C# Error: The call is ambiguous between the following methods or properties. Overloading operators

Question

I have 2 classes with overloaded operators in a namespace called Dinero, these are the 2 classes:

First one:

namespace Dinero
{
    class Dollar
    {
        #region Atributos

        public Double cant;
        
        #endregion

        #region Constructores

        public Dollar()
        {
            this.cant = 0;
        }

        public Dollar(Double amount)
        {
            this.cant = amount;
        }

        #endregion

        #region Sobrecarga de Operadores

        public static Dollar operator +(Euro eu, Dollar dol)
        {
            Dollar devolucion = new Dollar();

            devolucion.cant = eu.cant + (dol.cant * 1.3642);

            return devolucion;
        }

        public static Dollar operator -(Euro eu, Dollar dol)
        {
            Dollar devolucion = new Dollar();

            devolucion.cant = eu.cant + (dol.cant * 1.3642);

            return devolucion;
        }
        
        public static bool operator ==(Euro eu, Dollar dol)
        {
            if (eu.cant == (dol.cant * 1.3642))
                return true;
            else
                return false;
        }

        public static bool operator !=(Euro eu, Dollar dol)
        {
            if (eu.cant != (dol.cant * 1.3642))
                return true;
            else
                return false;
        }

        #endregion

    }
}

Second one:

namespace Dinero
{
    class Euro
    {
        #region Atributos

        public Double cant;

        #endregion

        #region Constructores

        public Euro()
        {
            this.cant = 0;
        }

        public Euro(Double amount)
        {
            this.cant = amount;
        }

        #endregion

        #region Sobrecarga de operadores

        public static Euro operator +(Euro eu, Dollar dol)
        {
            Euro devolucion = new Euro();

            devolucion.cant = eu.cant + (dol.cant * 1.3642);

            return devolucion;
        }

        public static Euro operator -(Euro eu, Dollar dol)
        {
            Euro devolucion = new Euro();

            devolucion.cant = eu.cant - (dol.cant * 1.3642);

            return devolucion;
        }

        public static bool operator ==(Euro eu, Dollar dol)
        {
            if (eu.cant == (dol.cant * 1.3642))
                return true;
            else
                return false;
        }

        public static bool operator !=(Euro eu, Dollar dol)
        {
            if (eu.cant != (dol.cant * 1.3642))
                return true;
            else
                return false;
        }
        
        #endregion

    }
}

And when I go to the main program ( I don't know how you guys call the main file, I'd like to know since I'm a total n00b ) and I type this:

namespace Ejercicio_21
{
    class Ejercicio_21
    {
        static void Main(string[] args)
        {
            Console.Title = "Ejercicio Nro 21";

            Euro euro00 = new Euro(1);
            Dollar dollar00 = new Dollar(1);

            Euro sumaEuros = euro00 + dollar00;

About the last line, the compiler says:

Error 11 The call is ambiguous between the following methods or properties: 'Dinero.Euro.operator +(Dinero.Euro, Dinero.Dollar)' and 'Dinero.Dollar.operator +(Dinero.Euro, Dinero.Dollar)'

I assume it has something to do with the different namespaces, but I couldn't figure it out, even using google.

This is the first question I ask here, so please, don't flame me to oblivion and please excuse my horrid English.

Note: I'm forced to keep Dollar and Euro classes in a different namespace than the main program.

Answer 1

No, it's got nothing to do with different namespaces - it's that you've got the same operator signature declared in two places:

public static Dollar operator +(Euro eu, Dollar dol)
public static Euro operator +(Euro eu, Dollar dol)

The compiler doesn't know which one you want to call.

To be honest, I don't think adding a dollar value to a Euro value makes much sense to start with - but even beyond that, having the same operation "adding dollars to Euros" should have a single logical result type.

If you really want to make the two operations valid, I'd suggest having instance methods called Plus:

// In Dollar
public Dollar Plus(Euro eu)

// In Euro
public Dollar Plus(Dollar dol)

Then:

Euro euro00 = new Euro(1);
Dollar dollar00 = new Dollar(1);

Euro sumaEuros = euro00.Plus(dollar00);

Pretty much as clear, but without the ambiguity.

Another alternative which I don't recommend is to make (say) the first operand's type take priority:

public static Dollar operator +(Dollar dol, Euro eu)    
public static Euro operator +(Euro eu, Dollar dol)

Then you could do:

Dollar dol1 = ...;
Euro eu1 = ...;

Dollar dol2 = dol1 + eu1;
Euro eu2 = eu1 + do1;

It's pretty horrible though.