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CA2242: Test for NaN correctly

TypeName

TestForNaNCorrectly

CheckId

CA2242

Category

Microsoft.Usage

Breaking Change

Non Breaking

An expression tests a value against Single.Nan or Double.Nan.

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

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

Do not suppress a warning from this rule.

The following example shows two expressions that incorrectly test a value against Double.NaN and an expression that correctly uses 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) );
      }
   }
}
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