Question

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?

Answer 1

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); }