Rule of thumb to test the equality of two doubles in C#?

Question

Let\'s say I have some code that does some floating point arithmetic and stores the values in doubles. Because some values can\'t be represented perfectly in binary, how do

Answer 1

double d1 = GetRandomDouble();
double d2 = GetRandomDouble();

if (Math.Abs(d1 - d2) < double.Epsilon)
{
   // the doubles are equal
}

Note that in practice this code is equivalent to just d1 == d2, because epsilon is defined as the smallest possible positive value > 0. Thus you'll never have a value between 0 and epsilon, and if you have the kind of rounding/precision error that would cause problems with the == operator you'll see it here, too.

But what you can do is use the technique to define your own precision level — you're own epsilon. I would expect double.Equals() to have an overloaded for this technique, but the documentation is clear that one strangely does not exist. So let's make our own:

public static bool IsEqual(this double d1, double d2, unsigned int precisionFactor)
{
   return Math.Abs(d1 - d2) < precisionFactor * double.Epsilon;
}