Moving to http://blogs.msdn.com/kirillosenkov

Hi all,

for the sake of consistency with the rest of the C# Team I'm moving my blog over to http://blogs.msdn.com/kirillosenkov. I hope you update your readers and bookmarks and sorry for the trouble. I would be glad to welcome you at my new place.

This blog will stay here as it is, all the posts and comments will remain just in case anyone needs them. However all the new stuff is going to happen there, so you won't miss anything if you unsubscribe from this one.

I have to say, Blogger is a very nice place and I liked it here a lot. I felt "at home" with this blog, I hope to have the same feeling for the new one. Thanks Blogger!

Hope to see you again soon!


How to create a generic List of anonymous types?

There was a question on one of the forums asking how to create a list of anonymous types given a single instance of an anonymous type:

            var Customer = new { FirstName = "John", LastName = "Doe" };
var customerList = new List<????>();

My first reaction was to answer that this was impossible, and even if it was, for what purpose, but thankfully people replied before me, who knew better. I was amazed by these ingenious tricks. For example, Kael Rowan suggested:

            var Customer = new { FirstName = "John", LastName = "Doe" };
var customerList = (new[] { Customer }).ToList();

customerList.Add(new { FirstName = "Bill", LastName = "Smith" });

I think this is brilliant! This technique is called casting by example. Here's another implementation, using a "list factory":

        static void Main(string[] args)
var Customer = new { FirstName = "John", LastName = "Doe" };
var customerList = MakeList(Customer);

customerList.Add(new { FirstName = "Bill", LastName = "Smith" });

public static List<T> MakeList<T>(T itemOftype)
List<T> newList = new List<T>();
return newList;

They use generic type inference here to "name" the anonymous type to T and return a List of it.

This somewhat made me think of var as a black hole - once a type is being "converted" to a var, it can never come back explicitly (well, unless you want to cast :) It can reappear in another method as another "var" (casting by example) or be remanifested as inferred generic type parameter, but it can't get an explicit name ever again.

There was a discussion a while ago (between Wilco Bauwer, Rick Strahl and yours truly), whether C# should extend type inference to return types of methods, types of fields, properties etc. : Returning var from a method in C# 3.0 Generally it was agreed that this would not be a good thing to do, for various reasons. Eric Lippert gave a good summary of all the trouble it might bring:

  • what if languages consume the method, which don't support type inference?

  • how to store this in CLR metadata?

  • how to detect versioning problems once you update your class?

  • how to deal with two structurally identical types from different assemblies?

  • etc.

My personal feeling about this all (which sort of corresponds with the general consensus), is that one shouldn't use anonymous types for more than trivial LINQ operations. If a type is going to be reused, give it a proper name and declaration. Anonymous types don't scale (at least, not in C# 3.0).

If there is ever going to be a C# feature to use var as a return type of members, this is going to be the first time ever I'll want the C# team not to implement a feature :)