Overloading == checking for null

Question

Suppose that I have a Foo class for which I've overloaded the == operator: // Class foo. public static bool operator == (Foo foo1 Foo foo2) { return (foo1.data == foo2.data); } Now I do this: Foo foo = something(); if ( foo == null ) ... Oops: NullReferenceException because the overloaded == operator was used and it tried to dereference the null that I passed in. So how do I test for a null value if the == operator is overloaded? Sure I could try to use it and then catch the NullReferenceException but is there a more elegant solution?

p3ya8my02 · Answer

I've found that there are in fact two answers to this question. The first answer is to cast the variable to an Object. Since the == operator is static (resolved at compile time) the Object class's == operator will be called. For example: /**/ // From the Apache Foundation's log4net.Priority class /**/ // (Formatting munged by the forum.) /**/ public static bool operator == (Priority l Priority r) /**/ { /**/ if (((object)l) != null && ((object)r) != null) /**/ { /**/ return l.m_level == r.m_level; /**/ } /**/ else /**/ { /**/ return ((object)l) == ((object)r); /**/ } /**/ } (The object with a lowercase o is a synonym for Object with a capital O.) The other answer is to use the Object class's static ReferenceEquals method. Because all classes inherit from Object you can use it as a function: /**/ if ( ReferenceEquals(foonull) ) { ... } Note that the ReferenceEquals method is NOT sealed (final) so it can be overridden. If for some reason you don't trust your base class explicitly use Object.ReferenceEquals: /**/ if ( Object.ReferenceEquals(foonull) ) { ... } Neither solution is particularly elegant. However if you take into account null values when overloading the == operator (as in the log4net class above) that will keep the clumsiness to a minimum.

Overloading ==, checking for null