LINQ to Entities through Interface Property

Question

I have a situation where I would like to use a single business logic class to perform similar operations on a variety of entity framework classes. I have defined an interface which these classes implement in a partial class file.

However when I try to write a LINQ to entities query against these interface methods I get a NotSupportedException since the query is not using the class's properties directly but through an interface.

I would like to keep the heavy lifting to the database tier so is there a way to achieve this without resorting to LINQ to objects?

Here is some code which demonstrates my problem (it is using a generic repository class created by a factory).

public interface INamedEntity
{
    int ID { get; set; }
    string Name { get; set; }
}

// This is an Entity Framework class which has CustomerID and CustomerName properties.
public partial class Customer: INamedEntity
{
    int INamedEntity.ID
    {
        get { return this.CustomerID; }
        set { this.CustomerID = value; }
    }
    string INamedEntity.Name
    {
        get { return this.CustomerName; }
        set { this.CustomerName = value; }
    }
}

...

public string GetName<T>(int entityID) where T: EntityObject, INamedEntity
{
    using(var repository = RepositoryFactory.CreateRepository<T>())
    {
        return repository
            .Where(e => e.ID == entityID)
            .Select(e.Name)
            .Single();
    }
}

Answer 1

This is not supported. Your Linq-to-entities query can use only mapped properties of your entities. If you use interface properties EF doesn't know how to convert them to SQL because it is not able to analyze your code in property implementation.

Don't use interfaces for entities - EF doesn't support it at all. In your special case it will even not work with any other ORM because you are querying on properties which are unknown to mapping. This would require you to build your own Linq provider translating your query to query with real mapped properties.

Answer 2

The following exception occures during query execution based on a generic source and with interface member used in the where clause.

NotSupportedException: The mapping of interface member [InterfaceName].[MemberName] is not supported.

The exception occures only when the query should return multiple items and when I used == operator. I could not reproduce the error when executed the query with First, FirstOrDefault or Single, or when I used equals or other operator in the where clause.

Reference : Interface not supported

Answer 3

You can use Dynamic Query Library (http://weblogs.asp.net/scottgu/archive/2008/01/07/dynamic-linq-part-1-using-the-linq-dynamic-query-library.aspx).

        if (typeof (INamedEntity).IsAssignableFrom(typeof (T)))
        {
            q = q.Where("ID ==@0", id);
        }