Linq to sql（二）：DataContext与实体（二）

强类型DataContext

public partial class NorthwindDataContext : DataContext

{

public Table<Customer> Customers;

public NorthwindDataContext(IDbConnection connection) : base(connection) { }

public NorthwindDataContext(string connection) : base(connection) { }

}

强类型数据上下文使代码更简洁：

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

GridView1.DataSource = from c in ctx.Customers where c.CustomerID.StartsWith("A") select new { 顾客ID = c.CustomerID, 顾客名 = c.Name, 城市 = c.City };

GridView1.DataBind();

DataContext其实封装了很多实用的功能，下面一一介绍。

日志功能

using System.IO;

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

StreamWriter sw = new StreamWriter(Server.MapPath("log.txt"), true); // Append

ctx.Log = sw;

GridView1.DataSource = from c in ctx.Customers where c.CustomerID.StartsWith("A") select new { 顾客ID = c.CustomerID, 顾客名 = c.Name, 城市 = c.City };

GridView1.DataBind();

sw.Close();

运行程序后在网站所在目录生成了log.txt，每次查询都会把诸如下面的日志追加到文本文件中：

SELECT [t0].[CustomerID], [t0].[ContactName], [t0].[City]

FROM [Customers] AS [t0]

WHERE [t0].[CustomerID] LIKE @p0

-- @p0: Input String (Size = 2; Prec = 0; Scale = 0) [A%]

-- Context: SqlProvider(Sql2005) Model: AttributedMetaModel Build: 3.5.20706.1

应该说这样的日志对于调试程序是非常有帮助的。

探究查询

using System.Data.Common;

using System.Collections.Generic;

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

var select = from c in ctx.Customers where c.CustomerID.StartsWith("A") select new { 顾客ID = c.CustomerID, 顾客名 = c.Name, 城市 = c.City };

DbCommand cmd = ctx.GetCommand(select);

Response.Write(cmd.CommandText + "<br/>");

foreach (DbParameter parm in cmd.Parameters)

Response.Write(string.Format("参数名:{0},参数值:{1}<br/>", parm.ParameterName, parm.Value));

Customer customer = ctx.Customers.First();

customer.Name = "zhuye";

IList<object> queryText = ctx.GetChangeSet().ModifiedEntities;

Response.Write(((Customer)queryText[0]).Name);

在这里，我们通过DataContext的GetCommand方法获取了查询对应的DbCommand，

并且输出了CommandText和所有的DbParameter。之后，我们又通过GetChangeSet方法获取了修改后的实体，

并输出了修改内容。