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

With a very large nod to the author of the original code in the link given above, here is what I ended up with. It passes basic scenarios but I haven’t yet tried out deeper navigational and related queries. Even if there is an issue, I reckon it will be bug fix rather than show stop.

Here goes. The following is reusable, works with multiple contexts and means you can go to the database and get something back in 2 lines of actual code in the client.

DataModel (Class Library Project)

Your EF Code First project with DbContext POCOs, etc.

DbContextManagement (Class Library Project)

DbContextManager.cs

namespace DbContextManagement
{
    using System.Data.Entity;

    /// <summary>
    /// Abstract base class for all other DbContextManager classes.
    /// </summary>
    public abstract class DbContextManager
    {
        /// <summary>
        /// Returns a reference to an DbContext instance.
        /// </summary>
        /// <typeparam name="TDbContext">The type of the db context.</typeparam>
        /// <returns>The current DbContext</returns>
        public abstract TDbContext GetDbContext<TDbContext>() 
            where TDbContext : DbContext, new();
    }
}

DbContextScope.cs

namespace DbContextManagement
{
    using System;
    using System.Collections.Generic;
    using System.Data.Common;
    using System.Data.Entity;
    using System.Data.Entity.Infrastructure;
    using System.Linq;
    using System.Threading;

    /// <summary>
    /// Defines a scope wherein only one DbContext instance is created, and shared by all of those who use it. 
    /// </summary>
    /// <remarks>Instances of this class are supposed to be used in a using() statement.</remarks>
    public class DbContextScope : IDisposable
    {
        /// <summary>
        /// List of current DbContexts (supports multiple contexts).
        /// </summary>
        private readonly List<DbContext> contextList;

        /// <summary>
        /// DbContext scope definitiion.
        /// </summary>
        [ThreadStatic]
        private static DbContextScope currentScope;

        /// <summary>
        /// Holds a value indicating whether the context is disposed or not.
        /// </summary>
        private bool isDisposed;

        /// <summary>
        /// Initializes a new instance of the <see cref="DbContextScope"/> class.
        /// </summary>
        /// <param name="saveAllChangesAtEndOfScope">if set to <c>true</c> [save all changes at end of scope].</param>
        protected DbContextScope(bool saveAllChangesAtEndOfScope)
        {
            if (currentScope != null && !currentScope.isDisposed)
            {
                throw new InvalidOperationException("DbContextScope instances cannot be nested.");
            }

            this.SaveAllChangesAtEndOfScope = saveAllChangesAtEndOfScope;

            this.contextList = new List<DbContext>();

            this.isDisposed = false;

            Thread.BeginThreadAffinity();

            currentScope = this;
        }

        /// <summary>
        /// Gets or sets a value indicating whether to automatically save all object changes at end of the scope.
        /// </summary>
        /// <value><c>true</c> if [save all changes at end of scope]; otherwise, <c>false</c>.</value>
        private bool SaveAllChangesAtEndOfScope { get; set; }

        /// <summary>
        /// Save all object changes to the underlying datastore.
        /// </summary>
        public void SaveAllChanges()
        {
            var transactions = new List<DbTransaction>();

            foreach (var context in this.contextList
                .Select(dbcontext => ((IObjectContextAdapter)dbcontext)
                    .ObjectContext))
            {
                context.Connection.Open();

                var databaseTransaction = context.Connection.BeginTransaction();

                transactions.Add(databaseTransaction);

                try
                {
                    context.SaveChanges();
                }
                catch
                {
                    /* Rollback & dispose all transactions: */
                    foreach (var transaction in transactions)
                    {
                        try
                        {
                            transaction.Rollback();
                        }
                        catch
                        {
                            // "Empty general catch clause suppresses any errors."
                            // Haven't quite figured out what to do here yet.
                        }
                        finally
                        {
                            databaseTransaction.Dispose();
                        }
                    }

                    transactions.Clear();

                    throw;
                }
            }

            try
            {
                /* Commit all complete transactions: */
                foreach (var completeTransaction in transactions)
                {
                    completeTransaction.Commit();
                }
            }
            finally
            {
                /* Dispose all transactions: */
                foreach (var transaction in transactions)
                {
                    transaction.Dispose();
                }

                transactions.Clear();

                /* Close all open connections: */
                foreach (var context in this.contextList
                    .Select(dbcontext => ((IObjectContextAdapter)dbcontext).ObjectContext)
                    .Where(context => context.Connection.State != System.Data.ConnectionState.Closed))
                {
                    context.Connection.Close();
                }
            }
        }

        /// <summary>
        /// Disposes the DbContext.
        /// </summary>
        public void Dispose()
        {
            // Monitor for possible future bugfix.
            // CA1063 : Microsoft.Design : Provide an overridable implementation of Dispose(bool) 
            // on 'DbContextScope' or mark the type as sealed. A call to Dispose(false) should 
            // only clean up native resources. A call to Dispose(true) should clean up both managed 
            // and native resources.
            if (this.isDisposed)
            {
                return;
            }

