Support conversion to Boolean but still have WriteLine display using ToString

Question

When I run this bit of code, Equation(10, 20) is output to the console:

public class Equation
{
    public int a;
    public int b;

    public override string ToString()
    { return "Equation(" + a + ", " + b + ")"; }
}

class Program
{
    static void Main(string[] args)
    {
        Console.WriteLine(new Equation() { a = 10, b = 20 });

        Console.ReadLine();
    }
}

I'd like to support Equation instances being used in the test of an if so I allowed for implicit conversion to Boolean:

public class Equation
{
    public int a;
    public int b;

    public override string ToString()
    { return "Equation(" + a + ", " + b + ")"; }

    public static implicit operator Boolean(Equation eq)
    { return eq.a == eq.b; }
}

class Program
{
    static void Main(string[] args)
    {
        if (new Equation() { a = 10, b = 10 })
            Console.WriteLine("equal");

        Console.WriteLine(new Equation() { a = 10, b = 20 });

        Console.ReadLine();
    }
}

However, the trouble is, now when I use WriteLine on an Equation, it get's converted to a Boolean instead of printing using ToString.

How can I allow for implicit conversion to Boolean and still have WriteLine display using ToString?

---

update

This question is inspired by the Equation class in SymbolicC++. The code below illustrates that an Equation can be displayed via cout as well as used in the test of an if:

auto eq = x == y;

cout << eq << endl;

if (eq)
    cout << "equal" << endl;
else
    cout << "not equal" << endl;

So this is somehow possible in C++.

Answer 1

You can't, as far as I can tell. You could also provide a conversion to string... but that would then make the call ambiguous between WriteLine(string) and WriteLine(bool`).

Personally I'd strongly recommend that you ditch the implicit conversion to Boolean. Implicit conversions are almost always a bad idea. They make the code far more confusing, as well as leading to unintended overload changes as you've found.

(I'd also change your bracing style, but that's a different matter.)

Answer 2

You can't do it with a bool conversion but you can overload the true and false operators for Equation. Of course Equation won't be implicitly convertible to bool any more but you can still use it in if, while, do, and for statements and conditional expressions (i.e. ?: operator).

public class Equation
{
    public int a;
    public int b;

    public override string ToString()
    { return "Equation(" + a + ", " + b + ")"; }

    public static bool operator true(Equation eq)
    {
        return eq.a == eq.b;
    }

    public static bool operator false(Equation eq)
    {
        return eq.a != eq.b;
    }       
}

From your example:

if (new Equation() { a = 10, b = 10 })
    Console.WriteLine("equal"); // prints "equal"

Console.WriteLine(new Equation() { a = 10, b = 20 }); // prints Equation(10, 20)