Question

I have the following code:

class Program
{
    static void Main(string[] args)
    {
        Test test = new Test();
        object objTest = test;

        Console.WriteLine(test.GetType());    // Output: "OperatorOverload.Test"
        Console.WriteLine(objTest.GetType()); // Output: "OperatorOverload.Test"

        Console.WriteLine(test == null);    // Output: "True"
        Console.WriteLine(objTest == null); // Output: "False"

        test.Equals(null);    // Output: "Hi!"
        objTest.Equals(null); // Output: "Hi!"

        Console.ReadLine();
    }
}

Test looks like this:

class Test
{

    public static bool operator ==(Test obj1, Test obj2)
    {
        return true;
    }

    public static bool operator !=(Test obj1, Test obj2)
    {
        return !(obj1 == obj2);
    }

    public override bool Equals(object obj)
    {
        Console.WriteLine("Hi!");
        return true;
    }

}

It appears that operator overloading only works when the type of the variable you’re dealing with is the class the operator overload is defined in. I can see why that would be the case, as I’m looking for a safe way of checking whether an object equals null.

My questions are:

Do overloaded operators only work if the type of the variable is the class the operator is defined in (my code tells me yes, but I could be making a mistake)?

Is the following code a safe way of checking whether an object equals null?

SomeClass obj = new SomeClass(); // may have overloaded operators
if ((object)obj == null)
{
    // ...
}

Answer 1

In C# overriding overloading an operator essentially creates a static method with its name like op_xxx(e.g. op_Add, op_Equality). In your question, test == null behaviors like Test.op_Equality(test, null) which returns true obviously. While objTest == null calls Object.op_Equality(test, null) that returns false. On the other hand, you override the virtual method Equals, so test.Equals and objTest.Equals have the same result.