Question

According to Pro LINQ: Language Integrated Query in C# 2008, Prototype of OrderBy operator is

public static IOrderedEnumerable<T> OrderBy<T, K>(     this IEnumerable<T> source,     Func<T, K> keySelector) where     K : IComparable<K> 

But the MSDN documentation does not have a generics contraint on TKey that it should be of type IComparable<TKey>

public static IOrderedEnumerable<TSource> OrderBy<TSource, TKey>(     this IEnumerable<TSource> source,     Func<TSource, TKey> keySelector ) 

I am basically sorting Inventory by Unit and then by Size.

    var sortedInventories = inventories                             .OrderBy(inventory => inventory.Unit)                             .OrderBy(inventory => inventory.Size); 

From the above code snippet, lambda expressions simply return inventory properties to sort by. It does not look like an expression that returns IComparer<T>

But according to logic, it looks like the lambda expression should be of type IComparer<T>.

Which one is the correct declaration of OrderBy?
(Apress.com Errata page has no information on it)

Here is a sample application I created to test OrderBy

public class Program {     public static void Main(string[] args)     {         var inventories = new[] {             new Inventory { Unit = 1, Size = 2 },             new Inventory { Unit = 2, Size = 4 },             new Inventory { Unit = 3, Size = 6 },         };         var sortedInventories = inventories                                 .OrderBy(inventory => inventory.Unit)                                 .OrderBy(inventory => inventory.Size);          foreach (var inventory in sortedInventories)             Console.WriteLine('Unit: {0}; Size = {1}', inventory.Unit, inventory.Size);     } }  public class Inventory {     public int Unit { get; set; }     public double Size { get; set; } } 

Answer 1

Change your second ‘OrderBy’ to ‘ThenBy’. You’re currently resorting everything, so it’s effectively by Size and then Unit, but inefficiently. I’m not sure where you think IComparer<T> should come in, unless you specify it as another argument. Basically it uses Comparer<T>.Default unless you specify a separate comparer.

Anyway, your query should be:

var sortedInventories = inventories                           .OrderBy(inventory => inventory.Unit)                           .ThenBy(inventory => inventory.Size); 

(With your test data you can’t tell the difference, because in each case Size = Unit * 2. Try it with one item which has a small Unit and a large Size though.)

Yes, it looks like the book got the signature slightly wrong – possibly due to it changing shortly before release. If you were basically worried about getting the wrong results though, the above is the explanation.