Question

How can I perform a sort on two different criteria?

For example, I have person objects like:

Person with properties FirstName (string), LastName, and Rank (int).

Example data like so:

Xavier    Smith 1
Alexander Smith 2
Alexander Smith 1
Bob       Hawke 2

It should sort on FirstName alphabetically, then on rank, e.g. resulting:

Alexander Smith 1
Alexander Smith 2
Bob       Hawke 2
Xavier    Smith 1

So far, I have tried the following, but it isn’t working properly:

peopleList is List<Person>

peopleList.Sort(new Comparison<Person>((x,y) => x.Rank.CompareTo(y.Rank)));
peopleList.Sort(new Comparison<Person>((x, y) => string.Compare(x.Name, y.Name)));

Thanks

edit: to avoid changing my code too much, I really want to keep list, if I change the above lines to:

peopleList.OrderBy(person => person.FirstName).ThenBy(person => person.Rank).ToList();

Would give the exact same list just sorted properly, correct?

Answer 1

LINQ Approach

With LINQ you can use OrderBy and ThenBy:

var result = peopleList.OrderBy(p => p.FirstName).ThenBy(p => p.Rank);

This will return an IEnumerable<T>. If you really need a List<T> add a .ToList() at the end.

If you want to use the Sort method then you’ll need to write a custom comparer.

EDIT: using ToList() returns a new list. If you want to sort your existing list then you should use the Sort method which does not return a list but rather operates on the current list (it is a void method).

Sort / Comparer Approach

Use: list.Sort(new PersonComparer());

Here’s the comparer code. It was adapted from the MSDN example so I recommend reading the comments they used to understand why it is structured this way.

public class PersonComparer : IComparer<Person>
{
    public int Compare(Person x, Person y)
    {
        if (x == null)
        {
            if (y == null)
            {
                return 0;
            }
            else
            {
                return -1;
            }
        }
        else
        {
            if (y == null)
            {
                return 1;
            }
            else
            {
                int retval = x.FirstName.CompareTo(y.FirstName);

                if (retval != 0)
                {
                    return retval;
                }
                else
                {
                    return x.Rank.CompareTo(y.Rank);
                }
            }
        }
    }
}