When is Double's == operator invoked?

Question

It all started with a trick question that someone posed to me.. (It\'s mentioned in the book - C# in a nutshell) Here\'s the gist of it.

Double a = Double.NaN;
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Answer 1

From msdn : http://msdn.microsoft.com/en-us/library/ya2zha7s.aspx

If two Double.NaN values are tested for equality by calling the Equals method, the method returns true. However, if two NaN values are tested for equality by using the equality operator, the operator returns false. When you want to determine whether the value of a Double is not a number (NaN), an alternative is to call the IsNaN method.