CODEWITHSUNDEEP
c#genericsdynamic
aw04
aw04

Reputation: 11187

Unexpected behavior with dynamic List

The following throws a runtime exception on the last line:

dynamic dynamicCollection = new List<ExpandoObject>();

dynamic itemOne = new ExpandoObject();
itemOne.Name = "itemOne";
dynamicCollection.Add(itemOne);

dynamic itemTwo = new ExpandoObject();
itemTwo.Name = "itemTwo";
dynamicCollection.Add(itemTwo);

var firstItem = dynamicCollection.First();

'System.Collections.Generic.List<System.Dynamic.ExpandoObject>' does not contain a definition for 'First'

What's happening here? This looks completely legal. It's clearly being recognized as a List<ExpandoObject>, and a generic collection should not care about the value of T.. right?

Upvotes: 0

Views: 70

Answers (2)

takemyoxygen
takemyoxygen

Reputation: 4394

Do you really need dynamicCollection to be dynamic? If not, you can use concrete type for the list itself: 

var dynamicCollection = new List<ExpandoObject>();
...
dynamic firstItem = dynamicCollection.First();
Console.WriteLine(firstItem.Name);

and then dynamicCollection.First() will be resolved as an extension method call as you expect. Return type of this method is ExpandoObject, so you also should cast it to dynamic to access Name property.

Upvotes: 1

David
David

Reputation: 10708

The First() method is an extention method located in System.Linq.Enumerable rather than System.Collections.Generic.List<T>. This means that a dyanmic object, which looks up members at runtime, cannot resolve what .First() means because List<T> does, as said, not contain that method. The big issue here is that it's looking for a member, and doesn't check for Extension Methods when it goes to resolve.

Syntactically, you can say List<T>.First() because it's syntactic sugar. You can also refer to the full method, and you'll have to get this code to work, because dynamic objects don't check for extension methods.

Enumerable.First(dyanmicCollection);

At compile time, this is what extension methods effectively expand to anyway, so you're basically bypassing the sugar,

Upvotes: 4

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