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
