EF Core 2.0中Transaction事务会对DbContext底层创建和关闭数据库连接的行为有所影响
2018-10-17 11:25
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数据库
我们先在SQL Server数据库中建立一个Book表：
CREATE TABLE [dbo].[Book]( [ID] [int] IDENTITY(1,1) NOT NULL, [BookName] [nvarchar](50) NULL, [BookDescription] [nvarchar](50) NULL, [ISBN] [nvarchar](20) NULL, [CreateTime] [datetime] NULL, CONSTRAINT [PK_Book] PRIMARY KEY CLUSTERED ( [ID] ASC )WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY] ) ON [PRIMARY] GO ALTER TABLE [dbo].[Book] ADD CONSTRAINT [DF_Book_CreateTime] DEFAULT (getdate()) FOR [CreateTime] GO
然后插入如下数据：
INSERT [dbo].[Book] ([BookName], [BookDescription], [ISBN]) VALUES (N'Chinese', N'Chinese', N'0001') GO INSERT [dbo].[Book] ([BookName], [BookDescription], [ISBN]) VALUES (N'English', N'English', N'0002') GO INSERT [dbo].[Book] ([BookName], [BookDescription], [ISBN]) VALUES (N'Japanese', N'Japanese', N'0003') GO INSERT [dbo].[Book] ([BookName], [BookDescription], [ISBN]) VALUES (N'Russian', N'Russian', N'0004') GO INSERT [dbo].[Book] ([BookName], [BookDescription], [ISBN]) VALUES (N'Italian', N'Italian', N'0005') GO
查询Book表的数据，如下图所示：
现在我们使用EF Core将Book表映射到.NET Core控制台项目中的Book实体上，Book实体如下所示：
using System;
using System.Collections.Generic;
namespace EFCoreDB.Entities
{
public partial class Book
{
public int Id { get; set; }
public string BookName { get; set; }
public string BookDescription { get; set; }
public string Isbn { get; set; }
public DateTime? CreateTime { get; set; }
}
}
不使用事务
然后我们在.NET Core控制台项目Program类的Main方法中，使用DbContext（也就是FinanceDigitalToolContext）读取BookName为"Chinese"的Book实体，然后使用DbContext.SaveChanges方法两次更改其BookDescription属性的值，再从数据库中将其查询出来显示，代码如下：
using EFCoreDB.Entities;
using System;
using System.Linq;
using System.Linq.Expressions;
namespace EFCoreDB
{
class Program
{
static void Main(string[] args)
{
using (FinanceDigitalToolContext dbContext = new FinanceDigitalToolContext())
{
Expression<Func<Book, bool>> bookExpression = b => b.BookName == "Chinese";//构造查询条件，来查询BookName为Chinese的Book
var chineseBook = dbContext.Book.First(bookExpression);//获取BookName为Chinese的Book实体chineseBook
chineseBook.BookDescription = "This is a Chinese book";//更改chineseBook的BookDescription属性
dbContext.SaveChanges();//用DbContext.SaveChanges方法保存更改到数据库
chineseBook.BookDescription = "This is a very good Chinese book";//再次更改chineseBook的BookDescription属性
dbContext.SaveChanges();//用DbContext.SaveChanges方法保存更改到数据库
chineseBook = dbContext.Book.First(bookExpression);//重新获取BookName为Chinese的Book实体chineseBook
//显示当前chineseBook的当前BookDescription属性值
Console.WriteLine(chineseBook.BookName + " book has description: \"" + chineseBook.BookDescription + "\"");
}
Console.WriteLine("Press key to quit....");
Console.ReadLine();
}
}
}
执行上面的代码，我们使用EF Core的日志功能，输出每次DbContext访问数据库时的后台日志信息：
首先在执行
var chineseBook = dbContext.Book.First(bookExpression);//获取BookName为Chinese的Book实体chineseBook
时，EF Core的日志如下所示：
=============================== EF Core log started =============================== Opening connection to database 'FinanceDigitalTool' on server 'CNGDCAAITSQL01'. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== Opened connection to database 'FinanceDigitalTool' on server 'CNGDCAAITSQL01'. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== Executing DbCommand [Parameters=[], CommandType='Text', CommandTimeout='30'] SELECT TOP(1) [b].[ID], [b].[BookDescription], [b].[BookName], [b].[CreateTime], [b].[ISBN] FROM [Book] AS [b] WHERE [b].[BookName] = N'Chinese' =============================== EF Core log finished =============================== =============================== EF Core log started =============================== Executed DbCommand (129ms) [Parameters=[], CommandType='Text', CommandTimeout='30'] SELECT TOP(1) [b].[ID], [b].[BookDescription], [b].[BookName], [b].[CreateTime], [b].[ISBN] FROM [Book] AS [b] WHERE [b].[BookName] = N'Chinese' =============================== EF Core log finished =============================== =============================== EF Core log started =============================== A data reader was disposed. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== Closing connection to database 'FinanceDigitalTool' on server 'CNGDCAAITSQL01'. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== Closed connection to database 'FinanceDigitalTool' on server 'CNGDCAAITSQL01'. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== Context 'Book' started tracking 'FinanceDigitalToolContext' entity. Consider using 'DbContextOptionsBuilder.EnableSensitiveDataLogging' to see key values. =============================== EF Core log finished ===============================