            // Monitor for possible future bugfix.
            // CA1063 : Microsoft.Design : Modify 'DbContextScope.Dispose()' so that it calls 
            // Dispose(true), then calls GC.SuppressFinalize on the current object instance 
            // ('this' or 'Me' in Visual Basic), and then returns.
            currentScope = null;

            Thread.EndThreadAffinity();

            try
            {
                if (this.SaveAllChangesAtEndOfScope && this.contextList.Count > 0)
                {
                    this.SaveAllChanges();
                }
            }
            finally
            {
                foreach (var context in this.contextList)
                {
                    try
                    {
                        context.Dispose();
                    }
                    catch (ObjectDisposedException)
                    {
                        // Monitor for possible future bugfix.
                        // CA2202 : Microsoft.Usage : Object 'databaseTransaction' can be disposed 
                        // more than once in method 'DbContextScope.SaveAllChanges()'. 
                        // To avoid generating a System.ObjectDisposedException you should not call 
                        // Dispose more than one time on an object.
                    }
                }

                this.isDisposed = true;
            }
        }

        /// <summary>
        /// Returns a reference to a DbContext of a specific type that is - or will be -
        /// created for the current scope. If no scope currently exists, null is returned.
        /// </summary>
        /// <typeparam name="TDbContext">The type of the db context.</typeparam>
        /// <returns>The current DbContext</returns>
        protected internal static TDbContext GetCurrentDbContext<TDbContext>()
            where TDbContext : DbContext, new()
        {
            if (currentScope == null)
            {
                return null;
            }

            var contextOfType = currentScope.contextList
                .OfType<TDbContext>()
                .FirstOrDefault();

            if (contextOfType == null)
            {
                contextOfType = new TDbContext();

                currentScope.contextList.Add(contextOfType);
            }

            return contextOfType;
        }
    }
}

FacadeBase.cs

namespace DbContextManagement
{
    using System;
    using System.Collections;
    using System.Collections.Generic;
    using System.Configuration;
    using System.Data.Entity;
    using System.Reflection;

    /// <summary>
    /// Generic base class for all other Facade classes.
    /// </summary>
    /// <typeparam name="TDbContext">The type of the db context.</typeparam>
    /// <typeparam name="TEntity">The type of the entity.</typeparam>
    /// <typeparam name="TEntityKey">The type of the entity key.</typeparam>
    /// <remarks>Not sure the handling of TEntityKey is something I've worked with properly.</remarks>
    public abstract class FacadeBase<TDbContext, TEntity, TEntityKey>
        where TDbContext : DbContext, new()
        where TEntity : class
    {
        /// <summary>
        /// Gets the db context.
        /// </summary>
        public TDbContext DbContext
        {
            get
            {
                if (DbContextManager == null)
                {
                    this.InstantiateDbContextManager();
                }

                return DbContextManager != null 
                    ? DbContextManager.GetDbContext<TDbContext>() 
                    : null;
            }
        }

        /// <summary>
        /// Gets or sets the DbContextManager.
        /// </summary>
        /// <value>The DbContextManager.</value>
        private DbContextManager DbContextManager { get; set; }

        /// <summary>
        /// Adds a new entity object to the context.
        /// </summary>
        /// <param name="newObject">A new object.</param>
        public virtual void Add(TEntity newObject)
        {
            this.DbContext.Set<TEntity>().Add(newObject);
        }

        /// <summary>
        /// Deletes an entity object.
        /// </summary>
        /// <param name="obsoleteObject">An obsolete object.</param>
        public virtual void Delete(TEntity obsoleteObject)
        {
            this.DbContext.Set<TEntity>().Remove(obsoleteObject);
        }

        /// <summary>
        /// Gets all entities for the given type.
        /// </summary>
        /// <returns>DbContext Set of TEntity.</returns>
        public virtual IEnumerable<TEntity> GetAll()
        {
            return this.DbContext.Set<TEntity>();
        }

        /// <summary>
        /// Gets the entity by the specified entity key.
        /// </summary>
        /// <param name="entityKey">The entity key.</param>
        /// <returns>Entity matching specified entity key or null if not found.</returns>
        public virtual TEntity GetByKey(TEntityKey entityKey)
        {
            return this.DbContext.Set<TEntity>().Find(entityKey);
        }

        /// <summary>
        /// Deletes the entity for the specified entity key.
        /// </summary>
        /// <param name="entityKey">The entity key.</param>
        public virtual void DeleteByKey(TEntityKey entityKey)
        {
            var entity = this.DbContext.Set<TEntity>().Find(entityKey);

            if (entity != null)
            {
                this.DbContext.Set<TEntity>().Remove(entity);
            }
        }

