Question

Using DynamicObject it’s possible to create a custom class and define its behavior when an instance is compared with something else. It’s done by overriding TryBinaryOperation.

However, when I try to compare it with null or any reference value the overridden method is never called.

public class Foo : DynamicObject
{
    public override bool TryBinaryOperation(BinaryOperationBinder binder, object arg, out object result)
    {
        if (binder.Operation == ExpressionType.Equal)
        {
            result = true;
            return true;
        }

        return base.TryBinaryOperation(binder, arg, out result);
    }
}

static void Main(string[] args)
{
    dynamic foo = new Foo();

    Console.WriteLine(foo == 1); // True
    Console.WriteLine(foo == new object()); // False
    Console.WriteLine(foo == null); // False
}

Is there a way to intercept this call? I would assume DynamicObject is not the correct abstraction for this.

Answer 1

Yes you can via operator overloading. In fact, I use this trick to implement a dynamic expression builder in my DynamicLinq project. See an example here:

https://github.com/davidfowl/DynamicLinq/blob/master/DynamicLINQ/DynamicExpressionBuilder.cs#L88