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float ff = (float)31.15; double dd = 31.15; var frst = Math.Round(ff, 1, MidpointRounding.AwayFromZero); var drst = Math.Round(dd, 1, MidpointRounding.AwayFromZero); frst: 31.1
drst: 31.2
Can someone explain why?
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The round( ) function in the C programming language provides the integer value that is nearest to the float, the double or long double type argument passed to it. If the decimal number is between “1 and. 5′′, it gives an integer number less than the argument.
So ( sizeof(int) + that remainder - one) is always >= sizeof(int). So it always rounds up.
Well, Math.Round wants double, not float, that's why
Math.Round(ff, 1, MidpointRounding.AwayFromZero); equals to
Math.Round((double)ff, 1, MidpointRounding.AwayFromZero); and if we inspect (double)ff value
Console.Write(((double)ff).ToString("R")); we'll see round up errors in action
31.149999618530273 Finally, Math.Round(31.149999618530273, 1, MidpointRounding.AwayFromZero) == 31.1 as expected
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