Implementing Repository Pattern With Entity Framework

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Introduction  

The Repository pattern is defined by Patterns of Enterprise Application Architecture as:   

Mediates between the domain and data mapping layers using a collection-like interface for accessing domain objects. 

Repository provides an in-memory like collection interface for accessing domain objects. So as far as the consuming component is concerned, it uses the repository just like a collection when working with Domain objects. The repository then neatly abstracts the internal mechanics of how the Add / Remove calls to the repository translate to the actual data access calls to the data store. Objects can be added to and removed from the Repository, as they can from a simple collection of objects, and the mapping code encapsulated by the Repository will carry out the appropriate operations behind the scenes. Conceptually, a Repository encapsulates the set of objects persisted in a data store and the operations performed over them, providing a more object-oriented view of the persistence layer. Repository also supports the objective of achieving a clean separation and one-way dependency between the domain and data mapping layers.

So with the repository we get a nice abstraction that provides us with persistence ignorance and a nice separation of concerns where the responsibility of persisting domain objects is encapsulated by the Repository leaving the domain objects to deal entirely with the domain model and domain logic.Here are some reasons for using repository pattern in data access layer in place direct access the database code-

• Duplicated code 
• A higher potential for programming errors
• Weak typing of the business data
• Difficulty in centralizing data-related policies such as caching
• An inability to easily test the business logic in isolation from external dependencies 

Using the Code 

Here I use the Composite Repository pattern:

    /// <summary>
    /// Repository Interface defines the base
    /// functionality required by all Repositories.
    /// </summary>
    /// <typeparam name="T">
    /// The entity type that requires a Repository.
    /// </typeparam>
    public interface IRepository<E>
    {
        string KeyProperty { get; set; }

        void Add(E entity);
        void AddOrAttach(E entity);
        void DeleteRelatedEntries(E entity);
        void DeleteRelatedEntries
        (E entity, ObservableCollection<string> keyListOfIgnoreEntites);
        void Delete(E entity);

        ObjectQuery<E> DoQuery();
        ObjectQuery<E> DoQuery(ISpecification<E> where);
        ObjectQuery<E> DoQuery(int maximumRows, int startRowIndex);
        ObjectQuery<E> DoQuery(Expression<Func<E, object>> sortExpression);
        ObjectQuery<E> DoQuery(Expression<Func<E, object>> sortExpression,
                    int maximumRows, int startRowIndex);

        IList<E> SelectAll(string entitySetName);
        IList<E> SelectAll();
        IList<E> SelectAll(string entitySetName, ISpecification<E> where);
        IList<E> SelectAll(ISpecification<E> where);
        IList<E> SelectAll(int maximumRows, int startRowIndex);
        IList<E> SelectAll(Expression<Func<E, object>> sortExpression);
        IList<E> SelectAll(Expression<Func<E, object>> sortExpression,
                    int maximumRows, int startRowIndex);

        E SelectByKey(string Key);

        bool TrySameValueExist(string fieldName, object fieldValue, string key);
        bool TryEntity(ISpecification<E> selectSpec);

        int GetCount();
        int GetCount(ISpecification<E> selectSpec);
    }

You can write your own Repository for each business object like RoleRepository, UserReporsitory, etc. Or you can implement this interface as a generic class of Repository something like this:   

    public class Repository<E, C> : IRepository<E>
        where E : class
        where C : ObjectContext
    {
        private readonly C _ctx;

        private string _KeyProperty = "ID";

        public string KeyProperty
        {
            get
            {
                return _KeyProperty;
            }
            set
            {
                _KeyProperty = value;
            }
        }

