int j = num * -1;

if (j - num  == j)
{
     // Num is equal to zero
}
else if (j < i)
{
      // Num is positive
}
else if (j > i) 
{
     // Num is negative
}

你只需要比较是否值和它的绝对值相等:

if (value == Math.abs(value))
    return "Positif"
else return "Negatif"

if (num < 0) {
  //negative
}
if (num > 0) {
  //positive
}
if (num == 0) {
  //neither positive or negative,
}

或者使用else if

    public static bool IsPositive<T>(T value)
        where T : struct, IComparable<T>
    {
        return value.CompareTo(default(T)) > 0;
    }

public static bool IsNegative<T>(T value)
   where T : struct, IComparable<T>
{
    return value.CompareTo(default(T)) < 0;
}

对于32位有符号整数，例如System。Int32，也就是c#中的int:

bool isNegative = (num & (1 << 31)) != 0;