Entity framework 4, Dynamic query.

Question

Is it possible to create a dynamic query with Entity Framework. I Have 18 tables, Each of them has the same structures. How can I create a dynamic query to reuse the same query for each tables. I would like to have a generic query for Create Read Update Delete. The read contains the same "Where" Clause. Thanks for your help.

Answer 1

Here you have simple example for pure CRUD scenario. Create interface which will contain shared properties for your queries. Implement this interface in all your entity classes. Than create repository. Repository is usually defined as generic but in your case I defined each method as generic so that you can use same repository instence for all entities.

public interface IWellKnownEntity
{
  int Type { get; set; }
}

public class Repository
{
  public T GetEntityByWellKnownQuery<T>() where T : IWellKnownEntity
  {
    using (var context = new MyContext())
    {
      return context.CreateObjectSet<T>().FirstOrDefault(e => e.Type == 1);
    }
  }

  public IEnumerable<T> GetEntitiesByCustomQuery<T>(Expression<Func<T, bool>> where)
  {
    using (var context = new MyContext())
    {
      return context.CreateObjectSet<T>().Where(where).ToList();
    }
  }

  public void Create<T>(T entity) where T : IWellKnownEntity
  {
    using (var context = new MyContext())
    {
      context.AddObject(entity);
      context.SaveChanges();
    }
  }

  public void Update<T>(T entity) where T : IWellKnownEntity
  {
    using (var context = new MyContext())
    {
      context.Attach(entity);
      context.ObjectStateManager.ChageObjecState(entity, EntityState.Modified);
      context.SaveChanges();
    }
  }

  public void Delete<T>(T entity) where T : IWellKnownEntity
  {
    using (var context = new MyContext())
    {
      context.Attach(entity);
      context.DeleteObject(entity);
      context.SaveChanges();
    }
  }
}

Than you suppose that you have entity product and catebory which impelment well known interface. You can simply call:

var repository = new Repository();

var product = repository.GetEntityByWellKnownQuery<Product>();
product.Name = "Updated";
repository.Update<Product>(product);

var category = repository.GetEntitiesByCustomQuery<Category>(c => c.Id == 1).First();
repository.Delete<Category>(category);

You can futher improve the sample code. This code doesn't use shared context so it is more usable for disconnected scenario (web application). If you use connected scenario like WinForms application or batch application you can implement IDisposable on repository and share context among all methods. Dispose method on repository will hanlde disposing of context. Code for Update and Delete methods will be different because there is no need to attach entity back to context or set entity state.