        public C Session
        {
            get { return _ctx; }
        }

        public Repository(C session)
        {
            _ctx = session;
        }

        #region IRepository<E,C> Members

        public int Save()
        {
            return _ctx.SaveChanges();
        }
        /// <summary>
        /// A generic method to return ALL the entities
        /// </summary>
        /// <param name=”entitySetName”>
        /// The EntitySet name of the entity in the model.
        /// </param>
        /// <typeparam name=”TEntity”>
        /// The Entity to load from the database.
        /// </typeparam>
        /// <returns>Returns a set of TEntity.</returns>
        private ObjectQuery<E> DoQuery(string entitySetName)
        {
            return _ctx.CreateQuery<E>("[" + entitySetName + "]");
        }
        /// <summary>
        /// A generic method to return ALL the entities
        /// </summary>
        /// <typeparam name=”TEntity”>
        /// The Entity to load from the database.
        /// </typeparam>
        /// <returns>Returns a set of TEntity.</returns>
        public ObjectQuery<E> DoQuery()
        {
            return _ctx.CreateQuery<E>("[" + this.GetEntitySetName( typeof(E).Name) + "]");
        }

        /// <summary>
        /// </summary>
        /// <param name=”entitySetName”>
        /// The EntitySet name of the entity in the model.
        /// </param>
        /// <typeparam name=”TEntity”>
        /// The Entity to load from the database.
        /// </typeparam>
        /// <returns>Returns a set of TEntity.</returns>
        private ObjectQuery<E> DoQuery(string entitySetName, ISpecification<E> where)
        {
            return
                (ObjectQuery<E>)_ctx.CreateQuery<E>("[" + entitySetName + "]")
                .Where(where.EvalPredicate);
        }

        /// <summary>
        /// </summary>
        /// <typeparam name=”TEntity”>
        /// The Entity to load from the database.
        /// </typeparam>
        /// <returns>Returns a set of TEntity.</returns>
        public ObjectQuery<E> DoQuery(ISpecification<E> where)
        {
            return
                (ObjectQuery<E>)_ctx.CreateQuery<E>("[" + this.GetEntitySetName( typeof(E).Name ) + "]")
                .Where(where.EvalPredicate);
        }
        /// <summary>
        /// Query Entity with Paging 
        /// </summary>
        /// <param name="maximumRows">Max no of row to Fetch</param>
        /// <param name="startRowIndex">Start Index</param>
        /// <returns>Collection of Entities</returns>
        public ObjectQuery<E> DoQuery(int maximumRows, int startRowIndex)
        {
            return (ObjectQuery<E>)_ctx.CreateQuery<E>
        ("[" + this.GetEntitySetName(typeof(E).Name) + "]").Skip<E>(startRowIndex).Take(maximumRows);
        }
        /// <summary>
        /// Query Entity in sorted Order
        /// </summary>
        /// <param name="sortExpression">Sort Expression/condition</param>
        /// <param name="ErrorCode">custom Error Message</param> 
        /// <returns>Collection of Entities</returns>
        public ObjectQuery<E> DoQuery(Expression<Func<E, object>> sortExpression)
        {
            if (null == sortExpression)
            {
                return this.DoQuery();
            }
            return (ObjectQuery<E>)((IRepository<E>)this).DoQuery().OrderBy
                        <E, object>(sortExpression);
        }
        /// <summary>
        /// Query All Entity in sorted Order with Paging support
        /// </summary>
        /// <param name="sortExpression">Sort Expression/condition</param>
        /// <param name="maximumRows">Max no of row to Fetch</param>
        /// <param name="startRowIndex">Start Index</param>
        /// <returns>Collection Of entities</returns>
        public ObjectQuery<E> DoQuery(Expression<Func<E, object>>
            sortExpression, int maximumRows, int startRowIndex)
        {
            if (sortExpression == null)
            {
                return ((IRepository<E>)this).DoQuery(maximumRows, startRowIndex);
            }
            return (ObjectQuery<E>)((IRepository<E>)this).DoQuery
            (sortExpression).Skip<E>(startRowIndex).Take(maximumRows);
        }
        /// <summary>
        /// A generic method to return ALL the entities
        /// </summary>
        /// <param name=”entitySetName”>
        /// The EntitySet name of the entity in the model.
        /// </param>
        /// <typeparam name=”TEntity”>
        /// The Entity to load from the database.
        /// </typeparam>
        /// <returns>Returns a set of TEntity.</returns>
        public IList<E> SelectAll(string entitySetName)
        {
            return DoQuery(entitySetName).ToList();
        }
        /// <summary>
        /// A generic method to return ALL the entities
        /// </summary>
        /// <typeparam name=”TEntity”>
        /// The Entity to load from the database.
        /// </typeparam>
        /// <returns>Returns a set of TEntity.</returns>
        public IList<E> SelectAll()
        {
            try
            {
                return DoQuery().ToList(); //_ctx.CreateQuery<E>("[" + typeof(E).Name + "]");
            }
            catch (Exception)
            {
                throw;
            }
        }