从日志中我们可以看出来，dbContext.Book.First(bookExpression)在数据库中开启了一个数据库连接，并使用SQL语句做了查询，然后关闭了数据库连接。
然后在执行下面的代码
chineseBook.BookDescription = "This is a Chinese book";//更改chineseBook的BookDescription属性 dbContext.SaveChanges();//用DbContext.SaveChanges方法保存更改到数据库
时，EF Core的日志如下所示：
=============================== EF Core log started =============================== SaveChanges starting for 'FinanceDigitalToolContext'. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== DetectChanges starting for 'FinanceDigitalToolContext'. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== Unchanged 'Book.BookDescription' detected as changed and will be marked as modified. Consider using 'DbContextOptionsBuilder.EnableSensitiveDataLogging' to see property values. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== An 'Book' entity tracked by 'FinanceDigitalToolContext' changed from 'Unchanged' to 'Modified'. Consider using 'DbContextOptionsBuilder.EnableSensitiveDataLogging' to see key values. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== DetectChanges completed for 'FinanceDigitalToolContext'. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== Opening connection to database 'FinanceDigitalTool' on server 'CNGDCAAITSQL01'. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== Opened connection to database 'FinanceDigitalTool' on server 'CNGDCAAITSQL01'. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== Beginning transaction with isolation level 'ReadCommitted'. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== Executing DbCommand [Parameters=[@p1='?' (DbType = Int32), @p0='?' (Size = 50)], CommandType='Text', CommandTimeout='30'] SET NOCOUNT ON; UPDATE [Book] SET [BookDescription] = @p0 WHERE [ID] = @p1; SELECT @@ROWCOUNT; =============================== EF Core log finished =============================== =============================== EF Core log started =============================== Executed DbCommand (86ms) [Parameters=[@p1='?' (DbType = Int32), @p0='?' (Size = 50)], CommandType='Text', CommandTimeout='30'] SET NOCOUNT ON; UPDATE [Book] SET [BookDescription] = @p0 WHERE [ID] = @p1; SELECT @@ROWCOUNT; =============================== EF Core log finished =============================== =============================== EF Core log started =============================== A data reader was disposed. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== Committing transaction. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== Closing connection to database 'FinanceDigitalTool' on server 'CNGDCAAITSQL01'. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== Closed connection to database 'FinanceDigitalTool' on server 'CNGDCAAITSQL01'. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== Disposing transaction. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== An 'Book' entity tracked by 'FinanceDigitalToolContext' changed from 'Modified' to 'Unchanged'. Consider using 'DbContextOptionsBuilder.EnableSensitiveDataLogging' to see key values. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== SaveChanges completed for 'FinanceDigitalToolContext' with 1 entities written to the database. =============================== EF Core log finished ===============================
从日志中我们可以看出来在执行dbContext.SaveChanges方法时，EF Core在数据库中开启了一个数据库连接，并使用SQL语句做了数据库更改，然后关闭了数据库连接。然后chineseBook这个Book实体的EntityState从Modified变为了Unchanged。
然后执行代码
chineseBook.BookDescription = "This is a very good Chinese book";//再次更改chineseBook的BookDescription属性 dbContext.SaveChanges();//用DbContext.SaveChanges方法保存更改到数据库
时，EF Core的日志如下所示：
