Extension Methods not working for an interface

Inspired by the MVC storefront the latest project I'm working on is using extension methods on IQueryable to filter results.

I have this interface;

IPrimaryKey
{
  int ID { get; }
}

and I have this extension method

public static IPrimaryKey GetByID(this IQueryable<IPrimaryKey> source, int id)
{
    return source(obj => obj.ID == id);
}

Let's say I have a class, SimpleObj which implements IPrimaryKey. When I have an IQueryable of SimpleObj the GetByID method doesn't exist, unless I explicitally cast as an IQueryable of IPrimaryKey, which is less than ideal.

Am I missing something here?

From stackoverflow Kirschstein

• This cannot work due to the fact that generics don't have the ability to follow inheritance patterns. ie. IQueryable<SimpleObj> is not in the inheritance tree of IQueryable<IPrimaryKey>

  From Garry Shutler

• Edit: Konrad's solution is better because its far simpler. The below solution works but is only required in situations similar to ObjectDataSource where a method of a class is retrieved through reflection without walking up the inheritance hierarchy. Obviously that's not happening here.

  This is possible, I've had to implement a similar pattern when I designed a custom entity framework solution for working with ObjectDataSource:

  public interface IPrimaryKey<T> where T : IPrimaryKey<T>
  {
      int Id { get; }
  }
  
  public static class IPrimaryKeyTExtension
  {
       public static IPrimaryKey<T> GetById<T>(this IQueryable<T> source, int id) where T : IPrimaryKey<T>
       {
           return source.Where(pk => pk.Id == id).SingleOrDefault();
       }
  }
  
  public class Person : IPrimaryKey<Person>
  {
      public int Id { get; set; }
  }
  

  Snippet of use:

  var people = new List<Person>
  {
      new Person { Id = 1 },
      new Person { Id = 2 },
      new Person { Id = 3 }
  };
  
  var personOne = people.AsQueryable().GetById(1);
  

  From cfeduke

• It works, when done right. cfeduke's solution works. However, you don't have to make the IPrimaryKey interface generic, in fact, you don't have to change your original definition at all:

  public static IPrimaryKey GetByID<T>(this IQueryable<T> source, int id) where T : IPrimaryKey
  {
      return source(obj => obj.ID == id);
  }
  

  Kirschstein : Excellent - I've unaccepted the original answer. I shall try this tomorrow. Thanks both of you.

  Garry Shutler : I'd just like to say that my answer was not wrong, it was the reason his code was not working, I just didn't take the time to work out a solution. Up voted your better answer.

  Kirschstein : Just a very minor point: it should be GetByID(this IQueryable source...
  From Konrad Rudolph