        /// <summary>
        /// A generic method to return ALL the entities
        /// </summary>
        /// <param name=”entitySetName”>
        /// The EntitySet name of the entity in the model.
        /// </param>
        /// <typeparam name=”TEntity”>
        /// The Entity to load from the database.
        /// </typeparam>
        /// <returns>Returns a set of TEntity.</returns>
        public IList<E> SelectAll(string entitySetName, ISpecification<E> where)
        {
            return DoQuery(entitySetName, where).ToList();
        }

        /// <summary>
        /// A generic method to return ALL the entities
        /// </summary>
        /// <typeparam name=”TEntity”>
        /// The Entity to load from the database.
        /// </typeparam>
        /// <returns>Returns a set of TEntity.</returns>
        public IList<E> SelectAll(ISpecification<E> where)
        {
            return DoQuery(where).ToList();
        }
        /// <summary>
        /// Select All Entity with Paging 
        /// </summary>
        /// <param name="maximumRows">Max no of row to Fetch</param>
        /// <param name="startRowIndex">Start Index</param>
        /// <returns>Collection of Entities</returns>
        public IList<E> SelectAll(int maximumRows, int startRowIndex)
        {
            return DoQuery(maximumRows, startRowIndex).ToList();
        }
        /// <summary>
        /// Select All Entity in sorted Order
        /// </summary>
        /// <param name="sortExpression">Sort Expression/condition</param>
        /// <param name="ErrorCode">custom Error Message</param> 
        /// <returns>Collection of Entities</returns>
        public IList<E> SelectAll(Expression<Func<E, object>> sortExpression)
        {
            if (null == sortExpression)
            {
                return DoQuery(sortExpression).ToList();
            }
            return DoQuery(sortExpression).ToList();
        }
        /// <summary>
        /// Select All Entity in sorted Order with Paging support
        /// </summary>
        /// <param name="sortExpression">Sort Expression/condition</param>
        /// <param name="maximumRows">Max no of row to Fetch</param>
        /// <param name="startRowIndex">Start Index</param>
        /// <returns>Collection Of entities</returns>
        public IList<E> SelectAll(Expression<Func<E, object>>
            sortExpression, int maximumRows, int startRowIndex)
        {
            if (sortExpression == null)
            {
                return DoQuery(maximumRows, startRowIndex).ToList();
            }
            return DoQuery(sortExpression, maximumRows, startRowIndex).ToList();
        }
        /// <summary>
        /// Get Entity By Primary Key
        /// </summary>
        /// <typeparam name="E">Entity Type</typeparam>
        /// <param name="Key">Primary Key Value</param>
        /// <returns>return entity</returns>
        public E SelectByKey(string Key)
        {
            // First we define the parameter that we are going to use the clause. 
            var xParam = Expression.Parameter(typeof(E), typeof(E).Name);
            MemberExpression leftExpr = MemberExpression.Property(xParam, this._KeyProperty);
            Expression rightExpr = Expression.Constant(Key);
            BinaryExpression binaryExpr = MemberExpression.Equal(leftExpr, rightExpr);
            //Create Lambda Expression for the selection 
            Expression<Func<E, bool>> lambdaExpr =
            Expression.Lambda<Func<E, bool>>(binaryExpr,
            new ParameterExpression[] { xParam });
            //Searching ....
            var resultCollection = (ObjectQuery<E>)_ctx.CreateQuery<E>("[" + this.GetEntitySetName(typeof(E).Name) + "]")
                .Where(lambdaExpr);
            if (null != resultCollection && resultCollection.Count() > 0)
            {
                //return valid single result
                return resultCollection.First<E>();
            }//end if 
            return null;
        }
        /// <summary>
        /// Check if value of specific field is already exist
        /// </summary>
        /// <typeparam name="E"></typeparam>
        /// <param name="fieldName">name of the Field</param>
        /// <param name="fieldValue">Field value</param>
        /// <param name="key">Primary key value</param>
        /// <returns>True or False</returns>
        public bool TrySameValueExist(string fieldName, object fieldValue, string key)
        {
            // First we define the parameter that we are going to use the clause. 
            var xParam = Expression.Parameter(typeof(E), typeof(E).Name);
            MemberExpression leftExprFieldCheck =
            MemberExpression.Property(xParam, fieldName);
            Expression rightExprFieldCheck = Expression.Constant(fieldValue);
            BinaryExpression binaryExprFieldCheck =
            MemberExpression.Equal(leftExprFieldCheck, rightExprFieldCheck);

