Linq expression does not allow Generic Property Type

Question

Error: Unable to create a constant value of type 'System.Object'. Only primitive types or enumeration types are supported in this context.

Ok here is the deal, I have a Generic Base Entity as shown below:

public abstract class Entity<Tkey> : IEntity<Tkey>
    where Tkey:IComparable
{
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public abstract Tkey Id { get; set;}

}

I prefer this, because you know, some table's Identity column type is Guid, some int and some long.

So, in my Generic repository I have some method like this one:

public class FRepository<TEntity,Tkey>:IDisposable where TEntity : Entity<Tkey>, new()
    where Tkey:IComparable

{

    public TEntity Find(Tkey id)
    {
        return this.GetAll().Where(e=> e.Id.CompareTo(id)==0).FirstOrDefault();
    }
}

GetAll() method returns IQueryable<TEntity>.

But when I use Linq to find specific entity, it throws exception as I write begining of this post.

Note: The entity which I am trying to find has Id with int type, so it is a primitive type...

Note-1: I can't able to use '==' operator with generic type TKey. I get compilation error.

Note-2: I also tried this Where(e=>e.Id.GetHashCode()==id.GetHashCode()). This throws another error(run-time).

LINQ to Entities does not recognize the method 'Int32 GetHashCode()' method, and this method cannot be translated into a store expression.

I have achieved this by adding a Where method to generic repository, which allows a predicate parameter. And then I push predicate to one-level-down as `return GetAll().Where(predicate).FirstOrDefault();` — Hasan Ünal, Feb 21 '17 at 19:30
BTW, why `IComparable`? Isn't better `IEquatable` for `Id` equality? — Matías Fidemraizer, Feb 21 '17 at 19:32
In the other hand, why are you using `IQueryable` when EF already has a `Find` method? I mean, repository's `GetById` should call EF's `Find` counterpart... — Matías Fidemraizer, Feb 21 '17 at 19:33
Remember that LINQ-to-entity translates a query to an SQL-command and in SQL there is no such thing like `CompareTo` or `GetHashCode`. Not every valid C# expression translates to SQL. — Olivier Jacot-Descombes, Feb 21 '17 at 19:33
@EdPlunkett Probably that one that cries when you want to use `==` or `!=` on generic parameters which is unknown that they may or may not implement these operators overloading.. — Matías Fidemraizer, Feb 21 '17 at 19:34
See: [EF object comparison with generic types](http://stackoverflow.com/questions/10402029/ef-object-comparison-with-generic-types) — Olivier Jacot-Descombes, Feb 21 '17 at 19:41
`GetAll().Where(e=>(object)e.Id==(object)id).FirstOrDefault();` solves problem. Actually my GetAll() method returns relevant DbSet. So as @Matias told, DbSet already has a Find method for this specific issue. So I changed my Find method into `this.dbset.Find(id)`. But anyway, if someone exprience this issue in another situation, just cast them into object :) — Hasan Ünal, Feb 22 '17 at 06:11

score 2 · Answer 1 · edited May 23 '17 at 12:31

Instead of attempting to solve the original issue, as some others users have already commented out that not all valid expressions are translatable to SQL under the hoods, I'll try to give you some advises...

Summary:

You want Id to implement IEquatable<TId>, not IComparable<TId>. Thus, you'll be able to write typed equalities like this: id.Equals(id2). BTW EF can't handle IEquatable<T>.Equals either (see Is it possible to use IComparable to compare entities in Entity Framework?). Anyway, you're not going to go this route anymore but the advise is still valid, you want IEquatable<T>, not a comparison. See the next point to know why.
You don't want a GetAll method on your repositories to later look for an entity by Id. Entity Framework already has IDbSet<T>.Find method for this exact use case.

Also, I would encourage you to read some answer I composed in 2011 (a long time ago) against GetAll in repositories: Repository design pattern. In addition, I've documented the repository pattern here where I explain the thing against GetAll too.

Linq expression does not allow Generic Property Type

1 Answers1