CA2242: Test for NaN correctly

Property	Value
Rule ID	CA2242
Title	Test for NaN correctly
Category	Usage
Fix is breaking or non-breaking	Non-breaking
Enabled by default in .NET 9	As suggestion

Cause

An expression tests a value against System.Single.NaN or System.Double.NaN.

Rule description

System.Double.NaN, which represents a value that's not a number, results when an arithmetic operation is undefined. Any expression that tests for equality between a value and System.Double.NaN always returns false. Any expression that tests for inequality (!= in C#) between a value and System.Double.NaN always returns true.

How to fix violations

To fix a violation of this rule and accurately determine whether a value represents System.Double.NaN, use System.Single.IsNaN or System.Double.IsNaN to test the value.

When to suppress warnings

Do not suppress a warning from this rule.

Example

The following example shows two expressions that incorrectly test a value against System.Double.NaN and an expression that correctly uses System.Double.IsNaN to test the value.

Imports System

Namespace ca2242

    Class NaNTests

        Shared zero As Double

        Shared Sub Main2242()
            Console.WriteLine(0 / zero = Double.NaN)
            Console.WriteLine(0 / zero <> Double.NaN)
            Console.WriteLine(Double.IsNaN(0 / zero))
        End Sub

    End Class

End Namespace

class NaNTests
{
    static double zero = 0;

    static void RunIt()
    {
        Console.WriteLine(0 / zero == double.NaN);
        Console.WriteLine(0 / zero != double.NaN);
        Console.WriteLine(double.IsNaN(0 / zero));
    }
}