            MemberExpression leftExprKeyCheck =
            MemberExpression.Property(xParam, this._KeyProperty);
            Expression rightExprKeyCheck = Expression.Constant(key);
            BinaryExpression binaryExprKeyCheck =
            MemberExpression.NotEqual(leftExprKeyCheck, rightExprKeyCheck);
            BinaryExpression finalBinaryExpr =
            Expression.And(binaryExprFieldCheck, binaryExprKeyCheck);

            //Create Lambda Expression for the selection 
            Expression<Func<E, bool>> lambdaExpr =
            Expression.Lambda<Func<E, bool>>(finalBinaryExpr,
            new ParameterExpression[] { xParam });
            //Searching ....            
            return _ctx.CreateQuery<E>("[" + this.GetEntitySetName(typeof(E).Name) + "]").Any<E>
                        (lambdaExpr);
        }
        /// <summary>
        /// Check if Entities exist with Condition
        /// </summary>
        /// <param name="selectExpression">Selection Condition</param>
        /// <returns>True or False</returns>
        public bool TryEntity(ISpecification<E> selectSpec)
        {
            return _ctx.CreateQuery<E>("[" + this.GetEntitySetName( typeof(E).Name ) + "]").Any<E>
                        (selectSpec.EvalPredicate);
        }
        /// <summary>
        /// Get Count of all records
        /// </summary>
        /// <typeparam name="E"></typeparam>
        /// <returns>count of all records</returns>
        public int GetCount()
        {
            return _ctx.CreateQuery<E>("[" + this.GetEntitySetName( typeof(E).Name ) + "]").Count();
        }
        /// <summary>
        /// Get count of selection
        /// </summary>
        /// <typeparam name="E">Selection Condition</typeparam>
        /// <returns>count of selection</returns>
        public int GetCount(ISpecification<E> selectSpec)
        {
            return _ctx.CreateQuery<E>("[" + this.GetEntitySetName( typeof(E).Name ) + "]")
                .Where(selectSpec.EvalPredicate).Count();
        }
        /// <summary>
        /// Delete data from context
        /// </summary>
        /// <typeparam name="E"></typeparam>
        /// <param name="entity"></param>
        public void Delete(E entity)
        {
            _ctx.DeleteObject(entity);
        }
        /// <summary>
        /// Delete data from context
        /// </summary>
        /// <typeparam name="E"></typeparam>
        /// <param name="entity"></param>
        public void Delete(object entity)
        {
            _ctx.DeleteObject(entity);
        }
        /// <summary>
        /// Insert new data into context
        /// </summary>
        /// <typeparam name="E"></typeparam>
        /// <param name="entity"></param>
        public void Add(E entity)
        {
            _ctx.AddObject( this.GetEntitySetName(entity.GetType().Name), entity);
        }
        /// <summary>
        /// Insert if new otherwise attach data into context
        /// </summary>
        /// <param name="entity"></param>
        public void AddOrAttach(E entity)
        {
            // Define an ObjectStateEntry and EntityKey for the current object.
            EntityKey key;
            object originalItem;
            // Get the detached object's entity key.
            if (((IEntityWithKey)entity).EntityKey == null)
            {
                // Get the entity key of the updated object.
                key = _ctx.CreateEntityKey(this.GetEntitySetName(entity.GetType().Name), entity);
            }
            else
            {
                key = ((IEntityWithKey)entity).EntityKey;
            }
            try
            {
                // Get the original item based on the entity key from the context
                // or from the database.
                if (_ctx.TryGetObjectByKey(key, out originalItem))
                {//accept the changed property
                    if (originalItem is EntityObject &&
                        ((EntityObject)originalItem).EntityState != EntityState.Added)
                    {
                        // Call the ApplyCurrentValues method to apply changes
                        // from the updated item to the original version.
                        _ctx.ApplyCurrentValues(key.EntitySetName, entity);
                    }
                }
                else
                {//add the new entity
                    Add(entity);
                }//end else
            }
            catch (Exception ex)
            {
                throw ex;
            }
        }
        /// <summary>
        /// Delete all related entries
        /// </summary>
        /// <param name="entity"></param>        
        public void DeleteRelatedEntries(E entity)
        {
            foreach (var relatedEntity in (((IEntityWithRelationships)entity).
        RelationshipManager.GetAllRelatedEnds().SelectMany(re =>
        re.CreateSourceQuery().OfType<EntityObject>()).Distinct()).ToArray())
            {
                _ctx.DeleteObject(relatedEntity);
            }//end foreach
        }
        /// <summary>
        /// Delete all related entries
        /// </summary>
        /// <param name="entity"></param>        
        public void DeleteRelatedEntries(E entity, ObservableCollection<string>
                            keyListOfIgnoreEntites)
        {
            foreach (var relatedEntity in (((IEntityWithRelationships)entity).
            RelationshipManager.GetAllRelatedEnds().SelectMany(re =>
            re.CreateSourceQuery().OfType<EntityObject>()).Distinct()).ToArray())
            {
                PropertyInfo propInfo = relatedEntity.GetType().GetProperty
                            (this._KeyProperty);
                if (null != propInfo)
                {
                    string value = (string)propInfo.GetValue(relatedEntity, null);
                    if (!string.IsNullOrEmpty(value) &&
                        keyListOfIgnoreEntites.Contains(value))
                    {
                        continue;
                    }//end if 
                }//end if
                _ctx.DeleteObject(relatedEntity);
            }//end foreach
        }

