Convert Library from ObjectContext to DbContext

Question

I have a library (based on code found in an old blog post) that allows me to very easily wrap a façade around my data access using Entity Framework. It uses ObjectContext and has performed well enough for my purposes.

But now, we are excitedly investigating code first using DbContext and of course would like to reuse / adapt as much of our existing effort as possible.

Everything went ok naively converting our Facade enabling library with IObjectContextAdapter until we tried to utilise our facade, when the following error was received:

The type 'Employee' cannot be used as type parameter 'TEntity' in the generic type or method 'DbContextManagement.FacadeBase'. There is no implicit reference conversion from 'Employee' to 'System.Data.Objects.DataClasses.EntityObject'

MSDN says:

EntityObject derived types are not supported by the DbContext API, to use these entity types you must use the ObjectContext API.

That's fine, but how would I then go ahead completing my refactor to bypass this inability?

Here's some code (line breaks introduced):

FacadeBase.cs

namespace DbContextManagement
{
    using System;
    using System.Collections;
    using System.Configuration;
    using System.Data.Entity;
    using System.Data.Entity.Infrastructure;
    using System.Data.Metadata.Edm;
    using System.Data.Objects.DataClasses;
    using System.Linq;
    using System.Reflection;

    public abstract class FacadeBase<TDbContext, TEntity>
        where TDbContext : DbContext, new()
        where TEntity : EntityObject
    {
        protected TDbContext DbContext
        {
            get
            {
                if (DbContextManager == null)
                {
                    this.InstantiateDbContextManager();
                }

                return DbContextManager.GetDbContext<TDbContext>();
            }
        }

        private DbContextManager DbContextManager { get; set; }

        public virtual void Add(TEntity newObject)
        {
            var context = ((IObjectContextAdapter)this.DbContext).ObjectContext;

            string entitySetName;

            if (newObject.EntityKey != null)
            {
                entitySetName = newObject.EntityKey.EntitySetName;
            }
            else
            {
                string entityTypeName = newObject.GetType().Name;

                var container = context.MetadataWorkspace.GetEntityContainer(
                                    context.DefaultContainerName, 
                                    DataSpace.CSpace);

                entitySetName = (from meta in container.BaseEntitySets
                                    where meta.ElementType.Name == 
                                       entityTypeName
                                    select meta.Name).First();
            }

            context.AddObject(entitySetName, newObject);
        }

        public virtual void Delete(TEntity obsoleteObject)
        {
            var context = ((IObjectContextAdapter)this.DbContext).ObjectContext;

            context.DeleteObject(obsoleteObject);
        }

        private void InstantiateDbContextManager()
        {
            var objectContextManagerConfiguration = 
               ConfigurationManager.GetSection("DbContext") as Hashtable;

            if (objectContextManagerConfiguration != null && 
                objectContextManagerConfiguration.ContainsKey("managerType"))
            {
                var managerTypeName = 
                   objectContextManagerConfiguration["managerType"] as string;

                if (string.IsNullOrEmpty(managerTypeName))
                {
                    throw new ConfigurationErrorsException(
                        "The managerType attribute is empty.");
                }

                managerTypeName = managerTypeName.Trim().ToLower();

                try
                {
                    var frameworkAssembly = 
                         Assembly.GetAssembly(typeof(DbContextManager));

                    var managerType = 
                         frameworkAssembly.GetType(managerTypeName, true, true);

                    this.DbContextManager = 
                        Activator.CreateInstance(managerType) as DbContextManager;
                }
                catch (Exception e)
                {
                    throw new ConfigurationErrorsException(
                        "The managerType specified in the 
                            configuration is not valid.", e);
                }
            }
            else
            {
                throw new ConfigurationErrorsException(
    "A Facade.DbContext tag or its managerType attribute
    is missing in the configuration.");
            }
        }
    }
}

EmployeeFacade.cs

namespace Facade
{
    using System.Collections.Generic;
    using System.Linq;
    using DataModel;
    using DataModel.Entities;
    using DbContextManagement;

    public sealed class EmployeeFacade : FacadeBase<FleetContext, Employee>
    {
        public Employee GetById(int? employeeId)
        {
            return employeeId == null
                ? null
                : this.DbContext.Employees.FirstOrDefault(m => m.Id == employeeId);
        }
    }
}

Employee.cs

namespace DataModel.Entities
{
    public class Employee
    {
        public int Id { get; set; }
        public string Surname { get; set; }
        public string Forename { get; set; }
        public string EmployeeNumber { get; set; }
    }
}

Answer 1

If your entities are derived from EntityObject and the code you want to reuse is dependent on EntityObject based entities then it is a show stopper. You cannot use DbContext API until your entities are POCOs (no EntityObject parent).

Btw. you can use code only mapping with ObjectContext (and POCOs) without ever using DbContext. You just need to:

• Create EntityTypeConfiguration or ComplexTypeConfiguration based class for each your POCO entity / complex type describing the mapping
• Use DbModelBuilder to collect your configurations and call Build to get DbModel instance
• Call Compile on your DbModel instance to get DbCompiledModel
• Cache compiled model for lifetime of your application
• When you need a new ObjectContext instance call CreateObjectContext on compiled model

Just be aware that code mapping has much more limited feature set so not everything you currently have in EDMX will be possible to achieve through code mapping.