=============================== EF Core log started =============================== SaveChanges starting for 'FinanceDigitalToolContext'. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== DetectChanges starting for 'FinanceDigitalToolContext'. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== Unchanged 'Book.BookDescription' detected as changed and will be marked as modified. Consider using 'DbContextOptionsBuilder.EnableSensitiveDataLogging' to see property values. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== An 'Book' entity tracked by 'FinanceDigitalToolContext' changed from 'Unchanged' to 'Modified'. Consider using 'DbContextOptionsBuilder.EnableSensitiveDataLogging' to see key values. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== DetectChanges completed for 'FinanceDigitalToolContext'. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== Opening connection to database 'FinanceDigitalTool' on server 'CNGDCAAITSQL01'. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== Opened connection to database 'FinanceDigitalTool' on server 'CNGDCAAITSQL01'. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== Beginning transaction with isolation level 'ReadCommitted'. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== Executing DbCommand [Parameters=[@p1='?' (DbType = Int32), @p0='?' (Size = 50)], CommandType='Text', CommandTimeout='30'] SET NOCOUNT ON; UPDATE [Book] SET [BookDescription] = @p0 WHERE [ID] = @p1; SELECT @@ROWCOUNT; =============================== EF Core log finished =============================== =============================== EF Core log started =============================== Executed DbCommand (19ms) [Parameters=[@p1='?' (DbType = Int32), @p0='?' (Size = 50)], CommandType='Text', CommandTimeout='30'] SET NOCOUNT ON; UPDATE [Book] SET [BookDescription] = @p0 WHERE [ID] = @p1; SELECT @@ROWCOUNT; =============================== EF Core log finished =============================== =============================== EF Core log started =============================== A data reader was disposed. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== Committing transaction. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== Closing connection to database 'FinanceDigitalTool' on server 'CNGDCAAITSQL01'. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== Closed connection to database 'FinanceDigitalTool' on server 'CNGDCAAITSQL01'. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== Disposing transaction. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== An 'Book' entity tracked by 'FinanceDigitalToolContext' changed from 'Modified' to 'Unchanged'. Consider using 'DbContextOptionsBuilder.EnableSensitiveDataLogging' to see key values. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== SaveChanges completed for 'FinanceDigitalToolContext' with 1 entities written to the database. =============================== EF Core log finished ===============================
同样从日志中我们可以看出来在执行dbContext.SaveChanges方法时，EF Core在数据库中开启了一个数据库连接，并使用SQL语句做了数据库更改，然后关闭了数据库连接。然后chineseBook这个Book实体的EntityState从Modified变为了Unchanged。
然后执行代码
chineseBook = dbContext.Book.First(bookExpression);//重新获取BookName为Chinese的Book实体chineseBook
时，EF Core的日志如下所示：
=============================== EF Core log started =============================== Opening connection to database 'FinanceDigitalTool' on server 'CNGDCAAITSQL01'. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== Opened connection to database 'FinanceDigitalTool' on server 'CNGDCAAITSQL01'. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== Executing DbCommand [Parameters=[], CommandType='Text', CommandTimeout='30'] SELECT TOP(1) [b].[ID], [b].[BookDescription], [b].[BookName], [b].[CreateTime], [b].[ISBN] FROM [Book] AS [b] WHERE [b].[BookName] = N'Chinese' =============================== EF Core log finished =============================== =============================== EF Core log started =============================== Executed DbCommand (21ms) [Parameters=[], CommandType='Text', CommandTimeout='30'] SELECT TOP(1) [b].[ID], [b].[BookDescription], [b].[BookName], [b].[CreateTime], [b].[ISBN] FROM [Book] AS [b] WHERE [b].[BookName] = N'Chinese' =============================== EF Core log finished =============================== =============================== EF Core log started =============================== A data reader was disposed. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== Closing connection to database 'FinanceDigitalTool' on server 'CNGDCAAITSQL01'. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== Closed connection to database 'FinanceDigitalTool' on server 'CNGDCAAITSQL01'. =============================== EF Core log finished ===============================
