Is this a hole in dynamic binding in C# 4?

Question

I\'ve seen a very interesting post on Fabio Maulo\'s blog. Here\'s the code and the bug if you don\'t want to jump to the url. I defined a new generic class like so:
<

Answer 1

The binder tries to work out an accessible class to treat the object as - in this case, the code making that call doesn't "know" about the MyClass class, so it doesn't "know" about PartitionKey either.

I don't know how thoroughly this is documented in the C# 4 spec - I know I've had an email conversation about it with Chris Burrows though, so the details may be somewhere on his blog :) Bear in mind that the dynamic binding has changed over time, so more recent posts are likely to be more accurate with respect to the RTM code.

I think that if you put PartitionKey into a public interface that the private class implements, that may work - but you'd have to try it.

There are various "gotchas" around dynamic typing. Explicit interface implementation has a similar problem:

public int Count(IList list)
{
   int count1 = list.Count; // Fine
   dynamic d = list;
   int count2 = d.Count; // Should work, right?
}

This will fail if you pass in an array - because although IList.Count exists, it's implemented explicitly in arrays - so it's a bit like this:

string[] array = new string[10];
Console.WriteLine(array.Count); // Won't compile

The binder tries to treat the object as its concrete type, not as IList, hence the failure...