The Moth

For the importance and context of this blog entry, please DO read my previous blog post including the disclaimer at the top.

The feature I have not explicitly addressed yet is extension methods. If you are a C# developer that has been using the "magic" this keyword read my explanation to understand what we are talking about here. You see, even though extension methods are a compiler trick, they do rely on the new System.Runtime.CompilerServices.ExtensionAttribute introduced in System.Core.dll. Since we've already established that System.Core is a Fx v3.5 assembly and hence is not usable from 2.0 projects, you might think this is the end of the road for extension method usage in 2.0 projects. Let's explore this further.

In a 2.0 Orcas project try and compile the following code:

static class Program
{
  //[ExtensionAttribute()]
  static void DisplayInterval(this Timer t)
  {
    Console.WriteLine(t.Interval.ToString());
  }

  static void Main(string[] args)
  {
    System.Timers.Timer t = new System.Timers.Timer(2000);
   
    Program.DisplayInterval(t);  // long hand usage of method
    t.DisplayInterval(); // short hand usage of method (extension)

    Console.ReadLine();
  }
}

You will see a compiler error as follows:

"Cannot use 'this' modifier on first parameter of method declaration without a reference to System.Core.dll. Add a reference to System.Core.dll or remove 'this' modifier from the method declaration"

What the compiler *really* wants to say is:

"You are using the 'this' modifier and it depends on the ExtensionAttribute that itself resides in the System.Core.dll and you do not have a reference to it"

Once you understand what it is really saying, and once you realise that it only needs this attribute at compile time (since at runtime the IL to be interpreted will still include the call to the utility method in long hand), then it all becomes clear what you need to do next. Declare the attribute yourself! It isn't such a big logical jump (or else I wouldn't have made it :-))

So add to your 2.0 project a new empty code file and paste in it the following code:

namespace System.Runtime.CompilerServices
{
  public class ExtensionAttribute : Attribute { }
}

Then, wherever you declare the extension methods, optionally add to the top:
using System.Runtime.CompilerServices;
Now your projects will compile and you can use extension methods!

So summing up the previous post and this one, all 6 language features behind LINQ are usable from Fx v2.0 projects. But the LINQ to objects/xml/data implementations clearly are not usable because they live in Fx 3.5 assemblies. You can of course offer your own LINQ implementations for your own data sources. For example, the following code will compile in a Fx 2.0 project...

using Moth.Linq;
static class Program
{
  static void Main(string[] args)
  {
    var ints = new []{1,2,3,4};
    var res2 = from p in ints
               where p > 2
               select p;

    System.Diagnostics.Debugger.Break();
    // examine res2
    Console.ReadLine();
  }
}

...if you also add to the project this (demo) code file here.


BTW, how soon are you guys out there planning to move your existing 2.0 projects to Visual Studio "Orcas"? ;-)