Comparing Double.NaN with itself

Question

I am stuck trying to find out why these two operations return different values:

1. Double.NaN == Double.NaN returns false
2. Double.NaN.Equals(Double.NaN) returns true

I have the answer to the first part but not the second and not to "why are these two comparisons returning different values"

Answer 1

The reason for the difference is simple, if not obvious.

If you use the equality operator ==, then you're using the IEEE test for equality.

If you're using the Equals(object) method, then you have to maintain the contract of object.Equals(object). When you implement this method (and the corresponding GetHashCode method), you have to maintain that contract, which is different from the IEEE behaviour.

If the Equals contract was not upheld, then the behaviour of hash tables would break.

var map = new Dictionary<double,string>(); map[double.NaN] = "NaN"; var s = map[double.NaN]; 

If !double.NaN.Equals(double.NaN), you'd never get your value out of the dictionary!

If the previous sentence does not make sense, then understand that the mechanics of hashing (used in Dictionary<T,U>, HashSet<T>, etc) use both the object.Equals(object) and object.GetHashCode() methods extensively, and rely upon guarantees of their behaviour.

Answer 2

At the very bottom of the remarks section of Double.Equals, you will find:

If two Double.NaN values are tested for equality by calling the Equals method, the method returns true. However, if two NaN values are tested for equality by using the equality operator, the operator returns false. When you want to determine whether the value of a Double is not a number (NaN), an alternative is to call the IsNaN method.