Entity Framework 程序设计入门二对数据进行CRUD操作和查询

前一篇文章介绍了应用LLBL Gen生成Entity Framework所需要的类型定义，用一行代码完成数据资料的读取，

《LLBL Gen + Entity Framework 程序设计入门》。如果已经对Entity Framework产生兴趣，则可以借助于这一篇，来学习Entity Framework如何对资料库进行操作。

连接字符串的写法 Database Connection String

string con ="name = ConnectionString.SQL Server (SqlClient)" ;

其中的”ConnectionString.SQL Server (SqlClient)”是配置文件中的连接字符串名称，App.config的文件内容如下

<?xml version="1.0" encoding="utf-8" ?>

<configuration>

    <connectionStrings>

        <!-- please adjust the connection string embedded in the element below to target the proper catalog / server using the proper user / password combination -->

        <add name="ConnectionString.SQL Server (SqlClient)" connectionString="metadata=res://*/Entity35.csdl|res://*/Entity35.ssdl|res://*/Entity35.msl;provider=System.Data.SqlClient;provider connection string=&quot;data source=.\sqlexpress;initial catalog=Framework;integrated security=SSPI;persist security info=False;packet size=4096&quot;" providerName="System.Data.EntityClient" />

    </connectionStrings>

</configuration>

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或者直接在代码中嵌入上面的连接字符串的值

string conn="ConnectionString.SQL Server (SqlClient)" connectionString="metadata=res://*/Entity35.csdl|res://*/Entity35.ssdl|res://*/Entity35.msl;provider=System.Data.SqlClient;provider connection string=&quot;data source=.\sqlexpress;initial catalog=Framework;integrated security=SSPI;persist security info=False;packet size=4096&quot;" providerName="System.Data.EntityClient";

如果担心字符串的方法会报错或是需要动态建立连接字符串，可以构造EntityConnectionStringBuilder类型来生成连接字符串。

// Specify the provider name, server and database.

string providerName = "System.Data.SqlClient";

string serverName = ".";

string databaseName = "AdventureWorks";

// Initialize the connection string builder for the underlying provider.

SqlConnectionStringBuilder sqlBuilder =new SqlConnectionStringBuilder();

// Set the properties for the data source.

sqlBuilder.DataSource = serverName;

sqlBuilder.InitialCatalog = databaseName;

sqlBuilder.IntegratedSecurity = true;

// Build the SqlConnection connection string.

string providerString = sqlBuilder.ToString();

// Initialize the EntityConnectionStringBuilder.

EntityConnectionStringBuilder entityBuilder =new EntityConnectionStringBuilder();

//Set the provider name.

entityBuilder.Provider = providerName;

// Set the provider-specific connection string.

entityBuilder.ProviderConnectionString = providerString;

// Set the Metadata location.

entityBuilder.Metadata = @"res://*/AdventureWorksModel.csdl|

                            res://*/AdventureWorksModel.ssdl|

                            res://*/AdventureWorksModel.msl";

Console.WriteLine(entityBuilder.ToString());

创建新数据 Create Record

创建一笔新的配置项数据，把它保存到数据库中，有二种方法：AddToX或是AddObject，然后调用SaveChanges

Entity35DataContext dataContext = new Entity35DataContext();

Configuration configuration = new Configuration();

configuration.MasterKey = "James";

configuration.Description = "My Profile";

// save configuration

dataContext.AddToConfigurations(configuration);.

dataContext.SaveChanges();

//save configruation

dataContext.AddObject("Configuration",configuration);

dataContext.SaveChanges();

注意必须输入的数据库字段(NOT NULL)必须要有值，否则会抛出数据库异常。

更新数据 Update Record

Configuration configuration.....

configuration.Description="New Profile";

dataContext.SaveChanges();

在更改实体的映射属性之后，调用SaveChanges即可将修改后的属性值保存到数据库中。

如果要更新的属性所映射的是数据表的主键，不应该直接更新主键，这样会报错，而应该先删除，再创建一笔新的记录。

删除数据 Delete Record

Configuration configuraion.....

dataContext.DeleteObject(configuration);

被删除的对象configuration，可以是已经读取了映射的表的实体，也可以没有映射过值的对象，构造一个新的内存对象，再把它传给DeleteObject方法。我认为这个方法的内部实现应该只需要去查找这个实体的主键，生成相应的SQL语句即可。

事务 Transaction

如果需要对多个对象进行操作，事务支持可以用dataContext的Connection对象，代码例子如下

Entity35DataContext dataContext = null;

System.Data.Common.DbTransaction tran = null;

try

{

                dataContext = new Entity35DataContext();

                dataContext.Connection.Open();

                tran = dataContext.Connection.BeginTransaction();

                Configuration cst = dataContext.Configurations.FirstOrDefault(cc => cc.MasterKey == "James");

                cst.Description = "China ";

                dataContext.SaveChanges();

                tran.Commit();

}

catch (Exception ex)

{

                if (tran != null) tran.Rollback();

                throw ex;

}

finally

{

                if (dataContext != null && dataContext.Connection.State != ConnectionState.Closed)

                    dataContext.Connection.Close();

}

应用Repository 模式 Apply Repository Pattern

将上面的CRUD操作应用Repository模式封装，以隔离变化。

public interface IConfigurationRepository

 {

        // Query Methods

        IQueryable<Configuration> FindAllConfiguration();

        Configuration GetConfiguration(string key);

        // Insert/Delete

        void Add(Configuration configuration);

        void Delete(Configuration configuration);

        // Persistence

        void Save();

}

可以考虑借助于Code Smith代码生成工具，直接生成这个接口模板，它的实现代码也非常容易。

查询 Entity Query

Entity Framework有二种方法写数据查询，一种是上面展示的Linq to Entity，它是延迟执行的。另一种是Entity SQL。这种写法维护起来相对复杂一些，不过性能会好一些。比如一个表有100个字段，Linq to Entity会将这100个字段的值会部读出来，映射到内存对象中，当数据量很大的时候，这种写法会产生性能瓶颈。有时候，我们也只需要读取表的若干个字段，而不是全部，所以有必要研究一下Entity SQL的写法。

对象查询 VALUES

SELECT VALUE it FROM FrameworkEntities.Configuration  as it

带参数查询

SELECT VALUE it FROM FrameworkEntities.Configuration  as it

where it.MasterKey=@MasterKey

SELECT count(1) FROM FrameworkEntities.Configuration

SELECT top(10) it.MasterKey FROM FrameworkEntities.Configuration as it

SELECT VALUES row(it.MasterKey,it.Description) FROM FrameworkEntities.Configuration as it

这种写法会构造一个匿名类型，只读取指定的字段值，返回匿名对象。

Key 主键

SELECT VALUE key(it)  FROM FrameworkEntities.Configuration as it