【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