LINQ OrderBy versus ThenBy

Can anyone explain what the difference is between:

tmp = invoices.InvoiceCollection
              .OrderBy(sort1 => sort1.InvoiceOwner.LastName)
              .OrderBy(sort2 => sort2.InvoiceOwner.FirstName)
              .OrderBy(sort3 => sort3.InvoiceID);

and

tmp = invoices.InvoiceCollection
              .OrderBy(sort1 => sort1.InvoiceOwner.LastName)
              .ThenBy(sort2 => sort2.InvoiceOwner.FirstName)
              .ThenBy(sort3 => sort3.InvoiceID);

Which is the correct approach if I wish to order by 3 items of data?

You should definitely use ThenBy rather than multiple OrderBy calls.

I would suggest this:

tmp = invoices.InvoiceCollection
              .OrderBy(o => o.InvoiceOwner.LastName)
              .ThenBy(o => o.InvoiceOwner.FirstName)
              .ThenBy(o => o.InvoiceID);

Note how you can use the same name each time. This is also equivalent to:

tmp = from o in invoices.InvoiceCollection
      orderby o.InvoiceOwner.LastName,
              o.InvoiceOwner.FirstName,
              o.InvoiceID
      select o;

If you call OrderBy multiple times, it will effectively reorder the sequence completely three times... so the final call will effectively be the dominant one. You can (in LINQ to Objects) write

foo.OrderBy(x).OrderBy(y).OrderBy(z)

which would be equivalent to

foo.OrderBy(z).ThenBy(y).ThenBy(x)

as the sort order is stable, but you absolutely shouldn't:

• It's hard to read
• It doesn't perform well (because it reorders the whole sequence)
• It may well not work in other providers (e.g. LINQ to SQL)
• It's basically not how OrderBy was designed to be used.

The point of OrderBy is to provide the "most important" ordering projection; then use ThenBy (repeatedly) to specify secondary, tertiary etc ordering projections.

Effectively, think of it this way: OrderBy(...).ThenBy(...).ThenBy(...) allows you to build a single composite comparison for any two objects, and then sort the sequence once using that composite comparison. That's almost certainly what you want.

I found this distinction annoying in trying to build queries in a generic manner, so I made a little helper to produce OrderBy/ThenBy in the proper order, for as many sorts as you like.

public class EFSortHelper
{
  public static EFSortHelper<TModel> Create<TModel>(IQueryable<T> query)
  {
    return new EFSortHelper<TModel>(query);
  }
}  

public class EFSortHelper<TModel> : EFSortHelper
{
  protected IQueryable<TModel> unsorted;
  protected IOrderedQueryable<TModel> sorted;

  public EFSortHelper(IQueryable<TModel> unsorted)
  {
    this.unsorted = unsorted;
  }

  public void SortBy<TCol>(Expression<Func<TModel, TCol>> sort, bool isDesc = false)
  {
    if (sorted == null)
    {
      sorted = isDesc ? unsorted.OrderByDescending(sort) : unsorted.OrderBy(sort);
      unsorted = null;
    }
    else
    {
      sorted = isDesc ? sorted.ThenByDescending(sort) : sorted.ThenBy(sort)
    }
  }

  public IOrderedQueryable<TModel> Sorted
  {
    get
    {
      return sorted;
    }
  }
}

There are a lot of ways you might use this depending on your use case, but if you were for example passed a list of sort columns and directions as strings and bools, you could loop over them and use them in a switch like:

var query = db.People.AsNoTracking();
var sortHelper = EFSortHelper.Create(query);
foreach(var sort in sorts)
{
  switch(sort.ColumnName)
  {
    case "Id":
      sortHelper.SortBy(p => p.Id, sort.IsDesc);
      break;
    case "Name":
      sortHelper.SortBy(p => p.Name, sort.IsDesc);
      break;
      // etc
  }
}

var sortedQuery = sortHelper.Sorted;

The result in sortedQuery is sorted in the desired order, instead of resorting over and over as the other answer here cautions.

if you want to sort more than one field then go for ThenBy:

like this

list.OrderBy(personLast => person.LastName)
            .ThenBy(personFirst => person.FirstName)

Yes, you should never use multiple OrderBy if you are playing with multiple keys. ThenBy is safer bet since it will perform after OrderBy.