Question

In C#, why don’t the rounding math functions Floor, Ceiling and Round return an int? Considering the result of the function will always be an integer, why does it return a float, double or decimal?

Answer 1

double has the range of ±5.0 × 10⁻³²⁴ to ±1.7 × 10³⁰⁸ and long has the range of –9,223,372,036,854,775,808 to 9,223,372,036,854,775,807. Unfortunately not all integral floating point values can be represented by an integer.

For example, 1e19 has already exceeded the range of a 64-bit signed integer.

(long)Math.Round(1e19) == -9223372036854775808L // WTF?

While it is true that the single-argument overloads Math.Round(double) and Math.Round(decimal) will always return an integral value, these overloads still cannot return an integer value type.

If you know that the value passed to the function will return a value representable by an integer value type, you can cast it yourself. The library won’t do that because it needs to consider the general case.