = 7, MenuName = "用户", ParentId = 4 });
    menus.Add(new Menus { Id = 8, MenuName = "采购退货", ParentId = 2 });
    menus.Add(new Menus { Id = 9, MenuName = "用户组", ParentId = 4 });
    menus.Add(new Menus { Id = 10, MenuName = "权限管理", ParentId = 4 });
    
    //首先查出顶级菜单，循环添加
    IEnumerable<Menus> menu = from m in menus where m.ParentId == 0 orderby m.ParentId select m;

    foreach (var item in menu)
    {
        TreeNode node = new TreeNode();
        node.Tag = item;
        node.Text = item.MenuName;
        treeView1.Nodes.Add(node);
        //将当前节点作为参数传入子节点
        AddChildMenu(node);
    }

}

private void AddChildMenu(TreeNode node)
{
    Menus menu = new Menus();
    menu = (Menus)node.Tag;   //将父节点转为Model
    //查出当前子节点中所有同父节点Id一样的数据
    IEnumerable<Menus> childMenu = from m in menus where m.ParentId == menu.Id orderby m.Id select m;
    foreach (var item in childMenu)
    {
        TreeNode childNode = new TreeNode();
        childNode.Tag = item;
        childNode.Text = item.MenuName;
        node.Nodes.Add(childNode);  //添加子节点
        AddChildMenu(childNode);    //递归调用自己，直到父节点添加所有子节点结束
    }
}