并发可能产生的三种问题

脏读

定义：A事务执行过程中B事务读取了A事务的修改，但是A事务并没有结束（提交），A事务后来可能成功也可能失败。

比喻：A修改了源代码并且并没有提交到源代码系统，A直接通过QQ将代码发给了B，A后来取消了修改。

代码示例
复制代码 代码如下:
[TestMethod]
         public void 脏读_测试()
         {
             //前置条件
             using (var context = new TestEntities())
             {
                 Assert.AreEqual(1, context.Tables.Count());
             }

             var autoResetEvent = new AutoResetEvent(false);

             var transactionOptions1 = new TransactionOptions { IsolationLevel = IsolationLevel.ReadCommitted };
             var transactionOptions2 = new TransactionOptions { IsolationLevel = IsolationLevel.ReadUncommitted };

             using (var ts1 = new TransactionScope(TransactionScopeOption.Required, transactionOptions1))
             {
                 //添加数据
                 using (var context = new TestEntities())
                 {
                     context.Tables.Add(new Table() { Id = Guid.NewGuid(), Name = "段光伟" });
                     context.SaveChanges();
                 }

                 ThreadPool.QueueUserWorkItem(data =>
                 {
                     using (var ts2 = new TransactionScope(TransactionScopeOption.Required, transactionOptions2))
                     {
                         //脏读测试
                         using (var context = new TestEntities())
                         {
                             Assert.AreEqual(2, context.Tables.Count());
                         }
                     }

                     autoResetEvent.Set();
                 });

                 autoResetEvent.WaitOne();
             }

             //前置条件
             using (var context = new TestEntities())
             {
                 Assert.AreEqual(1, context.Tables.Count());
             }
         }

不可重复读

定义：A事务读取了两次数据，在这两次的读取过程中B事务修改了数据，A事务的这两次读取出来的数据不一样了（不可重复读）。

比喻：A在做源代码审查，在审查的过程中获取了两次源代码，在这两次获取期间B修改了源代码，B修改的很可能是A审查过的代码，而这部分代码可能不符合规范了。

代码示例
复制代码 代码如下:
[TestMethod]
         public void 不可重复读_测试()
         {
             var autoResetEvent = new AutoResetEvent(false);

             var transactionOptions1 = new TransactionOptions { IsolationLevel = IsolationLevel.ReadCommitted };
             var transactionOptions2 = new TransactionOptions { IsolationLevel = IsolationLevel.ReadCommitted };

             using (var ts1 = new TransactionScope(TransactionScopeOption.Required, transactionOptions1))
             {
                 //前置条件
                 using (var context = new TestEntities())
                 {
                     Assert.AreEqual("李妞妞", context.Tables.First().Name);
                 }

                 ThreadPool.QueueUserWorkItem(data =>
                 {
                     using (var ts2 = new TransactionScope(TransactionScopeOption.Required, transactionOptions2))
                     {
                         //修改数据
                         using (var context = new TestEntities())
                         {
                             context.Tables.First().Name = "段光伟";
                             context.SaveChanges();
                         }

                         ts2.Complete();   
                     }

                     autoResetEvent.Set();
                 });

                 autoResetEvent.WaitOne();

                 //不可重复读测试
                 using (var context = new TestEntities())
                 {
                     Assert.AreEqual("段光伟", context.Tables.First().Name);
                 }
             }
         }

幻读

定义：A事务读取了两次数据，在这两次的读取过程中B事务添加了数据，A事务的这两次读取出来的集合不一样了（幻读）。

比喻：A在统计文件数据，为了统计精确A统计了两次，在这两次的统计过程中B添加了一个文件，A发现这两次统计的数量不一样（幻读），A会感觉自己的脑袋有点头疼。

代码示例
复制代码 代码如下:
[TestMethod]
         public void 幻读_测试()
         {
             var autoResetEvent = new AutoResetEvent(false);

             var transactionOptions1 = new TransactionOptions { IsolationLevel = IsolationLevel.RepeatableRead };
             var transactionOptions2 = new TransactionOptions { IsolationLevel = IsolationLevel.ReadCommitted };

             using (var ts1 = new TransactionScope(TransactionScopeOption.Required, transactionOptions1))
             {
                 //前置条件
                 using (var context = new TestEntities())
                 {
                     Assert.AreEqual(1, context.Tables.Count());
                 }

                 ThreadPool.QueueUserWorkItem(data =>
                 {
                     using (var ts2 = new TransactionScope(TransactionScopeOption.Required, transactionOptions2))
                     {
                         //添加数据
                         using (var context = new TestEntities())
                         {
                             context.Tables.Add(new Table() { Id = Guid.NewGuid(), Name = "段光伟" });
                             context.SaveChanges();
                         }

                         ts2.Complete();
                     }

                     autoResetEvent.Set();
                 });

                 autoResetEvent.WaitOne();

                 //幻读测试
                 using (var context = new TestEntities())
                 {
                     Assert.AreEqual(2, context.Tables.Count());
                 }
             }
         }

四种隔离级别如何处理并发问题
  脏读 不可重复读 幻读 读未提交 允许 允许 允许 读已提交 不允许 允许 允许 可重复读 不允许 不允许 允许 串行化 不允许 不允许 不允许