Extension method and dynamic object

Question

I am going to summarize my problem into the following code snippet.

List<int> list = new List<int>() { 5, 56, 2, 4, 63, 2 }; Console.WriteLine(list.First()); 

Above code is working fine.

Now I tried the following

dynamic dList = list;  Console.WriteLine(dList.First()); 

but I am getting RuntimeBinderException.Why is it so?

Answer 1

To expand on Jon's answer, the reason this doesn't work is because in regular, non-dynamic code extension methods work by doing a full search of all the classes known to the compiler for a static class that has an extension method that matches. The search goes in order based on the namespace nesting and available using directives in each namespace.

That means that in order to get a dynamic extension method invocation resolved correctly, somehow the DLR has to know at runtime what all the namespace nestings and using directives were in your source code. We do not have a mechanism handy for encoding all that information into the call site. We considered inventing such a mechanism, but decided that it was too high cost and produced too much schedule risk to be worth it.