        private string GetEntitySetName(string entityTypeName)
        {
            var container = this._ctx.MetadataWorkspace.GetEntityContainer
                    (this._ctx.DefaultContainerName, DataSpace.CSpace);

            return (from meta in container.BaseEntitySets

                                    where meta.ElementType.Name == entityTypeName

                                    select meta.Name).FirstOrDefault(); 
                     
        }

        #endregion
    }

Notice that, here I also implement IDispose interface to dispose the context manually.To get the name of Entityset, here I have used typeof, but you can do a  metadata query to retrieve the EntitySet name:

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

string entitySetName = (from meta in container.BaseEntitySets

                            where meta.ElementType.Name == entityTypeName

                            select meta.Name).FirstOrDefault();   

Here I am not going a little into the code. I have used ObjectQuery to get the features of MargeOption and EnablePlanCaching properties: 

ObjectQuery.EnablePlanCaching Property -indicates whether the query plan should be cached. Plan-caching caches information which is computed as part of putting together the query itself. By caching this, a subsequent execution of the same query (even if you change parameter values) will run much faster than the first one. This information is cached per app-domain so you will generally benefit from the query cache across multiple client requests to the same web app and the like. Here all DoQuery methods are responsible to queries and other query method like SelectAll or Select methods internally use these DoQuery methods with various parameters. 

Setting MergeOption to NoTracking has helped us to take advantage of the EF ability to return data that doesn’t need to be tracked by the context. Say I don’t have any plan to make changes to that data. Therefore, I like to avoid the performance hit taken when EF creates ObjectStateEntry instances for each object it’s tracking, as well as forcing the context to be aware of any changes made to those objects. 

