Overloading ==, checking for null

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?

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(foo,null) ) { ... } 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(foo,null) ) { ... } 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.

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p3ya8my0222y ago
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(foo,null) ) { ... } 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(foo,null) ) { ... } 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.