Comparison Nan

ID

csharp.comparison_nan

Severity

low

Remediation Complexity

trivial

Remediation Risk

low

Remediation Effort

low

Resource

Reliability

Language

CSharp

Tags

float, reliability

Description

Reports comparisons in which one operand is the IEEE-754 NaN constant (double.NaN, float.NaN, Double.NaN or Single.NaN). NaN is the only value that compares unequal to itself and unordered to every other value, so every such comparison always returns false regardless of the other operand.

Rationale

x == double.NaN is always false, even when x itself is NaN, because the IEEE-754 standard defines all comparisons against NaN to return false (or true for !=, which means x != double.NaN is always true). The intent is almost always to ask "is this value NaN", which has its own predicate.

double x = ComputeRatio();
if (x == double.NaN) { /* never reached */ }   // FLAW — always false
if (x != double.NaN) { /* always */ }          // FLAW — always true
if (x < double.NaN)  { /* never reached */ }   // FLAW — NaN is unordered

if (double.IsNaN(x)) { /* ... */ }              // OK

Remediation

Use double.IsNaN(x) / float.IsNaN(x) to detect NaN. To test for non-NaN, use !double.IsNaN(x).

References