How to handle a dynamic class differently based on expected return value

Question

Which method of DynamicObject do I have to override in order to get a different behavior (in the dynamic class) based on the context in which the instance is used?

Here is an example of what I am trying to accomplish:

class DynamicTest : DynamicObject
{
    public DynamicTest(string xyz)
    {
        _xyz = xyz;
    }

    private string _xyz;

    //TODO: what do I need to implement to get required behaviour?
}

class Program
{
    static void Main(string[] args)
    {
        dynamic foo = new DynamicTest("test);

        if (foo)  // treat foo as boolean
        { // jump in here when _xyz of foo has a value
            System.Console.WriteLine(foo); //treat foo as string
        }
        else
        { // jump in here when _xyz of foo is null
            System.Console.WriteLine("No Value In Object");
        }
    }
}

May I ask why do you want to do this? Why don't you want to access field in some predicate? — Ilya Ivanov, Jan 04 '13 at 10:51
In python a field with a value is considered true and with no value is false. I wanted a similar behavior. — rudimenter, Jan 04 '13 at 11:29

Ilya Ivanov · Answer 1 · 2013-01-04T11:16:57.790

I don't know why are you trying to do this and I will definitely NOT will recommend doing this, but you can override TryConvert method on DynamicObject like:

class DynamicTest : DynamicObject
{
    public DynamicTest(string xyz)
    {
        _xyz = xyz;
    }
    private string _xyz;


    public override bool TryConvert(ConvertBinder binder, out Object result)
    {
        Console.WriteLine ("TryConvert was called");
        Console.WriteLine ("Is explicit: "+binder.Explicit);
        if(binder.Type == typeof(bool))
        {
            result = true;
            return true;
        }
        else if(binder.Type == typeof(string))
        {   
            result = _xyz;
            return true;
        }

        result = null;
        return false;
    }

    public override string ToString()
    {
        return _xyz;
    }
}

Now there are some issues: ToString is required for Console.WriteLine, it doesn't try to convert if no implicit convertions exist (because WriteLine is overloaded), so it calls ToString. Implicit and explicit conversions to bool pass, but if you use foo inside if - you will get RuntimeBinderException: Cannot implicitly convert type 'DynamicTest' to 'bool'.

Examples:

dynamic foo = new DynamicTest("test:");
bool boolFoo = foo; //passes, TryConvert is called with `binder.Explicit` == false
bool boolFoo1 = (bool)foo; //passes, TryConvert is called with `binder.Explicit` == true
if(foo) //throws RuntimeBinderException

I want this behavior because in many dynamic languages an object with a value is considered true and with no value is false. In c# without dynamic keyword you need to compare explicitly. What is fine but not in the api i am working on. — rudimenter, Jan 04 '13 at 11:45

ígor · Accepted Answer · 2013-01-04T12:02:34.947

I think implicit operator can help you

Example code:

class DynamicTest : DynamicObject
{
    public DynamicTest(string xyz)
    {
        _xyz = xyz;
    }

    private string _xyz;

    public static implicit operator bool(DynamicTest rhs)
    {
        return rhs._xyz != null;
    }

    public static implicit operator string(DynamicTest rhs)
    {
        return rhs._xyz;

    }

    //TODO: what to override to get required behaviour
}



class Program
{
    static void Main(string[] args)
    {
        dynamic foo = new DynamicTest("test");


        if (foo)  // treat foo as boolean
        { // jump in here when _xyz of foo has a value
            System.Console.WriteLine((string)foo); //treat foo as string   //Importat: (string)foo to go operatorstring 
        }
        else
        { // jump in here when _xyz of foo is null
            System.Console.WriteLine("No Value In Object");
        }


    }
}

How to handle a dynamic class differently based on expected return value

2 Answers2