Sorting the list by two parameters simultaneously C#

Use custom sort comparison function.

Example 1:

    public struct MyItem
    {
        public int index;
        public DateTime date;
    }
    class Program
    {
        static void Main(string[] args)
        {
            var items = new MyItem[]{
                new MyItem{index=9, date=DateTime.Now},
                new MyItem{index=4, date=DateTime.Now},
                new MyItem{index=3, date=DateTime.Now},
                new MyItem{index=5, date=DateTime.Now},
                new MyItem{index=5, date=DateTime.Now + TimeSpan.FromDays(1)},
                new MyItem{index=6, date=DateTime.Now},
            };
            // sort by index, if equal then sort by date
            Array.Sort(items, (x, y) =>
            {
                if (x.index == y.index)
                    return x.date.CompareTo(y.date);
                return x.index.CompareTo(y.index);
            });

            foreach (var item in items)
                Console.WriteLine($"{item.index} {item.date}");
        }
    }

Example 2:

var items = new List<MyItem>{
    new MyItem{index=9, date=DateTime.Now},
    new MyItem{index=4, date=DateTime.Now},
    new MyItem{index=3, date=DateTime.Now},
    new MyItem{index=5, date=DateTime.Now},
    new MyItem{index=5, date=DateTime.Now + TimeSpan.FromDays(1)},
    new MyItem{index=6, date=DateTime.Now},
};
// sort by index, if equal then sort by date
items.Sort((x, y) => x.index.CompareTo(y.index) == 0 ? x.date.CompareTo(y.date) : x.index.CompareTo(y.index));

Example3: Linq

            var items = new List<MyItem>{
                new MyItem{index=9, date=DateTime.Now},
                new MyItem{index=4, date=DateTime.Now},
                new MyItem{index=3, date=DateTime.Now},
                new MyItem{index=5, date=DateTime.Now},
                new MyItem{index=5, date=DateTime.Now + TimeSpan.FromDays(1)},
                new MyItem{index=6, date=DateTime.Now},
            };
            // sort by index, if equal then sort by date
            var newItems = items.OrderBy(x => x.index).ThenBy(x => x.date);

If you want to Sort in place and you don't want to implement IComparable<Some> or IComparer<Some>:

List<Some> myList = ...

...

myList.Sort((left, right) => {
  // latest dates on the top (note - for descending sorting)
  int result = -left.date.CompareTo(right.date);

  // on tie when left and right have the same date we compare indexes
  return result == 0 
    ? -left.index.CompareTo(right.index)
    :  result; 
});

In case you have several fragments where you want to sort list in such a way, you can implement a comparer:

public sealed class SomeComparer : IComparer<Some> { 
  public int Compare(Some left, Some right) {
    // latest dates on the top (note - for descending sorting)
    int result = -left.date.CompareTo(right.date);

    // on tie when left and right have the same date we compare indexes
    return result == 0 
      ? -left.index.CompareTo(right.index)
      :  result;
  }
} 

Then whenever you want to sort the list:

myList.Sort(new SomeComparer());

Finally, if it's the only order you want to sort Some items, you can make Sort comparable:

public struct Some : IComparable<Some>
{
    public int index;
    public DateTime date;

    //TODO: implement Equals and GetHashCode 

    public bool CompareTo(Some other) {
      int result = -date.CompareTo(other.date);

      return result == 0 
        ? -index.CompareTo(other.index)
        : result;
    }
}

and you can put

myList.Sort();

You can use LINQ to easily sort items in a list by the values contained in a that list.

using System.Linq;

...

myStructs.OrderBy(s => s.index).ThenBy(s => s.date)

Will put everything in order by index. Items with equal index would be sorted by date.