.NET Framework - Color comparer mystery

Asked By Andrew

03-Jul-08 11:54 AM

The following code gets colors and sorts them.  When calling
GetColorsSorted() it uses the ColorComparer to compare each color.

In VS2008 on XP when running with the "Debug" build the compare has a
different number of iterations to when running under a "Release" build.  I
can't understand why this would be.

When the program was run on Vista the "Debug" version worked (again with a
different no of iterations) but the "Release" version hung forever because
the comparer never produced a zero return when it found the same items.

Why would the float variables get different values between the "Debug" and

Can anyone understand why it would never drop through to the zero return?

I have now fixed this, see the bottom for details.

Here is the code:

public static IEnumerable<Color> GetColors()
{
foreach (PropertyInfo PInfo in typeof(Color).GetProperties())
if (PInfo.PropertyType == typeof(Color))
if (PInfo.Name != "Transparent")
if (PInfo.Name != "Empty")
yield return Color.FromName(PInfo.Name);
}

public static IEnumerable<Color> GetColorsSorted()
{
List<Color> List = new List<Color>();

List.AddRange(GetColors());

List.Sort(new ColorComparer());

foreach (Color C in List)
yield return C;
}

internal class ColorComparer : IComparer<Color>
{
#region IComparer Members

public int Compare(Color x, Color y)
{
float hx, hy, sx, sy, bx, by;

// get saturation values
sx = x.GetSaturation();
sy = y.GetSaturation();
// get hue values
hx = x.GetHue();
hy = y.GetHue();
// get brightness values
bx = x.GetBrightness();
by = y.GetBrightness();

// determine order
// 1 : hue
if (hx < hy)
return -1;
else if (hx > hy)
return 1;
else
{
// 2 : saturation
if (sx < sy)
return -1;
else if (sx > sy)
return 1;
else
{
// 3 : brightness
if (bx < by)
return -1;
else if (bx > by)
return 1;
else
return 0;
}
}
}
#endregion
}


To fix this I added the following code at the top of the comparer:
if (x.ToArgb() == y.ToArgb())
return 0;



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Andrew Cutforth - AJC Software - www.ajcsoft.com
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03-Jul-08 01:19 PM

I don't know the precise answer.  But it's not surprising at all to me
that there may be subtle differences in floating point behavior between
the two versions.  Doing identity comparisons between floating point
values is almost never the right approach, and your experience
demonstrates why.

I think that your solution, converting to integers and then comparing
that, is a perfectly reasonable approach, especially given the problem
domain.  You could also have compared the difference between two floating
point values and considered them equal as long as the difference was small
enough.  That's a fairly common approach in other scenarios using floating
point, but in this case it's probably overkill.

Pete

03-Jul-08 01:30 PM

That's neat code, Andrew. You've got a few more years under your belt than I
do.

I am going to go see what "yield" means, now. ;)