Question

I like the new Dynamic keyword and read that it can be used as a replacement visitor pattern.

It makes the code more declarative which I prefer.

Is it a good idea though to replace all instances of switch on ‘Type’ with a class that implements dynamic dispatch.

class VistorTest
{
    public string DynamicVisit(dynamic obj)
    {
        return Visit(obj);
    }


    private string Visit(string str)
    {
        return "a string was called with value " + str;
    }


    private string Visit(int value)
    {
        return "an int was called with value " + value;
    }
}

Answer 1

It really depends on what you consider a “good idea”.

This works, and it works in a fairly elegant manner. It has some advantages and some disadvantages to other approaches.

On the advantage side:

1. It’s concise, and easy to extend
2. The code is fairly simple

For the disadvantages:

1. Error checking is potentially more difficult than a classic visitor implementation, since all error checking must be done at runtime. For example, if you pass visitorTest.DynamicVisit(4.2);, you’ll get an exception at runtime, but no compile time complaints.
2. The code may be less obvious, and have a higher maintenance cost.

Personally, I think this is a reasonable approach. The visitor pattern, in a classic implementation, has a fairly high maintenance cost and is often difficult to test cleanly. This potentially makes the cost slightly higher, but makes the implementation much simpler.

With good error checking, I don’t have a problem with using dynamic as an approach here. Personally, I’d probably use an approach like this, since the alternatives that perform in a reasonable manner get pretty nasty otherwise.

However, there are a couple of changes I would make here. First, as I mentioned, you really need to include error checking.

Second, I would actually make DynamicVisit take a dynamic directly, which might make it (slightly) more obvious what’s happening:

class VistorTest
{
    public string DynamicVisit(dynamic obj)
    {
        try
        {
            return Visit(obj);
        }
        catch (RuntimeBinderException e)
        {
            // Handle the exception here!
            Console.WriteLine("Invalid type specified");
        }
        return string.Empty;
    }

     // ...Rest of code