SelectByKey is creating a LINQ expression using Expression tree for Primary key and During creating the repository on an entity you can supply this Primary Key property as string. While using the POCO as you entity you can set attribute programming to serve this kind of job.

TrySameValueExist is doing the same job by allow you to set customize field and value comparison there. What will do is create the Expression for you and also add the PrimaryKey comparison so that it excludes the object that you are querying for (and where PrimaryKey != currentObjectPrimaryKey).

Add and Delete method simply respectively call the AddObject and DeleteObject Method of ObjectContext. AddOrAttach method is for special situation where you don’t know whether object is already added or not. It is expensive since it will do query to database to check the existence. 

The specification pattern can implement a re-usable business logic component that can be passed around to satisfy certain business criteria. The specification object has a clear and limited responsibility, which can be separated and decoupled from the domain object that uses it. I would highly recommend reading the white paper by Martin Fowler and Eric Evans on the Specification pattern.   

public interface ISpecification<E>
{
    /// <summary>
    /// Select/Where Expression
    /// </summary>
    Expression<Func<E, bool>> EvalPredicate { get; }
    /// <summary>
    /// Function to evaluate where Expression
    /// </summary>
    Func<E, bool> EvalFunc { get; }
}

You can write your own specification by implementing your interface like RoleSpecification and put down your business logic there. For general use, you can also implement the Interface; such composite specification like this:

    public class Specification<E> : ISpecification<E>
    {
        #region Private Members

        private Func<E, bool> _evalFunc = null;
        private Expression<Func<E, bool>> _evalPredicate;

        #endregion

        #region Virtual Accessors

        public virtual bool Matches(E entity)
        {
            return _evalPredicate.Compile().Invoke(entity);
        }

        public virtual Expression<Func<E, bool>> EvalPredicate
        {
            get { return _evalPredicate; }
        }

        public virtual Func<E, bool> EvalFunc
        {
            get { return _evalPredicate != null ? _evalPredicate.Compile() : null; }
        }

        #endregion

        #region Constructors

        public Specification(Expression<Func<E, bool>> predicate)
        {
            _evalPredicate = predicate;
        }

        private Specification() { }

        #endregion

        #region Private Nested Classes

        private class AndSpecification : Specification<E>
        {
            private readonly ISpecification<E> _left;
            private readonly ISpecification<E> _right;
            public AndSpecification(ISpecification<E> left, ISpecification<E> right)
            {
                this._left = left;
                this._right = right;

                this._evalFunc =
                    (Func<E, bool>)Delegate.Combine
                    (left.EvalPredicate.Compile(),
                    right.EvalPredicate.Compile());

                _evalPredicate = left.EvalPredicate.And(right.EvalPredicate);
            }
            public override bool Matches(E entity)
            {
                return EvalPredicate.Compile().Invoke(entity);
            }
        }

        private class OrSpecification : Specification<E>
        {
            private readonly ISpecification<E> _left;
            private readonly ISpecification<E> _right;
            public OrSpecification(ISpecification<E> left, ISpecification<E> right)
            {
                this._left = left;
                this._right = right;

                this._evalFunc =
                    (Func<E, bool>)Delegate.Combine
                    (left.EvalPredicate.Compile(),
                    right.EvalPredicate.Compile());

                _evalPredicate = left.EvalPredicate.Or(right.EvalPredicate);
            }
            public override bool Matches(E entity)
            {
                return EvalPredicate.Compile().Invoke(entity);
            }
        }

        #endregion

        #region Operator Overloads

        public static Specification<E> operator &(Specification<E> left, ISpecification<E> right)
        {
            return new AndSpecification(left, right);
        }

        public static Specification<E> operator |(Specification<E> left, ISpecification<E> right)
        {
            return new OrSpecification(left, right);
        }

        #endregion

    } 

Here some operator overloading has been applied here is to combine the lambda expression (or predicate) of each specification (left and right side of a composite specification) to create a new lambda expression.This new lambda expression is going to use for querying. So It will help you while you are working with multiple specification already defined as your business rule.  

