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:

public class TableStorageInitializer<TTableEntity> where TTableEntity : class, new()
    {
        public void Initialize()
        {
            InitializeInstance(new TTableEntity());
        }

        public void InitializeInstance(dynamic entity)
        {
            entity.PartitionKey = Guid.NewGuid().ToString();
            entity.RowKey = Guid.NewGuid().ToString();
        }

    }

Note that InitializeInstance accepts one parameter, which is of type dynamic. Now to test this class, I defined another class that is nested inside my main Program class like so:

class Program
        {
            static void Main(string[] args)
            {
               TableStorageInitializer<MyClass> x = new TableStorageInitializer<MyClass>();
                x.Initialize();
            }
            private class MyClass
            {
                public string PartitionKey { get; set; }
                public string RowKey { get; set; }
                public DateTime Timestamp { get; set; }
            }
        }

Note: the inner class "MyClass" is declared private.
Now if i run this code I get a Microsoft.CSharp.RuntimeBinder.RuntimeBinderException on the line "entity.PartitionKey = Guide.NewGuid().ToString()".
The interesting part, though is that the message of the exception says "Object doesn't contain a definition for PartitionKey".
alt text http://img697.imageshack.us/img697/4188/testdl.png

Also note that if you changed the modifier of the nested class to public, the code will execute with no problems. So what do you guys think is really happening under the hood? Please refer to any documentation -of course if this is documented anywhere- that you may find?

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...