C# dynamic and working with IEnumerable collections

Question

After encountering some problems with Massive today, I decided to create a simple test program to illustrate the problem. I wonder, what's the mistake I'm doing in this code:

var list = new List<string>
               {
                   "Hey"
               };

dynamic data = list.Select(x => x);

var count = data.Count();

The last line throws an error: 'object' does not contain a definition for 'Count'

Why is the "data" treated as an object? Does this problem occur because I'm calling an extension method?

The following code works:

var list = new List<string>
               {
                   "Hey"
               };

dynamic data = list.Select(x => x);

foreach (var s in data)
{
}

Why in this case "data" is correctly treated as IEnumerable?

Answer 1

Yes, that's because Count() is an extension method.

extension methods aren't supported by dynamic typing in the form of extension methods, ie called as if they were instance methods. ( source )

foreach (var s in data) works, because data has to implements IEnumerable to be a foreach source - there is (IEnumerable)data conversion performed during execution.

You can see that mechanish when trying to do following:

dynamic t = 1;

foreach (var i in t)
    Console.WriteLine(i.ToString());

There is an exception thrown at runtime: Cannot implicitly convert type 'int' to 'System.Collections.IEnumerable'

Answer 2

Seems that extension methods do not work on dynamic objects (see Jon' answer ). However, you can call those directly as static methods:

var count = Enumerable.Count(data); // works