从日志中我们可以看出来，dbContext.Book.First(bookExpression)在数据库中还是开启了一个数据库连接，并使用SQL语句做了查询，然后关闭了数据库连接。
总结下来，上面的代码和日志发生的事情如下：
- dbContext.Book.First(bookExpression)开启和关闭了一个数据库连接，查询BookName为Chinese的Book实体chineseBook
- 第一个dbContext.SaveChanges()开启和关闭了一个数据库连接，更改chineseBook的BookDescription属性值"This is a Chinese book"到数据库
- 第二个dbContext.SaveChanges()开启和关闭了一个数据库连接，更改chineseBook的BookDescription属性值"This is a very good Chinese book"到数据库
- dbContext.Book.First(bookExpression)开启和关闭了一个数据库连接，重新查询BookName为Chinese的Book实体chineseBook
所以综上所述DbContext一共开启了和关闭了四个数据库连接。
使用事务
现在我们更改Program类Main方法中的代码，将两次DbContext.SaveChanges方法的调用都放在一个TransactionScope事务范围中，所以现在两次DbContext.SaveChanges方法提交的SQL都会在同一个数据库事务中，代码如下所示：
using EFCoreDB.Entities;
using System;
using System.Linq;
using System.Linq.Expressions;
using System.Transactions;
namespace EFCoreDB
{
class Program
{
static void Main(string[] args)
{
using (FinanceDigitalToolContext dbContext = new FinanceDigitalToolContext())
{
Expression<Func<Book, bool>> bookExpression = b => b.BookName == "Chinese";//构造查询条件，来查询BookName为Chinese的Book
var chineseBook = dbContext.Book.First(bookExpression);//获取BookName为Chinese的Book实体chineseBook
//使用TransactionScope事务范围来开启一个数据库事务
using (TransactionScope transactionScope = new TransactionScope(TransactionScopeOption.RequiresNew))
{
chineseBook.BookDescription = "This is a Chinese book";//更改chineseBook的BookDescription属性
dbContext.SaveChanges();//用DbContext.SaveChanges方法保存更改到数据库，由于TransactionScope事务范围的存在，所以DbContext.SaveChanges方法提交的SQL语句都存在于TransactionScope的事务当中
chineseBook.BookDescription = "This is a very good Chinese book";//再次更改chineseBook的BookDescription属性
dbContext.SaveChanges();//用DbContext.SaveChanges方法保存更改到数据库，由于TransactionScope事务范围的存在，所以DbContext.SaveChanges方法提交的SQL语句都存在于TransactionScope的事务当中
transactionScope.Complete();//提交TransactionScope事务范围中的SQL语句到数据库，数据库事务结束
}
chineseBook = dbContext.Book.First(bookExpression);//重新获取BookName为Chinese的Book实体chineseBook
//显示当前chineseBook的当前BookDescription属性值
Console.WriteLine(chineseBook.BookName + " book has description: \"" + chineseBook.BookDescription + "\"");
}
Console.WriteLine("Press key to quit....");
Console.ReadLine();
}
}
}
执行上面的代码，我们还是使用EF Core的日志功能，输出每次DbContext访问数据库时的后台日志信息：
首先执行
var chineseBook = dbContext.Book.First(bookExpression);//获取BookName为Chinese的Book实体chineseBook
时，EF Core的日志如下所示：
=============================== EF Core log started =============================== Opening connection to database 'FinanceDigitalTool' on server 'CNGDCAAITSQL01'. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== Opened connection to database 'FinanceDigitalTool' on server 'CNGDCAAITSQL01'. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== Executing DbCommand [Parameters=[], CommandType='Text', CommandTimeout='30'] SELECT TOP(1) [b].[ID], [b].[BookDescription], [b].[BookName], [b].[CreateTime], [b].[ISBN] FROM [Book] AS [b] WHERE [b].[BookName] = N'Chinese' =============================== EF Core log finished =============================== =============================== EF Core log started =============================== Executed DbCommand (139ms) [Parameters=[], CommandType='Text', CommandTimeout='30'] SELECT TOP(1) [b].[ID], [b].[BookDescription], [b].[BookName], [b].[CreateTime], [b].[ISBN] FROM [Book] AS [b] WHERE [b].[BookName] = N'Chinese' =============================== EF Core log finished =============================== =============================== EF Core log started =============================== A data reader was disposed. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== Closing connection to database 'FinanceDigitalTool' on server 'CNGDCAAITSQL01'. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== Closed connection to database 'FinanceDigitalTool' on server 'CNGDCAAITSQL01'. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== Context 'Book' started tracking 'FinanceDigitalToolContext' entity. Consider using 'DbContextOptionsBuilder.EnableSensitiveDataLogging' to see key values. =============================== EF Core log finished ===============================
从日志中我们可以看出来，dbContext.Book.First(bookExpression)在数据库中开启了一个数据库连接，并使用SQL语句做了查询，然后关闭了数据库连接。
然后执行
using (TransactionScope transactionScope = new TransactionScope(TransactionScopeOption.RequiresNew))
