C# Overloading operator== outside the class

Question

I have the following scenario:

    public class SomeClass { 
       // Have some other data members as well  
       public int i ; 
    } 

    public class TestClass { 

        public bool SomeFunction() { 
             SomeClass a = new SomeClass(); 
             SomeClass b = new SomeClass(); 
             if (a == b) // this is where I am getting compile error
               return true; 
             return false; 
         } 

        public static bool operator==(SomeClass a, SomeClass b) { 
             if (a.i == b.i)
               return true; 
             // compare some other members as well
             return false; 
        } 
    }

Is this possible to achieve in C#?

Thanks for the help!

Because in the actual code the class, in the example above "SomeClass", is someone else's and I am trying to validate it — test123
– test123, Commented Sep 11, 2011 at 6:34

Guffa · Accepted Answer · 2011-09-11 06:57:22Z

3

No, it's not possible to override an operator from a class that is not involved in the operation.

You can make a class that implements IEualityComparer<SomeClass>, which can be used instead of the standard comparison in some cases, for example in a dictionary:

var x = new Dictionary<SomeClass, string>(new SomeClassEqualityComparer());

If you just want to use the comparison in your own class, you could make it a regular static method instead of overriding an operator:

public static bool SomeClassEqual(SomeClass a, SomeClass b) { 
  if (a.i == b.i) {
    return true;
  }
  // compare some other members as well
  return false; 
}

Usage:

if (SomeClassEqual(a, b))

edited Sep 11, 2011 at 6:57

answered Sep 11, 2011 at 6:32

Guffa

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2 Comments

test123 Over a year ago

Thanks for the answer. That's what I wanted to know. So I guess I will have to write stupid functions to do the comparison. Darn it.

cdhowie Over a year ago

I think you mean IEqualityComparer<T>, not IEquatable<T>. The former can be used to compare two objects of a particular type, while the latter is used to compare some other object of a specific type to the object implementing IEquatable<T>. In other words, an equality comparer is not a party to the equality test, while an equatable is.

Ofer Zelig · Accepted Answer · 2011-09-11 06:33:12Z

1

To begin with, you can't use return true; on a void method.

Second, overriding operators should be applied to the host class. In your case, inside SomeClass rather than inside TestClass.

Third, when you implement == overriding operator, you should also implement != .

Here is your code, revised and working:

public class SomeClass
{
    // Have some other data members as well  
    public int i;

    public static bool operator ==(SomeClass a, SomeClass b)
    {
        if (a.i == b.i)
            return true;
        // compare some other members as well
        return false;
    }

    public static bool operator !=(SomeClass a, SomeClass b)
    {
        return !(a == b);
    }
}

public class TestClass
{

    public bool SomeFunction()
    {
        SomeClass a = new SomeClass();
        SomeClass b = new SomeClass();
        if (a == b) // this is where I am getting compile error
            return true;

        return false;
    }
}

edited Sep 11, 2011 at 6:33

answered Sep 11, 2011 at 6:27

Ofer Zelig

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3 Comments

test123 Over a year ago

Sorry my bad. This is just a sample code. I will fix it. Thanks!

test123 Over a year ago

Yes that's what I wanted to confirm is it possible to override operator outside the host class?

test123 Over a year ago

Thanks for the updated code. Actually I know that putting those methods inside the actual class would work. What my question actually was was that whether or not it could work if I try to override them outside the host class. But I guess it is not possible as answered by @Guffa. Thanks for the reply anyways.

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