Implement Group by LINQ query in two different Tables using C# in ASP.Net

Please refer below code.

Code

protected void Page_Load(object sender, EventArgs e)
{
    DataTable dtParentMenu = new DataTable();
    dtParentMenu.Columns.Add("ParentMenuId", typeof(int));
    dtParentMenu.Columns.Add("MenuName", typeof(string));
    dtParentMenu.Rows.Add(1, "Admin");
    dtParentMenu.Rows.Add(2, "Inventory");
    dtParentMenu.Rows.Add(3, "Report");

    DataTable dtSubMenu = new DataTable();
    dtSubMenu.Columns.Add("SubmenuId", typeof(int));
    dtSubMenu.Columns.Add("MenuName", typeof(string));
    dtSubMenu.Columns.Add("ParentMenuId", typeof(int));
    dtSubMenu.Rows.Add(1, "Dashboard", 1);
    dtSubMenu.Rows.Add(2, "Add user", 1);
    dtSubMenu.Rows.Add(3, "Report", 3);

    List<ParentMenu> result = (from pm in dtParentMenu.AsEnumerable()
                                join sm in dtSubMenu.AsEnumerable()
                                on pm.Field<int>("ParentMenuId") equals sm.Field<int>("ParentMenuId")
                                select new ParentMenu
                                {
                                    ParentMenuId = pm.Field<int>("ParentMenuId"),
                                    ParentMenuName = pm.Field<string>("MenuName")
                                }).ToList();
    List<ParentMenu> parents = new List<ParentMenu>();
    foreach (var item in result)
    {
        List<SubMenu> ChildMenus = new List<SubMenu>();
        ChildMenus.AddRange(from sm in dtSubMenu.AsEnumerable()
                            where sm.Field<int>("ParentMenuId").Equals(item.ParentMenuId)
                            select new SubMenu
                            {
                                SubMenuName = sm.Field<string>("MenuName")
                            });

        parents.Add(new ParentMenu
        {
            ParentMenuName = item.ParentMenuName,
            ChildMenus = ChildMenus
        });
    }
}

public class ParentMenu
{
    public List<SubMenu> ChildMenus { get; set; }
    public int ParentMenuId { get; set; }
    public string ParentMenuName { get; set; }
}

public class SubMenu
{
    public int SubmenuId { get; set; }
    public string SubMenuName { get; set; }
    public int ParentmenuId { get; set; }
}