Question

我对Linq相对较新，我正在使用.NET开发一个函数，该函数使用linq查询的选定字段实例化POCO。

问题是必须在“运行时”转换其中一个字段。

这是我的错误示例：

Private Shared Function GetInfectionHistory(HiveId As Long) As IList(Of HiveInfectionDetail)
    Using DB As LandDataModelUnitOfWork = Context.CreateUnitOfWork
            Dim historyResult = From I In DB.HiveAfbInfections
                                Where I.HiveId = HiveId
                                Select New HiveInfectionDetail With {
                                    .DateInfected = I.DateInfected,
                                    .DateCleaned = I.DateCleared,
                                    .PotentialAfbInfection = I.PotentialInfection,
                                    .AfbInfection = Not I.PotentialInfection
                                }
            If IsListEmpty(historyResult.ToList()) Then
                Return Nothing
            End If
            Return historyResult.ToList()
        End Using
End Function

和

Private Shared Function IsListEmpty(Of T)(source As IEnumerable(Of T)) As Boolean
        If source Is Nothing Then
            Return True
        End If
        Return Not source.Any()
    End Function
enter code here

有问题的一行是我为属性AfbInfection指定一个值。此属性将与数据库中的PotentialInfection相反。因此，如果I.PotentialInfection为True，那么我的属性AfbInfection应为False。如上所述将导致 IndexOutOfRangeException - 索引超出数组的范围不确定LinQ使用NOT表达式做什么，但肯定不是我想要的。

在将数据库字段值存储到自定义对象中时，有没有办法修改数据库字段值？

谢谢！

Answer 1

试试这个：

Private Shared Function GetInfectionHistory(HiveId As Long) As IList(Of HiveInfectionDetail)
    Using DB As LandDataModelUnitOfWork = Context.CreateUnitOfWork
            Dim historyQuery1 = From I In DB.HiveAfbInfections
                                Where I.HiveId = HiveId
                                Select New With {
                                    .DateInfected = I.DateInfected,
                                    .DateCleaned = I.DateCleared,
                                    .PotentialInfection = I.PotentialInfection
                                }
            Dim historyQuery2 = From I In historyQuery1.ToArray()
                                Select New HiveInfectionDetail With {
                                    .DateInfected = I.DateInfected,
                                    .DateCleaned = I.DateCleaned,
                                    .PotentialAfbInfection = I.PotentialInfection,
                                    .AfbInfection = Not I.PotentialInfection
                                }
            Dim historyResult = historyQuery2.ToList()
            If IsListEmpty(historyResult) Then
                Return Nothing
            End If
            Return historyResult
        End Using
End Function

LINQ - 如何根据DB将返回的值修改select

1 个答案: