float/double Math.Round in C# [duplicate]

Question

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?

Answer 1

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