The problem is that .NET will always round a double
to 15 significant decimal digits before applying your formatting, regardless of the precision requested by your format and regardless of the exact decimal value of the binary number.
I'd guess that the Visual Studio debugger has its own format/display routines that directly access the internal binary number, hence the discrepancies between your C# code, your C code and the debugger.
There's nothing built-in that will allow you to access the exact decimal value of a double
, or to enable you to format a double
to a specific number of decimal places, but you could do this yourself by picking apart the internal binary number and rebuilding it as a string representation of the decimal value.
Alternatively, you could use Jon Skeet's DoubleConverter
class (linked to from his "Binary floating point and .NET" article). This has a ToExactString
method which returns the exact decimal value of a double
. You could easily modify this to enable rounding of the output to a specific precision.
double i = 10 * 0.69;
Console.WriteLine(DoubleConverter.ToExactString(i));
Console.WriteLine(DoubleConverter.ToExactString(6.9 - i));
Console.WriteLine(DoubleConverter.ToExactString(6.9));
// 6.89999999999999946709294817992486059665679931640625
// 0.00000000000000088817841970012523233890533447265625
// 6.9000000000000003552713678800500929355621337890625