Question

Ok, I have a simple IEnumerable<HtmlString> things and I want to divide it up into four equal groups.

var quarter = things.OrderBy(t => t.Foo).Count() / 4;

should do the trick, but instead I get this funkiness:

Server Error in ‘/’ Application. At least one object must implement
IComparable. Description: An unhandled exception occurred during the
execution of the current web request. Please review the stack trace
for more information about the error and where it originated in the
code.

Exception Details: System.ArgumentException: At least one object must
implement IComparable.

Line 36: int quarter = things.OrderBy(t => t.Foo).Count() / 4;

Anyone know what the heck is going on here? Why would I need to implement IComparable to get a simple count?

Answer 1

My guess is that this is to do with lazy evaluation of LINQ’s OrderBy. For example, if you have:

var things = unsortedThings.OrderBy(foo => foo.Bar);
var quarter = things.Count() / 4;

then if the foo.Bar properties can’t be compared with each other, that will throw exactly that exception.

For example:

using System;
using System.Linq;

class Foo {}

class Program
{
    public static void Main()
    {
        var foos = new[] { new Foo(), new Foo() };
        var ordered = foos.OrderBy(x => x);
        Console.WriteLine(ordered.Count());
    }
}

Output:

Unhandled Exception: System.ArgumentException: At least one object must implement IComparable.
   at System.Collections.Comparer.Compare(Object a, Object b)
   at System.Linq.EnumerableSorter`2.CompareKeys(Int32 index1, Int32 index2)
   at System.Linq.EnumerableSorter`1.QuickSort(Int32[] map, Int32 left, Int32 right)
   at System.Linq.EnumerableSorter`1.Sort(TElement[] elements, Int32 count)
   at System.Linq.OrderedEnumerable`1.<GetEnumerator>d__0.MoveNext()
   at System.Linq.Enumerable.Count[TSource](IEnumerable`1 source)
   at Program.Main()