Re: Interesting behavior of double.NaN

In addition to what other's have said, I would be very careful in deciding whether NaN == NaN makes sense in your scenario.

The reason a NaN value is neither equal to or not equal to another NaN value is because NaN doesn't represent a real value.

For example, you're saying 

DoubleEquals(Math.Sqrt(-1.0), Math.Sqrt(-10000.0));

...will be true.

Conceptually the result of the square of two different values should not be equal (in a perfect world where rounding error didn't exist).

-- Peter

On Mon, 23 Mar 2009 15:40:42 +0100, Brink, Paul ten <[email protected]> wrote:

>Hello,
>
>After a long search I finally found why my program is not working properly: double.NaN != double.NaN
>
>You can try this for yourself in the Quick Watch window.
>
>This interesting behavior is by design. From the documentation:
>
>----------------------------
>Double.NaN Field
>
>Represents a value that is not a number (NaN). This field is constant.
>
>Use IsNaN<http://msdn.microsoft.com/en-us/library/system.double.isnan.aspx> to determine whether a value is not a number. It is not possible to determine whether a value is not a number by comparing it to another value equal to NaN.
>----------------------------
>
>Hm, a constant that is not equal to itself. Interesting...
>
>But it is very dangerous:
>
>If (width != this.Width)
>
>returns true if both are double.NaN
>
>Don't say Microsoft did not warn you...     ;-)
>
>I now use:
>
>private bool DoubleEquals(double d1, double d2)
>{
>    bool result = false;
>
>    bool isNaN1 = double.IsNaN(d1);
>    bool isNaN2 = double.IsNaN(d2);
>
>    if (isNaN1)
>    {
>        result  = isNaN2;
>    }
>    else
>    {
>        if (! isNaN2)
>        {
>            result = (d1 == d2);
>        }
>    }
>
>    return result;
>}
>

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