Why do I get arithmetic discrepancies between Debug and Release modes and how to fix them?

Question

In my program, this snippet:

trace.log( String.Format("a= {0:R} b= {1:R} a<b= {2}", b.GetPixel(447, 517).GetBrightness(), (100F / 255F), b.GetPixel(447, 517).GetBrightness() < (100F / 255F)) ); 

displays this in Debug \ prog.exe:

  a= 0.392156869 b= 0.392156869 a<b= False 

but this other result in Release \ prog.exe:

  a= 0.392156869 b= 0.392156869 a<b= True 

Can someone explain why the same operands give a different comparison result? And recommend a security tool, ideally the same software as the compiler? Thanks.

EDIT: Explanation: The above results run Debug \ prog.exe and Release \ prog.exe in Windows Explorer.

EDIT: Additional information: running from VS, “Start debugging” gives False (ie, the exact result is the same as WE-launch Debug \ prog.exe) and “Start without debugging” True (i.e. Inaccurate the result is the same as WE-launch Release \ prog.exe.

EDIT: Alternate Test Examples with Replaced Literals

These two cases

  trace.log( String.Format("a= {0:R} b= {1:R} a<b= {2}", 0.392156869F, 0.392156869F, 0.392156869F < 0.392156869F) ); trace.log( String.Format("a= {0:R} b= {1:R} a<b= {2}", 0.392156869F, (100F / 255F), 0.392156869F < (100F / 255F)) ); 

discrepancies in the outputs of Debug and Release are not displayed. This case:

  trace.log( String.Format("a= {0:R} b= {1:R} a<b= {2}", b.GetPixel(447, 517).GetBrightness(), 0.392156869F, b.GetPixel(447, 517).GetBrightness() < 0.392156869F) ); 

shows the same mismatch (and inaccuracy in Release) as an initial test case.

EDIT: Hidden Minimal Test Case Demonstrating the Problem

 Color c = Color.FromArgb( 255, 100, 100, 100 ); trace.log( String.Format("a= {0} b= {1} a<b= {2}", c.GetBrightness(), 0.392156869F, c.GetBrightness() < 0.392156869F) ); 

outputs this correct result in Debug \ prog.exe:

  a= 0.392156869 b= 0.392156869 a<b= False 

but this incorrect result in Release \ prog.exe:

  a= 0.392156869 b= 0.392156869 a<b= True 

EDIT: Remedies

1) From Peter, answer below:

 trace.log( String.Format("a= {0:R} b= {1:R} a<b= {2}", c.GetBrightness(), 0.392156869F, c.GetBrightness().CompareTo(0.392156869F)<0) ); 

2) From ChrisJJ, questionnaire ( UPDATED ):

 float comp = c.GetBrightness(); trace.log( String.Format("a= {0:R} b= {1:R} a<b= {2}", comp, 0.392156869F, comp < 0.392156869F) ); 

I think this adds evidence of the Release mode compiler error.

Answer 1

Most likely, this is the result of various floating point rules between debugging and release modes (see here and here ).

Update:

Thanks to your new update to the question, I was able to reproduce the problem on my machine (64-bit Windows). This only happens when installing the platform on X86; X64 and AnyCPU platforms show the correct result in release mode. It is likely that when the platform is X86, the common language runtime uses X86 emulation in 64-bit machines and seems to be corrupted in the comparison operator.

However, I found a possible workaround: use CompareTo instead of "<" and ">", for example:

 c.GetBrightness().CompareTo(0.392156869F)<0 

On my machine, this will provide the same correct results on X86 as on X64 and AnyCPU.