Linq .使用 LinqPad 选择方法问题

Question

我无法理解为什么在以下代码中出现错误。我确信我错过了一些简单的东西，但我需要帮助理解。

我一直在使用 LinqToSql 在 LinqPad 中运行此代码。

我在 LinqPad 中有以下代码：

涉及三个表：Shipments、ShipmentDetails 和 ShipmentWeights。所有三个表均通过shipmentID 链接，该ID 是Shipments 表的PK。

在此代码中，最终查询（“权重”）失败并显示错误：“不支持的异常：不支持使用本地集合的查询。”我发现 LinqToSql 中的某些内容不支持使用 .Contains 但我不明白名为“Details”的查询是如何工作的。对我来说这似乎是同一类查询。有人可以帮我填补空白吗？

对于那些不熟悉 LinqPad 的人来说，.Dump 方法只是 IQueryable 上的一个扩展方法，它以格式化的方式打印出内容。

var shipments = Shipments.Where(s => s.OrderID == "Some OrderID");
shipments.Dump("Shipments");

var shipmentIds = shipments.Select(s => s.ShipmentID);
shipmentIds.Dump();

    //This query works.  What is different about this query than the one that fails?
var shipmentDetails = ShipmentDetails.Where(d => shipmentIds.Contains(d.ShipmentID));
shipmentDetails.Dump("Details");

var detailShipmentIds = shipmentDetails.Select(sd => sd.ShipmentID);
detailShipmentIds.Dump();

//This is the query that generates the error
    var shipmentWeights = ShipmentWeights.Where(w => detailShipmentIds.Contains(w.ShipmentID));
shipmentWeights.Dump("Weights");

Answer 1

您的列表是 iQueryable 的 - 即查询尚未执行，因此它们不能用作 Contains 查询的一部分。只需先将它们更改为本地列表即可。例如，

var shipmentIds = shipments.Select(s => s.ShipmentID).ToList();

对所有本地列表执行相同的操作。

这是完整的清单

var shipments = Shipments.Where(s => s.OrderID == "Some OrderID");
shipments.Dump("Shipments");

var shipmentIds = shipments.Select(s => s.ShipmentID).ToList();
shipmentIds.Dump();

//This query works.  What is different about this query than the one that fails?
var shipmentDetails = ShipmentDetails.Where(d => shipmentIds.Contains(d.ShipmentID));
shipmentDetails.Dump("Details");

var detailShipmentIds = shipmentDetails.Select(sd => sd.ShipmentID).ToList();
detailShipmentIds.Dump();

//This is the query that generates the error
var shipmentWeights = ShipmentWeights.Where(w => detailShipmentIds.Contains(w.ShipmentID));
shipmentWeights.Dump("Weights");

Answer 2

是的，正如所指出的，你必须这样做

var shipments = Shipments.Where(s => s.OrderID == "Some OrderID");
shipments.Dump("Shipments");

var shipmentIds = shipments.Select(s => s.ShipmentID).ToList();
shipmentIds.Dump();

    //This query works.  What is different about this query than the one that fails?
var shipmentDetails = ShipmentDetails.Where(d => shipmentIds.Contains(d.ShipmentID));
shipmentDetails.Dump("Details");

var detailShipmentIds = shipmentDetails.Select(sd => sd.ShipmentID).ToList();
detailShipmentIds.Dump();

//This is the query that generates the error
    var shipmentWeights = ShipmentWeights.Where(w => detailShipmentIds.Contains(w.ShipmentID));
shipmentWeights.Dump("Weights");