overload ==（和！=，当然）运算符，我可以绕过==来确定对象是否为空

Question

when I try to overload operator == and != in C#, and override Equal as recommended, I found I have no way to distinguish a normal object and null. For example, I defined a class Complex.

public static bool operator ==(Complex lhs, Complex rhs)
{
    return lhs.Equals(rhs);
}

public static bool operator !=(Complex lhs, Complex rhs)
{
    return !lhs.Equals(rhs);
}

public override bool Equals(object obj)
{
    if (obj is Complex)
    {
        return (((Complex)obj).Real == this.Real &&
                   ((Complex)obj).Imaginary == this.Imaginary);
    }
    else
    {
        return false;
    }
}

But when I want to use

if (temp == null)

When temp is really null, some exception happens. And I can't use == to determine whether the lhs is null, which will cause infinite loop.

What should I do in this situation.

One way I can think of is to us some thing like Class.Equal(object, object) (if it exists) to bypass the == when I do the check.

What is the normal way to solve the problem?

Thank you.

Answer 1

You can use the following at the top of your Equals override:

if (Object.ReferenceEquals(obj, null))
    return false;

The exception you are getting is probably a StackOverflowException because your == operator will cause infinite recursion.

EDIT:

If Complex is a struct you should not have any problems with NullReferenceExceptions. If Complex is a class you can change your implementation of the == and != operator overloads to avoid the exception (Laurent Etiemble already pointed this out in his answer):

public static bool operator ==(Complex lhs, Complex rhs)
{
    return Equals(lhs, rhs);
}

public static bool operator !=(Complex lhs, Complex rhs)
{
    return !Equals(lhs, rhs);
} 

Answer 2

You should consider using the static Equals method in the operator overloads (which will call the instance Equals method):

public static bool operator ==(Complex lhs, Complex rhs)
{
    return Equals(lhs, rhs);
}

public static bool operator !=(Complex lhs, Complex rhs)
{
    return !Equals(lhs, rhs);
}

Note: You may also check for null in the Equals method.

You can also read the Object.Equals Topic on MSDN , which is a great source of samples.

Answer 3

public static bool operator ==(Complex lhs, Complex rhs)
{
    if (Object.ReferenceEquals(lhs, null))
    {
        return Object.ReferenceEquals(rhs, null);
    }

    return lhs.Equals(rhs);
}

public static bool operator !=(Complex lhs, Complex rhs)
{
    return !(lhs == rhs);
}

Poor man's unit test

Action<Complex, Complex> tester = (left, right) =>
{
    Console.WriteLine(left == right);
    Console.WriteLine(left != right);
    Console.WriteLine(left == null);
    Console.WriteLine(left != null);
    Console.WriteLine("---");
};

tester(new Complex(), new Complex());
tester(null, new Complex());
tester(null, null);
tester(new Complex(), null);

Answer 4

There is a better approach then using operators is and cast :

Complex c = obj as Complex;
return (c != null) && (c.Real == this.Real) && (c.Imaginary == this.Imaginary);

Here is a quick test concerning Equals operator override and comparing with null :

class Complex
{
    public override bool Equals(object obj)
    {
        if (obj is Complex)
        {
            return true;
        }
        else
        {
            return false;
        }
    }
}

Debugging doesn't step into operator's body:

var b = (new Complex() == new Complex());

Answer 5

I think you shoud test for null in the == operator implementation. Otherwise, when lhs is null, you'd call Complex(null).Equals (I don't know for C#, but in Java this would be a Nullpointer Exception)

To test for null, I suggest something like:

if (null == lhs && null == rhs) return true
else if (null == lhs) return false
else return lhs.Equals(rhs);

So Object.Equals will be called for all == comparisons above.