public struct Data
{
    public int item1;
    public int item2;
    public int category_id;
    public string category_name;
}

List<Data> listObject = getData();
string[] catNames = listObject.Select(i=> i.category_name).Distinct().OrderByDescending(s => s).ToArray();

var cats = listObject
    .Select(i => new { i.category_id, i.category_name })
    .Distinct()
    .OrderByDescending(i => i.category_name)
    .ToArray();

Data[] cats = listObject
    .GroupBy(i => new { i.category_id, i.category_name })
    .OrderByDescending(g => g.Key.category_name)
    .Select(g => g.First())
    .ToArray();

.Select(i => new { i.name, i.category_name })

i => new { Id = i.category_id, Name = i.category_name }

var result = listObject.Select( i => new{ i.category_name, i.category_id } )

var selectedCategories =
	from value in
		(from data in listObject
		orderby data.category_name descending
		select new { ID = data.category_id, Name = data.category_name })
	group value by value.Name into g
	select g.First();

foreach (var category in selectedCategories) Console.WriteLine(category);

new { Property1 = value1, Property2 = value2, ... }

var listObject = getData();
var catNames = listObject.Select(i =>
    new { CatName = i.category_name, Item1 = i.item1, Item2 = i.item2 })
    .Distinct().OrderByDescending(s => s).ToArray();

(from i in list
 select new { i.category_id, i.category_name })
 .Distinct()
 .OrderBy(i => i.category_name);