{
TransactionScope的事务范围开始，此时数据库事务已经开始，EF Core没有输出日志
然后执行
chineseBook.BookDescription = "This is a Chinese book";//更改chineseBook的BookDescription属性 dbContext.SaveChanges();//用DbContext.SaveChanges方法保存更改到数据库，由于TransactionScope事务范围的存在，所以DbContext.SaveChanges方法提交的SQL语句都存在于TransactionScope的事务当中
时，EF Core的日志如下所示：
=============================== EF Core log started =============================== SaveChanges starting for 'FinanceDigitalToolContext'. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== DetectChanges starting for 'FinanceDigitalToolContext'. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== Unchanged 'Book.BookDescription' detected as changed and will be marked as modified. Consider using 'DbContextOptionsBuilder.EnableSensitiveDataLogging' to see property values. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== An 'Book' entity tracked by 'FinanceDigitalToolContext' changed from 'Unchanged' to 'Modified'. Consider using 'DbContextOptionsBuilder.EnableSensitiveDataLogging' to see key values. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== DetectChanges completed for 'FinanceDigitalToolContext'. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== Opening connection to database 'FinanceDigitalTool' on server 'CNGDCAAITSQL01'. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== Opened connection to database 'FinanceDigitalTool' on server 'CNGDCAAITSQL01'. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== Enlisted in an ambient transaction with isolation level 'Serializable'. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== Executing DbCommand [Parameters=[@p1='?' (DbType = Int32), @p0='?' (Size = 50)], CommandType='Text', CommandTimeout='30'] SET NOCOUNT ON; UPDATE [Book] SET [BookDescription] = @p0 WHERE [ID] = @p1; SELECT @@ROWCOUNT; =============================== EF Core log finished =============================== =============================== EF Core log started =============================== Executed DbCommand (79ms) [Parameters=[@p1='?' (DbType = Int32), @p0='?' (Size = 50)], CommandType='Text', CommandTimeout='30'] SET NOCOUNT ON; UPDATE [Book] SET [BookDescription] = @p0 WHERE [ID] = @p1; SELECT @@ROWCOUNT; =============================== EF Core log finished =============================== =============================== EF Core log started =============================== A data reader was disposed. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== An 'Book' entity tracked by 'FinanceDigitalToolContext' changed from 'Modified' to 'Unchanged'. Consider using 'DbContextOptionsBuilder.EnableSensitiveDataLogging' to see key values. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== SaveChanges completed for 'FinanceDigitalToolContext' with 1 entities written to the database. =============================== EF Core log finished ===============================
从日志中我们可以看出来在执行dbContext.SaveChanges方法时，EF Core在数据库中开启了一个数据库连接，并使用SQL语句做了数据库更改，但是没有关闭数据库连接。然后chineseBook这个Book实体的EntityState从Modified变为了Unchanged。
然后执行
chineseBook.BookDescription = "This is a very good Chinese book";//再次更改chineseBook的BookDescription属性 dbContext.SaveChanges();//用DbContext.SaveChanges方法保存更改到数据库，由于TransactionScope事务范围的存在，所以DbContext.SaveChanges方法提交的SQL语句都存在于TransactionScope的事务当中
时，EF Core的日志如下所示：
=============================== EF Core log started =============================== SaveChanges starting for 'FinanceDigitalToolContext'. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== DetectChanges starting for 'FinanceDigitalToolContext'. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== Unchanged 'Book.BookDescription' detected as changed and will be marked as modified. Consider using 'DbContextOptionsBuilder.EnableSensitiveDataLogging' to see property values. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== An 'Book' entity tracked by 'FinanceDigitalToolContext' changed from 'Unchanged' to 'Modified'. Consider using 'DbContextOptionsBuilder.EnableSensitiveDataLogging' to see key values. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== DetectChanges completed for 'FinanceDigitalToolContext'. