我的查询有什么铸造问题

发布于 2024-10-30 09:54:41 字数 376 浏览 0 评论 0原文

在 C# vs2008 上工作。我的以下查询显示错误。任何人都可以告诉我问题是什么以及如何解决这个问题。提前致谢。

 NorthwindDataContext db = new NorthwindDataContext();
 List<Order> r = (from p in db.Orders
                  select new { p.OrderID, p.OrderDate });

错误信息：

无法隐式转换类型 'System.Linq.IQueryable' 到 '系统.集合.通用.列表'。存在显式转换（您是缺少演员？）

原文

Work on C# vs2008.my bellow query show me error.Can any body tell me what's the problem and how to solve this problem.Thanks in advance.

 NorthwindDataContext db = new NorthwindDataContext();
 List<Order> r = (from p in db.Orders
                  select new { p.OrderID, p.OrderDate });

Error message:

Cannot implicitly convert type
'System.Linq.IQueryable'
to
'System.Collections.Generic.List'.
An explicit conversion exists (are you
missing a cast?)

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颜漓半夏 2024-11-06 09:54:41

您正在尝试将查询表达式分配给 List 对象。那是错误的。

您需要调用 ToList() 将查询结果转换为找到的订单列表，并使用匿名类型，因为您仅选择部分数据并创建新的匿名对象：

var r = (from p in db.Orders
         select new { p.OrderID, p.OrderDate }).ToList();

请注意匿名类型仍然是可枚举的，因为它仍然是通用的 List，因此它仍然实现通用的 IEnumerable 接口。

要形成 List，您需要检索完整的对象，因此请按照 John Rasch 的说法选择 p：

List<Order> r = (from p in db.Orders
                 select p).ToList();

或者选择新的 Order并根据您选择的字段构建它们。

You are trying to assign a query expression to a List<> object. That's wrong.

You need to call ToList() to convert the results of the query to a list of orders found, and use an anonymous type since you're selecting only partial data and creating new anonymous objects:

var r = (from p in db.Orders
         select new { p.OrderID, p.OrderDate }).ToList();

Note that the anonymous type will still be enumerable as it's still a generic List<>, and as such it still implements the generic IEnumerable<> interface.

To form a List<Order>, you need to either retrieve complete objects, so select p instead as John Rasch says:

List<Order> r = (from p in db.Orders
                 select p).ToList();

Or select new Orders and build them out of the fields that you're selecting.

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旧瑾黎汐 2024-11-06 09:54:41

 NorthwindDataContext db = new NorthwindDataContext();
        List<Order> r = (from p in db.Orders
                 select p).ToList();

选择 new {...} 创建匿名类型的新实例。如果您想选择Order，则必须从查询中返回 Orders。最后，您必须调用 ToList()，因为 Linq 查询返回 IEnumberable。

 NorthwindDataContext db = new NorthwindDataContext();
        List<Order> r = (from p in db.Orders
                 select p).ToList();

Selecting new {...} creates a new instance of an anonymous type. If you want to select an Order you will have to return Orders from your query. Finally you have to call ToList() because Linq queries return IEnumberable<T>.

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萌化 2024-11-06 09:54:41

或者根据您稍后要对该查询执行的操作，将其保留为 IQueryable（这将允许您在服务器上进一步过滤和查询，而无需提取所有订单），方法是：

using (var context = new NorthwindDataContext())
{
  var allOrders = from p in db.Orders
               select new { p.OrderID, p.OrderDate };

  // Here you can do further processing (to be executed in DB)
  var someOrders = allOrders.Where(ao => ao.OrderDate < DateTime.Today.AddDays(-1));
  Console.WriteLine(someOrders.Count()); // <-- Query will execute here
}

Or depending what you will do with that query later, keep it as a IQueryable (which will allow you further filtering and querying on the server without pulling all orders), by doing:

using (var context = new NorthwindDataContext())
{
  var allOrders = from p in db.Orders
               select new { p.OrderID, p.OrderDate };

  // Here you can do further processing (to be executed in DB)
  var someOrders = allOrders.Where(ao => ao.OrderDate < DateTime.Today.AddDays(-1));
  Console.WriteLine(someOrders.Count()); // <-- Query will execute here
}

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