IComparer issue

Question

I have a weird issue and I don't have a clue to track the reason. I will try to descript my issue clearly.

I have a RTree class, in this class, I want to compare two rectanlge (here I called envelope, it contains minX, minY, maxX, maxY), so we have a comparer class as following:

private class AnonymousXComparerImpl : IComparer
{
    public AnonymousXComparerImpl()
    { }

    public int Compare(object o1, object o2) 
    {
        IEnvelope ea = (IEnvelope)((IBoundable)o1).Bounds;
        IEnvelope eb = (IEnvelope)((IBoundable)o2).Bounds;
        double a = (ea.MinX + ea.MaxX) / 2d;
        double b = (eb.MinX + eb.MaxX) / 2d;
        return a > b ? 1 : a < b ? -1 : 0;
    }
}

With this comparer, we can maintain a ArrayList of envelope and sort it easily, the envelopes are randomly added. When we call the following code and we met the

Unable to sort because the IComparer.Compare() method returns inconsistent results. Either a value does not compare equal to itself, or one value repeatedly compared to another value yields different results.

sortedChildBoundables.Sort(new AnonymousXComparerImpl());

Here is the weird part. This error only occurs in .net 4.0 which doesn't install the VistualStudio. If the machine installed the VS or .net 4.5, this issue cannot reproceduce again.

In this case, I cannot figure out why it happens. It will be great if you have any experience on debuging this kind of issue, I appreciate.

Thanks, Howard

Answer 1

If e.g. ea.MinX is NaN, a will be NaN and both a > b and a < b will be false. This means, there are objects which compare equal to every other object.

You first have to decide, how you want objects containing NaN to be sorted.

An easy workaround might be to insert

if (double.IsNaN(a)) a = 0.0;
if (double.IsNaN(b)) b = 0.0;

As noted by @Seph and @Jeppe in comments, double.CompareTo does the right thing, so the last line can be replaced by return a.CompareTo(b);.

Answer 2

One likely reason is that your information is actually changed during compare.

If you do the sorting in a background thread you will certainly get this error if the comparison get different values for the same item when asking for it twice.

If your main thread updates one of the values (maybe by databinding) while the comparison runs for example.

Make sure you cache the value compared so that you always return consistent results. Or accept that the error may happen from time to time and redo the sort if it does.

This will also explain your feeling of machine/os dependency. Multithreading issues occur differently depending of software and hardware differences.