Why does Assert.AreEqual(1.0, double.NaN, 1.0) pass?

Question

Short question, why does Assert.AreEqual(1.0, double.NaN, 1.0) pass? Whereas Assert.AreEqual(1.0, double.NaN) fails.

Is it a bug in MSTest (Microsoft.VisualStudio.QualityTools.UnitTestFramework) or am I missing something here?

Best regards, Egil.

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Update: Should probably add, that the reason behind my question is, that I have a bunch of unit tests that unfortunately passed due to the result of some linear algebraic matrix operation being NaN or (+/-)Infinity. The unit tests are fine, but since Assert.AreEqual on doubles with a delta will pass when actual or/and expected are NaN or Infinity, I was left to believe that the code I was testing was correct.

Answer 1

Be careful. NaN is weird, somewhat like null in many DBMSs, and you shouldn't be comparing values to it (either directly, or with Assert.AreEqual). From the docs for Double.NaN:

Use IsNaN 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.

double zero = 0;
Console.WriteLine((0 / zero) == Double.NaN);  // prints false
Console.WriteLine(Double.IsNaN(0 / zero));  // prints true

You'd have to peer at the internals of Assert(double, double, double) to see what's going on, but in general, you're depending on undefined behavior relative to NaN.