LINQ OrderBy versus ThenBy

Question

Can anyone explain what the difference is between:

tmp = invoices.InvoiceCollection
              .OrderBy(sort1 => sort1.InvoiceOwner.LastName)
                  


        

Answer 1

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 Create(IQueryable query)
  {
    return new EFSortHelper(query);
  }
}  

public class EFSortHelper : EFSortHelper
{
  protected IQueryable unsorted;
  protected IOrderedQueryable sorted;

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

  public void SortBy(Expression> 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 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.