Linq to sql：延迟执行

IQueryable query =from c inctx.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 inctx.Customers select c;

 

foreach (Customer cinquery)

 

           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 inctx.Customers select c;

 

foreach (Customer cinquery)

 

           Response.Write(c.CustomerID);

 

foreach (Customer cinquery)

 

           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 cin ctx.Customersselect

 

c).ToList();

 

foreach (Customer cincustomers)

 

           Response.Write(c.CustomerID);

 

foreach (Customer cincustomers)

 

           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 cinquery select c).OrderBy(c => c.CustomerID);