Linq to sql（六）：探究特性（一）

延迟执行

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

       这样的查询句法不会导致语句立即执行，它仅仅是一个描述，对应一个SQL。

仅仅在需要使用的时候才会执行语句，比如：

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

        foreach (Customer c in query)

            Response.Write(c.CustomerID);

       如果你执行两次foreach操作，将会捕获到两次SQL语句的执行：

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

        foreach (Customer c in query)

            Response.Write(c.CustomerID);

        foreach (Customer c in query)

            Response.Write(c.ContactName);

       对应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]

 

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]

       对于这样的需求，建议你先使用ToList()等方法把查询结果先进行保存，然后再对集合进行查询：

      IEnumerable<Customer> customers = (from c in ctx.Customers select c).ToList();

        foreach (Customer c in customers)

            Response.Write(c.CustomerID);

        foreach (Customer c in customers)

            Response.Write(c.ContactName);

       延迟执行的优点在于我们可以像拼接SQL那样拼接查询句法，然后再执行：

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

        var newquery = (from c in query select c).OrderBy(c => c.CustomerID);