Linq to sql：探究特性-技术开发专区

Linq to sql：探究特性

作者：lovecherry博客编辑： nancy 2008-01-25 00:00

【IT168技术文档】

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);

DataLoadOptions

var products = from p in ctx.Products select p;
foreach (var p in products)
{
if (p.UnitPrice > 10)
ShowDetail(p.Order_Details);
}
private void ShowDetail(EntitySet<Order_Detail> orderdetails)
{}

由于ShowDetail方法并没有使用到订单详细信息，所以这个操作只会执行下面的SQL：

SELECT [t0].[ProductID], [t0].[ProductName],
[t0].[SupplierID], [t0].[CategoryID],
[t0].[QuantityPerUnit], [t0].[UnitPrice],
[t0].[UnitsInStock], [t0].[UnitsOnOrder],
[t0].[ReorderLevel], [t0].[Discontinued]
FROM [dbo].[Products] AS [t0]

现在修改一下ShowDetail方法：

private void ShowDetail(EntitySet<Order_Detail> orderdetails)
{
foreach (var o in orderdetails)
{
Response.Write(o.Quantity + "<br>");
}
}

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