linq null parameter - linq

Question

how can i do the code
static string BuildMenu(List<Menu> menu, int? parentId)
{
foreach (var item in menu.Where(i => i.ParentMenu == parentId || i.ParentMenu.MenuId == parentId).ToList())
{
}
}
return BuildMenu(menuList,null);
so if parentId==null then return only records i => i.ParentMenu == null but when parentId is >0 then return records with i.ParentMenu.MenuId == parentId

Answer 1

You could do that in a single statement, probably something like this
var query = from menu in menus
where (parentId == null ? menu.ParentMenu == null : menu.ParentMenu != null && menu.ParentMenu.MenuId == parentId)
select menu;
But I think it would be more readable by breaking it up like the following
var query = menus.AsEnumerable();
if (parentId.HasValue)
query = query.Where(m => m.ParentMenu != null && m.ParentMenu.MenuId == parentId);
else
query = query.Where(m => m.ParentMenu == null);
Full example:
public class Menu
{
public Menu ParentMenu { get; set; }
public int MenuId { get; set; }
}
...
static void Main(string[] args)
{
List<Menu> menus = new List<Menu>();
for (int i = 0; i < 4; i++)
{
menus.Add(new Menu() { MenuId = i });
}
menus[2].ParentMenu = menus[0];
menus[3].ParentMenu = menus[1];
Console.WriteLine(BuildMenu(menus, 1));
Console.Read();
}
static string BuildMenu(List<Menu> menus, int? parentId)
{
var query = menus.AsEnumerable();
if (parentId.HasValue)
query = query.Where(m => m.ParentMenu != null && m.ParentMenu.MenuId == parentId);
else
query = query.Where(m => m.ParentMenu == null);
StringBuilder builder = new StringBuilder();
foreach (Menu menu in query)
{
// build your string
builder.Append(menu.MenuId + ";");
}
return builder.ToString();
}