Equivalent C# statement for this VB6 operation creating problems

Question

I have this code line in VB:

Dim Sqrt As Double
Sqrt = Radius ^ 2 - (CenterX - X) ^ 2

The parameters in the statement above are being passed the values below:

X=  -7.3725025845036161 Double
CenterX =0.0            Double
Radius= 8.0             Double

On executing the statement above, the value of Sqrt is below:

Sqrt    9.646205641487505   Double

Now I wrote a similar C# logic using the Math class:

double Sqrt = 0;
Sqrt = Math.Pow(Radius, 2) - Math.Pow((CenterX - X), 2);

with the same set of values, the output in C# code was:

Sqrt    9.6462056414874979  double

I need help because of this single change in C# code, all my values are getting affected. Is there anything I can do to get the similar value as of the *VB* source?

Answer 1

There is a difference in the precision between the VB6 and the .NET double type. Both are IEEE 64-bit double-precision types, but the .NET CLR uses 80-bit extended-precision internally, i.e. your computations will be more accurate in .NET.

If you have to be backward-compatible with the VB6 precision, you can force your FPU (floating point unit) to use the (less accurate) 64-bit values. This can be achieved using the native _controlfp_s function.

Below is a code snippet that you can use to temporarily "downgrade" floating point precision for backward compatibility. You can use it like this:

Usage

// default floating point precision 

using (new FloatingPoint64BitPrecision())
{
    // floating-point precision is set to 64 bit
}

// floating-point precision is reset to default

Code Snippet

/// <summary>
/// This class changes floating-point precision to 64 bit
/// </summary>
internal class FloatingPoint64BitPrecision : IDisposable
{
    private readonly bool _resetRequired;

    public FloatingPoint64BitPrecision()
    {
        int fpFlags;
        var errno = SafeNativeMethods._controlfp_s(out fpFlags, 0, 0);
        if (errno != 0)
        {
            throw new Win32Exception(
                errno, "Unable to retrieve floating-point control flag.");
        }

        if ((fpFlags & SafeNativeMethods._MCW_PC) != SafeNativeMethods._PC_64)
        {
            Trace.WriteLine("Change floating-point precision to 64 bit");
            errno = SafeNativeMethods._controlfp_s(
                out fpFlags, SafeNativeMethods._PC_64, SafeNativeMethods._MCW_PC);

            if (errno != 0)
            {
                throw new Win32Exception(
                    errno, "Unable to change floating-point precision to 64 bit.");
            }

            _resetRequired = true;
        }
    }

    public void Dispose()
    {
        if (_resetRequired)
        {
            Trace.WriteLine("Resetting floating-point precision to default");
            SafeNativeMethods._fpreset();
        }
    }
}

internal static class SafeNativeMethods
{
    [DllImport("msvcr120.dll")]
    public static extern void _fpreset();

    [DllImport("msvcr120.dll", CallingConvention = CallingConvention.Cdecl)]
    public static extern int _controlfp_s(
        out int currentControl, int newControl, int mask);

    public static int _CW_DEFAULT = 
        (_RC_NEAR | _PC_53 | _EM_INVALID | _EM_ZERODIVIDE | _EM_OVERFLOW 
        | _EM_UNDERFLOW | _EM_INEXACT | _EM_DENORMAL);

    public const int _MCW_EM = 0x0008001f;          // interrupt Exception Masks 
    public const int _EM_INEXACT = 0x00000001;      //   inexact (precision) 
    public const int _EM_UNDERFLOW = 0x00000002;    //   underflow 
    public const int _EM_OVERFLOW = 0x00000004;     //   overflow 
    public const int _EM_ZERODIVIDE = 0x00000008;   //   zero divide 
    public const int _EM_INVALID = 0x00000010;      //   invalid 
    public const int _EM_DENORMAL = 0x00080000;     // denormal exception mask 
                                                    // (_control87 only) 

    public const int _MCW_RC = 0x00000300;          // Rounding Control 
    public const int _RC_NEAR = 0x00000000;         //   near 
    public const int _RC_DOWN = 0x00000100;         //   down 
    public const int _RC_UP = 0x00000200;           //   up 
    public const int _RC_CHOP = 0x00000300;         //   chop 

    public const int _MCW_PC = 0x00030000;          // Precision Control 
    public const int _PC_64 = 0x00000000;           //    64 bits 
    public const int _PC_53 = 0x00010000;           //    53 bits 
    public const int _PC_24 = 0x00020000;           //    24 bits 

    public const int _MCW_IC = 0x00040000;          // Infinity Control 
    public const int _IC_AFFINE = 0x00040000;       //   affine 
    public const int _IC_PROJECTIVE = 0x00000000;   //   projective 
}

Answer 2

There no need to use the Math class, simply write your calculus this way :

sqrt = Radius * Radius - (CenterX - x) * (CenterX - x);