一步一步学Linq to sql（九）：其它补充

外部映射文件

我们可以使用sqlmetal命令行工具来生成外部映射文件，使用方法如下：

1、开始菜单－》 VS2008 －》VS工具－》VS2008命令行提示

2、输入命令：

D:\Program Files\Microsoft Visual Studio 9.0\VC>sqlmetal /conn:server=xxx;

database=Northwind;uid=xxx;pwd=xxx /map:c:\northwind.map /code:c:\northwind.cs

3、这样，我们就可以在C盘下得到一个xml映射文件和C#的实体类代码

4、把.cs文件添加到项目中来（放到App_Code目录），然后使用下面的代码加载映射文件：

String path = @"C:\Northwind.map";

XmlMappingSource xms = XmlMappingSource.FromXml(File.ReadAllText(path));

Northwind ctx = new Northwind("server=xxx;database=Northwind;uid=xxx;pwd=xxx", xms);

5、现在就可以照常进行其它工作了。使用sqlmetal可以很方便的同步数据库与实体和映射文件。每次修改数据库结构，从dbml设计器上删除表、存储过程然后再重新添加也是很麻烦的事情。

处理空值

var count = (from c in ctx.Customers where c.Region == null select c).Count();

Response.Write(count + " ");

var query = from emp in ctx.Employees select emp.ReportsTo;

foreach (Nullable<int> r in query)

{

Response.Write(r.HasValue ? r.Value.ToString() + " " : "没有 ");

}

代码执行后捕获到下面的SQL被执行：

SELECT COUNT(*) AS [value]

FROM [dbo].[Customers] AS [t0]

WHERE [t0].[Region] IS NULL

SELECT [t0].[ReportsTo]

FROM [dbo].[Employees] AS [t0]

已编译查询

对于一些在项目中经常被用到的查询可以封装成已编译查询，这样就能提高执行效率：

static class Queries

{

public static Func<NorthwindDataContext, string, IQueryable<Customer>>

CustomersByCity = CompiledQuery.Compile((NorthwindDataContext ctx, string city) => from c in ctx.Customers where c.City == city select c);

}

调用查询方式如下：

GridView1.DataSource = Queries.CustomersByCity(ctx, "London");

GridView1.DataBind();

获取一些信息

var query = from c in ctx.Customers select c;

Response.Write("Provider类型：" + ctx.Mapping.ProviderType + " ");

Response.Write("数据库：" + ctx.Mapping.DatabaseName + " ");

Response.Write("表：" + ctx.Mapping.GetTable(typeof(Customer)).TableName + " ");

Response.Write("表达式：" + query.Expression.ToString() + " ");

Response.Write("sql：" + query.Provider.ToString() + " ");

上面的代码执行结果如下：

Provider类型：System.Data.Linq.SqlClient.SqlProvider
数据库：Northwind
表：dbo.Customers
表达式：Table(Customer).Select(c => c)
sql：SELECT [t0].[CustomerID], [t0].[CompanyName], [t0].[ContactName], [t0].[ContactTitle], [t0].[Address], [t0].[City], [t0].[Region], [t0].[PostalCode], [t0].[Country], [t0].[Phone], [t0].[Fax] FROM [dbo].[Customers] AS [t0]窗体顶端

窗体底端

撤销提交

var customer = ctx.Customers.Single(c => c.CustomerID == "AROUT");

customer.ContactName = "zhuye";

customer.Country = "Shanghai";

Response.Write(string.Format("Name:{0},Country:{1} ", customer.ContactName, customer.Country));

customer = ctx.Customers.GetOriginalEntityState(customer);

Response.Write(string.Format("Name:{0},Country:{1} ", customer.ContactName, customer.Country));

上面的代码执行效果如下：

Name:zhuye,Country:Shanghai
Name:Thomas Hardy,Country:UK

批量操作

下面的代码会导致提交N次DELETE操作：

var query = from c in ctx.Customers select c;

ctx.Customers.RemoveAll(query);

ctx.SubmitChanges();

应该使用sql语句进行批操作：

string sql = String.Format("delete from {0}", ctx.Mapping.GetTable(typeof(Customer)).TableName);

ctx.ExecuteCommand(sql);

对于批量更新操作也是同样道理。

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