Question

我刚刚在我的代码中发现了一个错误，因为我认为在调用查询中的方法时，当用作out（byref）参数时，Linq查询中声明的变量（通过Let关键字）不会被设置。
有谁知道这只对VB.Net有效还是对C＃有效？这是在某处记录的吗？

Let关键字的MSDN文档说明：

计算一个值并将其分配给查询中的新变量。

稍后的文档说明：

使用Let子句可以计算每个查询结果的值，并使用别名引用它们。

以下是我的代码失败的示例：

From c In Me.AllContractsInHourBlock
Let isForcedFullTrading As Boolean = False
Let noTradingReason As ContractNoTradingReason = ContractNoTradingReason.None
Let targetMode = c.CalculateTradingMode(instant, sendWarningMessageOnNoPrognosis, isForcedFullTrading, noTradingReason)
Select New With {
   .CurrentMode = c.TradingMode,
   .TargetMode = targetMode,
   .TradingModeChanged = (c.TradingMode <> targetMode),
   .IsForcedFullTrading = isForcedFullTrading,
   .CurrentNoTradingReason = c.NoTradingReason,
   .NoTradingReason = noTradingReason,
   .NoTradingReasonChanged = (targetMode = ContractTradingMode.NoTrading AndAlso c.NoTradingReason <> noTradingReason

在上面的代码示例中，变量noTradingReason和isForcedFullTrading将始终在达到Select New时最初分配。
如果我在Linq之外提取变量，那么一切正常。

Dim isForcedFullTrading As Boolean = False
Dim noTradingReason As ContractNoTradingReason = ContractNoTradingReason.None      
From c In Me.AllContractsInHourBlock
Let targetMode = c.CalculateTradingMode(instant, sendWarningMessageOnNoPrognosis, isForcedFullTrading, noTradingReason)
Select New With {
   .CurrentMode = c.TradingMode,
   .TargetMode = targetMode,
   .TradingModeChanged = (c.TradingMode <> targetMode),
   .IsForcedFullTrading = isForcedFullTrading,
   .CurrentNoTradingReason = c.NoTradingReason,
   .NoTradingReason = noTradingReason,
   .NoTradingReasonChanged = (targetMode = ContractTradingMode.NoTrading AndAlso c.NoTradingReason <> noTradingReason

这是CalculateTradingMode方法的签名：

Function CalculateTradingMode(instant As DateTimeOffset, sendWarningMessageOnNoPrognosis As Boolean, ByRef isForcedFullTrading As Boolean, ByRef noTradingReason As ContractNoTradingReason) As ContractTradingMode

VB.Net Linq查询中的变量可以用作方法调用/ byref参数吗？

0 个答案: