【EntityFramwork--处理数据并发问题】

EntityFramwork--处理数据并发问题时支持乐观并发，即假定最佳场景(这里是指数据在更新过程中没有发生变化)

具体看《Beginning ASP.NET 4.5 Databases》P188-189：

 

 

本书源代码下载地址：

http://www.codeplex.com/Download?ProjectName=CommonServiceLocator&DownloadId=45067

以一下是截取源代码的实现Unit of work、并发、事务的BaseRepository：

BaseRepository.cs：

using System;
using System.Collections.Generic;
using System.Data;
using System.Data.Entity;
using System.Data.Entity.Infrastructure;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Transactions;
using Repository;

namespace DataAccess
{
    public class s : IDisposable
    {
        protected IUnitOfWork UnitOfWork { get; set; }
        private bool disposed = false;

        protected StoreEntities Context
        {
            get { return (EfStoreDataContext)this.UnitOfWork; }
        }

        /// <summary>
        /// .ctor
        /// </summary>
        /// <param name="unitOfWork"></param>
        public BaseRepository(IUnitOfWork unitOfWork)
        {
            if (unitOfWork == null) throw new ArgumentNullException("unitOfWork");
            this.UnitOfWork = unitOfWork;
        }

        public void Save()
        {
            try
            {
                using(var scope = new TransactionScope(TransactionScopeOption.RequiresNew))
                {
                    this.Context.SaveChanges();
                    scope.Complete();
                }
            }
            catch (DbUpdateConcurrencyException concurrencyException)
            {
                concurrencyException.Entries.Single().OriginalValues.SetValues(concurrencyException.Entries.Single().GetDatabaseValues());
                throw;
            }
        }

        protected virtual DbSet<TEntity> GetDbSet<TEntity>() where TEntity : class
        {
            return this.Context.Set<TEntity>();
        }

        protected virtual void SetEntityState(object entity, EntityState entityState)
        {
            this.Context.Entry(entity).State = entityState;
        }

        protected virtual void Dispose(bool disposing)
        {
            if (!this.disposed)
            {
                if (disposing)
                {
                    this.Context.Dispose();
                }
            }
            this.disposed = true;
        }

        public void Dispose()
        {
            Dispose(true);
            GC.SuppressFinalize(this);
        } 
    }
}

---------------------------
（一）.事务

38                 using(var scope = new TransactionScope(TransactionScopeOption.RequiresNew))
39                 {
40                     this.Context.SaveChanges();
41                     scope.Complete();
42                 }

使用事务的目的：在某个场景中,需要更新多个数据上下文实例，并想要把这些更新放在一个事务中。

2.

---------------
（二）并发

44             catch (DbUpdateConcurrencyException concurrencyException)
45             {
46                 concurrencyException.Entries.Single().OriginalValues.SetValues(concurrencyException.Entries.Single().GetDatabaseValues());
47                 throw;
48             }

EF框架处理并发的方式：

方式1：

让客户端决定状态----让项的原始值设置为从数据库中获取的值：

concurrencyException.Entries.Single().OriginalValues.SetValues(concurrencyException.Entries.Single().GetDatabaseValues());

方式2：

让客户端决定状态----用存储中的值刷新新实体值：

concurrencyException.Entries.Single().Reload（）;

方式3：
自定义一种方案，选择合适的选项。


---------------------
（三）工作单元 Unit of work

EF中的Unit Of Work机制

（假设StoreEntities 实例的变量名为contexDb）

contexDb.Entry(Order).State = System.Data.EntityState.Modified; //

------------------------------------------------
下面代码是设计一个实现了Unit of work机制和Repository模式的具体实现：

56         protected virtual void SetEntityState(object entity, EntityState entityState)
57         {
58             this.Context.Entry(entity).State = entityState;
59         }

this.Context是

19         protected StoreEntities Context
20         {
21             get { return (EfStoreDataContext)this.UnitOfWork; }
22         }

EfStoreDataContext.cs

    public class EfStoreDataContext : StoreEntities, IUnitOfWork
    {
        public new void SaveChanges()
        {
            base.SaveChanges();
        }
    }

EF生成的访问数据的DbContext

namespace DataAccess
{
    using System;
    using System.Data.Entity;
    using System.Data.Entity.Infrastructure;
    using Entities;
    
    public partial class StoreEntities : DbContext
    {
        public StoreEntities()
            : base("name=StoreEntities")
        {
        }
    
        protected override void OnModelCreating(DbModelBuilder modelBuilder)
        {
            throw new UnintentionalCodeFirstException();
        }
    
        public DbSet<Contact> Contacts { get; set; }
        public DbSet<Order> Orders { get; set; }
    }
}

IUnitofWork.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Repository
{
    public interface IUnitOfWork
    {
        void SaveChanges();
    }
}

使用了工作单元 （Unit of work）机制的OrderReposiory:

OrderReposiory.cs:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using Entities;
using Repository;

namespace DataAccess.Repositories
{
    public class OrderRepository : BaseRepository, IOrderRepository
    {

        public OrderRepository(IUnitOfWork unitOfWork)
            : base(unitOfWork)
        {

        }

        public IEnumerable<Order> GetAllOrders()
        {
            return this.GetDbSet<Order>();
        }

        public Order GetByOrderId(int orderId)
        {
            return this.GetDbSet<Order>().Find(orderId);
        }

        public void Create(Order order)
        {
            this.GetDbSet<Order>().Add(order);
        }

        public void Update(Order order)
        {
            this.SetEntityState(order, System.Data.EntityState.Modified);
        }

        public void Delete(int orderId)
        {
            var order = this.GetDbSet<Order>().Find(orderId);
            this.GetDbSet<Order>().Remove(order);
        }
    }
}

根据上面的设计，这行代码：

37             this.SetEntityState(order, System.Data.EntityState.Modified);
最终调用EF框架生成的

  public partial class StoreEntities : DbContext
的方法

contexDb.Entry(Order).State = System.Data.EntityState.Modified; //EF中的Unit Of Work机制

假设StoreEntities 实例的变量名为contexDb