Entity Framework 4 - Generic method for retrieving entity by ID

Question

I am writing method for fetching single entities by their ID :

public Customer GetCustomer(int i_CustomerID)
{
  return (from c in context.CustomerSet 
         where c.Id == i_CustomerID 
        select c).SingleOrDefault();            
}

public Movie GetMovie(int i_MovieID)
{
  return (from m in context.MovieSet 
         where m.Id == i_MovieID 
        select m).SingleOrDefault();
}

But I have many entities and this code repeats itself. I want to write a method like this:

public T GetEntityByID<T>(int i_EntityID)
{
  return (from e in context.T_Set 
         where e.Id == i_EntityID 
        select e).SingleOrDefault();
}

Is there a way to achieve that ?

You can find a similar question [here](http://stackoverflow.com/questions/248682/create-llblgen-linq-query-dynamicly-with-strings) — Menahem, Mar 27 '11 at 09:58

score 3 · Accepted Answer · edited Nov 30 '11 at 14:39

3

I haven't actually executed this but it compiles and is probably something along the lines of what you are trying to achieve:

    public static void Testing()
    {
        SelectEntity<MyObject>(r => r.MyObjectId == 1);
    }

    public static T SelectEntity<T>(Expression<Func<T, bool>> expression) where T : EntityObject
    {
        MyContext db = new MyContext();
        return db.CreateObjectSet<T>().SingleOrDefault(expression);
    }

edited Nov 30 '11 at 14:39

Gil Stal

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answered Mar 27 '11 at 09:26

Rob

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It's a nice solution, but don't like the idea of using the lambda. Would like somehow to just give an int. – Yaron Levi Mar 27 '11 at 21:45
You could declare a second partial class for your entities that implements an interface (e.g. IPrimaryKey) that hooks up the primary key to say MyEntity.PrimaryId and then make your generic method where T : IPrimaryKey. Alternatively have you considered trying to use the EntityKey? I think it might be possible to dig out the primary key from the entity key info... – Rob Mar 28 '11 at 11:35

score 1 · Answer 2 · answered Mar 27 '11 at 09:26

The problem is that there is no common super type that has the relevant properties that you seek. It is easy, however, to code generate your fetch methods using the in-built T4 code generation tool that EF is using. Here is a good link on how to hook in and generate the sort of code you need.

http://msdn.microsoft.com/en-us/data/gg558520

score 0 · Answer 3 · edited May 23 '17 at 12:00

If you know that your generic repository will be always used with entity types which have PK with the same name and the same type you can simply define interface like this:

public interface IEntity
{
    int Id { get; }
}

and either implement this interface in partial part of your generated entities or modify T4 template to include it automatically. Your repository will be then defined as:

public interface IRepository<TEntity> where T : IEntity
{
    ...
}

If the type of PK can change but the name is still the same you can improve the entity interface to:

public interface IEntity<TKey> 
{
    TKey Id { get; set; }
}

and the definition of repository will be:

public interface IRepository<TEntity, TKey> where TEntity : IEntity<TKey>
{
    ...
}

If you want generic repository which is able to work with entities with different PK's name and type check this answer. That solution should probably also work (or with small modification) with composite PKs.

I didn't understand why do I need IRepository for? how do I use it ? — Yaron Levi, Mar 27 '11 at 21:42
Repository is a pattern used to abstract the data layer. If you think about it, it is doing the same thing as the DBContext. — Burt, Apr 22 '15 at 10:19

Entity Framework 4 - Generic method for retrieving entity by ID

3 Answers3