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.