When will a Comparer make Sort throw an ArgumentException?

Question

The documentation for Sort says that Sort will throw an ArgumentException if "The implementation of comparer caused an error during the sort. For example, comparer might not return 0 when comparing an item with itself."

Apart from the example given, can anyone tell me when this would otherwise happen?

Answer 1

The sort algorithm (QuickSort) relies on a predictable IComparer implementation. After a few dozen layers of indirection in the BCL you end up at this method:

public void Sort(T[] keys, int index, int length, IComparer<T> comparer)
{
    try
    {
        ...
        ArraySortHelper<T>.QuickSort(keys, index, index + (length - 1), comparer);

    }
    catch (IndexOutOfRangeException)
    {
        ...
        throw new ArgumentException(Environment.GetResourceString("Arg_BogusIComparer", values));
    }
}

Going a bit further into the QuickSort implementation, you see code like this:

    while (comparer.Compare(keys[a], y) < 0)
    {
        a++;
    }
    while (comparer.Compare(y, keys[b]) < 0)
    {
        b--;
    }

Basically if the IComparer misbehaves the Quicksort call with throw an IndexOutOfRangeException, which is wrapped in n ArgumentException.

Here is another example of bad IComparer's

class Comparer: IComparer<int>
{
    public int Compare(int x, int y)
    {
        return -1;
    }
}

So I guess, the short answer is, anytime your IComparer implementation does not consistently compare values as defined in the documentation:

Compares two objects and returns a value indicating whether one is less than, equal to or greater than the other.