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== Executing DbCommand [Parameters=[@p1='?' (DbType = Int32), @p0='?' (Size = 50)], CommandType='Text', CommandTimeout='30'] SET NOCOUNT ON; UPDATE [Book] SET [BookDescription] = @p0 WHERE [ID] = @p1; SELECT @@ROWCOUNT; =============================== EF Core log finished =============================== =============================== EF Core log started =============================== Executed DbCommand (22ms) [Parameters=[@p1='?' (DbType = Int32), @p0='?' (Size = 50)], CommandType='Text', CommandTimeout='30'] SET NOCOUNT ON; UPDATE [Book] SET [BookDescription] = @p0 WHERE [ID] = @p1; SELECT @@ROWCOUNT; =============================== EF Core log finished =============================== =============================== EF Core log started =============================== A data reader was disposed. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== An 'Book' entity tracked by 'FinanceDigitalToolContext' changed from 'Modified' to 'Unchanged'. Consider using 'DbContextOptionsBuilder.EnableSensitiveDataLogging' to see key values. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== SaveChanges completed for 'FinanceDigitalToolContext' with 1 entities written to the database. =============================== EF Core log finished ===============================
从日志中我们可以看出来在执行dbContext.SaveChanges方法时，EF Core在数据库中并没有开启新的数据库连接，而是沿用了上一个DbContext.SaveChanges方法开启的数据库连接来提交SQL语句到数据库。然后chineseBook这个Book实体的EntityState从Modified变为了Unchanged。
然后执行
transactionScope.Complete();//提交TransactionScope事务范围中的SQL语句到数据库，数据库事务结束 }
提交TransactionScope的事务到数据库，此时数据库事务结束，TransactionScope的事务范围也结束，EF Core没有输出日志
然后执行
chineseBook = dbContext.Book.First(bookExpression);//重新获取BookName为Chinese的Book实体chineseBook
时，EF Core的日志如下所示：
=============================== EF Core log started =============================== Executing DbCommand [Parameters=[], CommandType='Text', CommandTimeout='30'] SELECT TOP(1) [b].[ID], [b].[BookDescription], [b].[BookName], [b].[CreateTime], [b].[ISBN] FROM [Book] AS [b] WHERE [b].[BookName] = N'Chinese' =============================== EF Core log finished =============================== =============================== EF Core log started =============================== Executed DbCommand (24ms) [Parameters=[], CommandType='Text', CommandTimeout='30'] SELECT TOP(1) [b].[ID], [b].[BookDescription], [b].[BookName], [b].[CreateTime], [b].[ISBN] FROM [Book] AS [b] WHERE [b].[BookName] = N'Chinese' =============================== EF Core log finished =============================== =============================== EF Core log started =============================== A data reader was disposed. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== Closing connection to database 'FinanceDigitalTool' on server 'CNGDCAAITSQL01'. =============================== EF Core log finished =============================== =============================== EF Core log started =============================== Closed connection to database 'FinanceDigitalTool' on server 'CNGDCAAITSQL01'. =============================== EF Core log finished ===============================
从日志中我们可以看出来，dbContext.Book.First(bookExpression)在数据库中也没有开启新的数据库连接，而是继续沿用了第一个DbContext.SaveChanges方法开启的数据库连接，使用SQL语句做了查询，之后关闭了第一个DbContext.SaveChanges方法开启的数据库连接，所以可以看到第一个DbContext.SaveChanges方法开启的数据库连接，现在才被关闭，这和不使用事务时是完全不一样的。
总结下来，使用事务后的代码和日志发生的事情如下：
- dbContext.Book.First(bookExpression)开启和关闭了一个数据库连接，查询BookName为Chinese的Book实体chineseBook
- using (TransactionScope transactionScope = new TransactionScope(TransactionScopeOption.RequiresNew)){开启了TransactionScope事务范围，数据库事务开始
- 第一个dbContext.SaveChanges()开启一个数据库连接，但是没有关闭数据库连接，更改chineseBook的BookDescription属性值"This is a Chinese book"到数据库
- 第二个dbContext.SaveChanges()没有开启新的数据库连接，而是沿用了上一个DbContext.SaveChanges方法开启的数据库连接，更改chineseBook的BookDescription属性值"This is a very good Chinese book"到数据库
- transactionScope.Complete()提交TransactionScope事务范围的数据库事务到数据库，数据库事务结束
- }TransactionScope事务范围结束
- dbContext.Book.First(bookExpression)没有开启新的数据库连接，而是沿用了第一个DbContext.SaveChanges方法开启的数据库连接，重新查询BookName为Chinese的Book实体chineseBook，最后关闭了第一个DbContext.SaveChanges方法开启的数据库连接。
所以综上所述DbContext这次一共只开启和关闭了两个数据库连接，并且第二个数据库连接执行了三次数据库操作(两次DbContext.SaveChanges更改数据，一次Book.First查询数据)才被关闭。
由此我们可以看到EF Core在处于事务中时，会优化底层开启和关闭数据库连接的机制，因为EF Core觉得两次DbContext.SaveChanges方法提交的SQL语句既然都在同一个事务中，所以就没有必要每次都开启和关闭一个数据库连接，而是沿用了第一次DbContext.SaveChanges方法开启的数据库连接，所以这和没有使用事务的时候是完全不一样的。
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