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学Linq to sql(六):探究特性

时间:2010-11-04 博客园 博客园 lovecherry

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)

    {}