To use this Repository class from your business layer, one thing I must say that I am sharing a single context in the CRUD process/operation. You must keep alive the context in your entire process of a crud operation. For this, I make a UnitOfWork on top of all.

    public interface IUnitOfWork : IDisposable
    {
        IRepository<TSet> GetRepository<TSet>() where TSet : class;
        DbTransaction BeginTransaction();
        int Save();
    }

Here Unit of Work  has been introduced as umbrella over multiple repositories and shared by all repository in place of the object context directly and method like save, transaction can be switched over there. And here is that implementation. 

    public class UnitOfWork<C> : IUnitOfWork where C : ObjectContext
    {
        private DbTransaction _transaction;
        private Dictionary<Type, object> _repositories;
        private C _ctx;

        public UnitOfWork()
        {
            _ctx = Activator.CreateInstance<C>();
            _repositories = new Dictionary<Type, object>();
        }

        public IRepository<TSet> GetRepository<TSet>() where TSet : class
        {
            if (_repositories.Keys.Contains(typeof(TSet)))
                return _repositories[typeof(TSet)] as IRepository<TSet>;

            var repository = new Repository<TSet, C>(_ctx);
            _repositories.Add(typeof(TSet), repository);
            return repository;
        }
        /// <summary>
        /// Start Transaction
        /// </summary>
        /// <returns></returns>
        public DbTransaction BeginTransaction()
        {
            if (null == _transaction)
            {
                if (_ctx.Connection.State != ConnectionState.Open)
                {
                    _ctx.Connection.Open();
                }
                this._transaction = _ctx.Connection.BeginTransaction();
            }
            return _transaction;
        }

        public int Save()
        {
            return _ctx.SaveChanges();
        }

        #region IDisposable Members

        public void Dispose()
        {
            if (null != _transaction)
            {
                _transaction.Dispose();
            }

            if (null != _ctx)
            {
                _ctx.Dispose();
            }
        }

        #endregion

    }

Repository is kind of mediator to connect business layer with data access. So your business layer should be aware of this repository class. Now, In my BLLRoleManagement class, say I have a method to delete User. That will be something like this:  

///  <summary>
/// Delete User
/// </summary>
/// <param name="userID">User ID</param>
/// <returns>True Or False</return>	   
    public static bool DeleteUser(
    string UserID)
        {
            try
            {
                var unitOfWork = new UnitOfWork<ProjectCodeEntities>();
                var userRepository = unitOfWork.GetRepository<User>();
                using (unitOfWork)
                {
                    using (var transaction = unitOfWork.BeginTransaction())
                    {
                        User UserTobeRemoved = userRepository.SelectByKey(UserID);
                        if (UserTobeRemoved == null)
                            return false;

                        userRepository.DeleteRelatedEntries(UserTobeRemoved);
                        userRepository.Delete(UserTobeRemoved);
                        if (unitOfWork.Save() >= 0)
                        {
                            transaction.Commit();
                            return true;
                        }
                    }
                }
            }
            catch (Exception ex)
            {
                throw ErrorHandler.WrapException(ex, ErrorCodes.DB_DeleteUser_Error_Code);
            }
            return false;
        } 

Here It will initialize a ObjectContext instance which will be used in you entire process/workflow of an operation. For this, I have provided a factory method inside Unit of Work which create a repository for you using that ObjectContext . Delete all related entities associated with User and then delete User. To share the same context in this delete process, I will call this method like this  

new BLLRoleManagement().DeleteUser(userID);   

 Surely it has a lot of area to improve. I have used some interfaces here those can be used with dependency injection to decouple your code from EF. Hope its help. So here is my end of the discussion on repository pattern implementation with Entity framework. Good luck. 

References: 

• http://en.wikipedia.org/wiki/Specification_pattern
• http://msdn.microsoft.com/en-us/library/ff649690.aspx
• http://www.martinfowler.com/apsupp/spec.pdf  
• http://martinfowler.com/eaaCatalog/repository.html