How do I check for nulls in an '==' operator overload without infinite recursion?

Question

Use ReferenceEquals:

Foo foo1 = null;
Foo foo2 = new Foo();
Assert.IsFalse(foo1 == foo2);

public static bool operator ==(Foo foo1, Foo foo2) {
    if (object.ReferenceEquals(null, foo1))
        return object.ReferenceEquals(null, foo2);
    return foo1.Equals(foo2);
}

Cast to object in the overload method:

public static bool operator ==(Foo foo1, Foo foo2) {
    if ((object) foo1 == null) return (object) foo2 == null;
    return foo1.Equals(foo2);
}

Use ReferenceEquals. From the MSDN forums:

public static bool operator ==(Foo foo1, Foo foo2) {
    if (ReferenceEquals(foo1, null)) return ReferenceEquals(foo2, null);
    if (ReferenceEquals(foo2, null)) return false;
    return foo1.field1 == foo2.field2;
}

If you are using C# 7 or later you can use null constant pattern matching:

public static bool operator==(Foo foo1, Foo foo2)
{
    if (foo1 is null)
        return foo2 is null;
    return foo1.Equals(foo2);
}

This gives you slightly neater code than the one calling object.ReferenceEquals(foo1, null)

Try Object.ReferenceEquals(foo1, null)

Anyway, I wouldn't recommend overloading the ==operator; it should be used for comparing references, and use Equals for "semantic" comparisons.