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?
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.
I ran into this today, and after investigating, I found that sometimes my comparer was being called with x and y being references to the same object, and my comparer was not returning 0. Once I fixed that, I stopped getting the exception.
HTH,
Eric
