CA2242: Test for NaN correctly

Cause

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

Rule Description

System.Double.NaN, which represents not-a-number, results when an arithmetic operation is undefined. Any expression that tests equality between a value and System.Double.NaN always returns false. Any expression that tests inequality 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.

using System;

namespace UsageLibrary
{
   class NaNTests
   {
      static double zero;
      
      static void Main()
      {
         Console.WriteLine( 0/zero == double.NaN );
         Console.WriteLine( 0/zero != double.NaN );
         Console.WriteLine( double.IsNaN(0/zero) );
      }
   }
}

Imports System

Namespace UsageLibrary

   Class NaNTests
   
      Shared zero As Double
      
      Shared Sub Main()
         Console.WriteLine( 0/zero = Double.NaN )
         Console.WriteLine( 0/zero <> Double.NaN )
         Console.WriteLine( Double.IsNaN(0/zero) )
      End Sub

   End Class

End Namespace