        /// <summary>
        /// Instantiates a new DbContextManager based on application configuration settings.
        /// </summary>
        private void InstantiateDbContextManager()
        {
            /* Retrieve DbContextManager configuration settings: */
            var contextManagerConfiguration = ConfigurationManager.GetSection("DbContext") as Hashtable;

            if (contextManagerConfiguration == null)
            {
                throw new ConfigurationErrorsException("A Facade.DbContext tag or its managerType attribute is missing in the configuration.");
            }

            if (!contextManagerConfiguration.ContainsKey("managerType"))
            {
                throw new ConfigurationErrorsException("dbManagerConfiguration does not contain key 'managerType'.");
            }

            var managerTypeName = contextManagerConfiguration["managerType"] as string;

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

            managerTypeName = managerTypeName.Trim().ToUpperInvariant();

            try
            {
                /* Try to create a type based on it's name: */
                var frameworkAssembly = Assembly.GetAssembly(typeof(DbContextManager));

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

                /* Try to create a new instance of the specified DbContextManager type: */
                this.DbContextManager = Activator.CreateInstance(managerType) as DbContextManager;
            }
            catch (Exception e)
            {
                throw new ConfigurationErrorsException("The managerType specified in the configuration is not valid.", e);
            }
        }
    }
}

ScopedDbContextManager.cs

namespace DbContextManagement
{
    using System.Collections.Generic;
    using System.Data.Entity;
    using System.Linq;

    /// <summary>
    /// Manages multiple db contexts.
    /// </summary>
    public sealed class ScopedDbContextManager : DbContextManager
    {
        /// <summary>
        /// List of Object Contexts.
        /// </summary>
        private List<DbContext> contextList;

        /// <summary>
        /// Returns the DbContext instance that belongs to the current DbContextScope.
        /// If currently no DbContextScope exists, a local instance of an DbContext
        /// class is returned.
        /// </summary>
        /// <typeparam name="TDbContext">The type of the db context.</typeparam>
        /// <returns>Current scoped DbContext.</returns>
        public override TDbContext GetDbContext<TDbContext>()
        {
            var currentDbContext = DbContextScope.GetCurrentDbContext<TDbContext>();

            if (currentDbContext != null)
            {
                return currentDbContext;
            }

            if (this.contextList == null)
            {
                this.contextList = new List<DbContext>();
            }

            currentDbContext = this.contextList.OfType<TDbContext>().FirstOrDefault();

            if (currentDbContext == null)
            {
                currentDbContext = new TDbContext();

                this.contextList.Add(currentDbContext);
            }

            return currentDbContext;
        }
    }
}

UnitOfWorkScope.cs

namespace DbContextManagement
{
    /// <summary>
    /// Defines a scope for a business transaction. At the end of the scope all object changes can be persisted to the underlying datastore. 
    /// </summary>
    /// <remarks>Instances of this class are supposed to be used in a using() statement.</remarks>
    public sealed class UnitOfWorkScope : DbContextScope
    {
        /// <summary>
        /// Initializes a new instance of the <see cref="UnitOfWorkScope"/> class.
        /// </summary>
        /// <param name="saveAllChangesAtEndOfScope">if set to <c>true</c> [save all changes at end of scope].</param>
        public UnitOfWorkScope(bool saveAllChangesAtEndOfScope)
            : base(saveAllChangesAtEndOfScope)
        {
        }
    }
}

Facade (Class Library Project)

YourEntityFacade.cs

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

    public class YourEntityFacade : FacadeBase<YourDbContext, YourEntity, int>
    {
        public override IEnumerable<YourEntity> GetAll()
        {
            return base.GetAll()
                .Distinct()
                .ToList();
        }
    }
}

TestConsole (Console Application Project)

App.config

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
    <configSections>
        <section name="DbContext" type="System.Configuration.SingleTagSectionHandler" />
    </configSections>
    <DbContext managerType="DbContextManagement.ScopedDbContextManager" />
    <connectionStrings>
        <add 
            name="YourDbContext" 
            providerName="System.Data.SqlClient" 
            connectionString="Your connection string" />
    </connectionStrings>
</configuration>

Program.cs

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

    public static class Program
    {
        public static void Main()
        {
            TestGetAll();

            Console.ReadLine();
        }

        private static void TestGetAll()
        {
            Console.WriteLine();
            Console.WriteLine("Test GetAll()");
            Console.WriteLine();

            IEnumerable<YourEntity> yourEntities;

            using (new UnitOfWorkScope(false))
            {
                yourEntities= new YourEntityFacade().GetAll();
            }

            if (yourEntities != null)
            {
                foreach (var yourEntity in yourEntities)
                {
                    Console.WriteLine(
                       string.Format("{0}, {1}", 
                       yourEntity.Id, 
                       yourEntity.Name));
                }
            }
            else
            {
                Console.WriteLine("GetAll() NULL");
            }
        }
    }
}

So, usage is very simple and you can work with multiple contexts and facades within a scope. With this approach, the only code you are writing is custom queries. No more endless updating UnitOfWorks with repository references and authoring copy-cat repositories. I think it’s great but be aware this is beta code and I’m sure it has a large hole somewhere that will need plugging 🙂

Thank you to all and in particular Ladislav for patience and help on this and many other related questions I ask. I hope this code is of interest. I contacted the author at the blog above but no reply yet and these days I think he’s into NHibernate.

Richard