Can the behavior for == be defined for an interface reference?

Question

If an interface inherits IEquatable the implementing class can define the behavior of the Equals method. Is it possible to define the behavior of == operations?

public interface IFoo : IEquatable  
{}  

public class Foo : IFoo  
{  
    // IEquatable.Equals  
    public bool Equals(IFoo other)  
    {  
        // Compare by value here...
    }  
}

To check that two IFoo references are equal by comparing their values:

IFoo X = new Foo();  
IFoo Y = new Foo();

if (X.Equals(Y))  
{  
     // Do something  
}

Is it possible to make if (X == Y) use the Equals method on Foo?

Answer 1

No - you can't specify operators in interfaces (mostly because operators are static). The compiler determines which overload of == to call based purely on their static type (i.e. polymorphism isn't involved) and interfaces can't specify the code to say "return the result of calling X.Equals(Y)".

Answer 2

No, because interface can't contain operator functions. A solution would be to make IFoo an abstract class instead of an interface :

abstract class IFoo : IEquatable<IFoo> 
{
    public static bool operator ==(IFoo i1, IFoo i2) { return i1.Equals(i2); }
    public static bool operator !=(IFoo i1, IFoo i2) { return !i1.Equals(i2); }
    public abstract bool Equals(IFoo other);
}

class Foo : IFoo
{
    public override bool Equals(IFoo other)
    {
        // Compare
    }
}

Of course, this makes you lose the flexibility provided by interfaces.