Ruminations of idle rants and ramblings of a code monkey

Linq and Anonymous Types

.NET Stuff | Linq | Performance

I've been playing with Linq quite a bit recently. I have to say ... it's some cool stuff and revolutionizes data access on the .Net platform. One of the things in Linq that I'm really fascinated with is anonymous types. These classes are created based on a Linq statement and only have the properties that you specified. They're nicely type-safe and work with IntelliSense. Beauty and goodness.

Now, for a time, I just played with them and used them without much thought about what's going on behind the scenes. But ... my curiosity got the better of me and I decided to dig a bit and see what's going on. And the best way to do this? Lutz Roeder's Reflector of course!

So first ... the code. Not much, pretty simple.

 using (DataClasses1DataContext dc = new DataClasses1DataContext())
var contactNames = from c in dc.Contacts
select new { c.FirstName, c.LastName };foreach(var contactName in contactNames)
Console.WriteLine(contactName.FirstName + contactName.LastName);

I could have made it even simpler ... remove the foreach loop. But that let's me know that all's well.

So ... what happens with the anonymous type? It's actually compiled in the assembly. Yup, that's right ... it's a compiled class, just like a case that you create. But there is some black voodoo majik going on and, I'm certain, some significant compiler changes to make this happen.

Here's the raw IL generated for the class (with attributes):

.class private auto ansi sealed beforefieldinit lt;<FirstName>j__TPar, <LastName>j__TPar>
extends [mscorlib]System.Object
.custom instance void [mscorlib]System.Diagnostics.DebuggerDisplayAttribute::.ctor(string) = 
         { string('\\{ FirstName = {FirstName}, LastName = {LastName} }') Type=string('<Anonymous Type>') } .custom instance void [mscorlib]System.Runtime.CompilerServices.CompilerGeneratedAttribute::.ctor()

And here's the C# version if the IL:

[DebuggerDisplay(@"\{ FirstName = {FirstName}, LastName = {LastName} }", Type="<Anonymous Type>"), CompilerGenerated]
internal sealed class <>f__AnonymousType0<<FirstName>j__TPar, <LastName>j__TPar>


If you had any doubt at all, the "CompilerGenerated" attribute pretty much says it all. All of the references to the anonymous type in the code are replaced by this class in the compiled IL. And the return value from the query? It's a generic class:
[mscorlib]System.Collections.Generic.IEnumerable`1<class <>f__AnonymousType0`2<string, string>>.

Pretty cool, eh?

Now I'm off to dig into the performance of these beasties when compared to a DataReader and a DataSet. Early results look promising, but I've got some work to do to make sure it's a valid and fair comparison.