外部映射文件
我們可以使用 sqlmetal 命令行工具來生成外部映射文件,使用方法如下:
1 、開始菜單 -》 VS2008 -》 VS 工具 -》 VS2008 命令行提示
2 、輸入命令:
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 、這樣,我們就可以在 C 盤下得到一個 xml 映射文件和 C# 的實體類代碼
4 、把 .cs 文件添加到項目中來(放到 App_Code 目錄),然後使用下面的代碼加載映射文件:
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 、現在就可以照常進行其它工作了。使用 sqlmetal 可以很方便的同步數據庫與實體和映射文件。每次修改數據庫結構,從 dbml 設計器上刪除表、存儲過程然後再重新添加也是很麻煩的事情。
處理空值
var count = (from c in ctx.Customers where c.Region == null select c).Count(); Response.Write(count + "<br/>" ); var query = from emp in ctx.Employees select emp.ReportsTo; foreach (Nullable <int > r in query) { Response.Write(r.HasValue ? r.Value.ToString() + "<br/>" : " 沒有 <br/>" ); } |
代碼執行後捕獲到下面的 SQL 被執行:
SELECT COUNT(*) AS [value] FROM [dbo].[Customers] AS [t0] WHERE [t0].[Region] IS NULL
SELECT [t0].[ReportsTo] FROM [dbo].[Employees] AS [t0] |
已編譯查詢
對於一些在項目中經常被用到的查詢可以封裝成已編譯查詢,這樣就能提高執行效率:
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); } |
調用查詢方式如下:
GridView1.DataSource = Queries .CustomersByCity(ctx, "London" ); GridView1.DataBind(); |
獲取一些信息
var query = from c in ctx.Customers select c; Response.Write("Provider 類型: " + ctx.Mapping.ProviderType + "<br/>" ); Response.Write(" 數據庫: " + ctx.Mapping.DatabaseName + "<br/>" ); Response.Write(" 表: " + ctx.Mapping.GetTable(typeof (Customer )).TableName + "<br/>" ); Response.Write(" 表達式: " + query.Expression.ToString() + "<br/>" ); Response.Write("sql : " + query.Provider.ToString() + "<br/>" ); |
上面的代碼執行結果如下:
Provider
類型:
System.Data.Linq.SqlClient.SqlProvider
|
撤銷提交
var customer = ctx.Customers.Single(c => c.CustomerID == "AROUT" ); customer.ContactName = "zhuye" ; customer.Country = "Shanghai" ; Response.Write(string .Format("Name:{0},Country:{1}<br/>" , customer.ContactName, customer.Country)); customer = ctx.Customers.GetOriginalEntityState(customer); Response.Write(string .Format("Name:{0},Country:{1}<br/>" , customer.ContactName, customer.Country)); |
上面的代碼執行效果如下:
Name:zhuye,Country:Shanghai |
批量操作
下面的代碼會導致提交 N 次 DELETE 操作:
var query = from c in ctx.Customers select c; ctx.Customers.RemoveAll(query); ctx.SubmitChanges(); |
應該使用 sql 語句進行批操作:
string sql = String .Format("delete from {0}" , ctx.Mapping.GetTable(typeof (Customer )).TableName); ctx.ExecuteCommand(sql); |
對於批量更新操作也是同樣道理。