Question

为什么此代码会生成null异常：

Public Function MessageCount(ByVal mId As Long) As Integer
    Dim messages As List(Of InboxMessage) = Nothing
    Using ctx As New UFCWEntities.UFCWEntities

        Dim allMessageIds = ctx.InboxLinks.Where(Function(o) o.MemberId = mId).ToList()
        For Each i As InboxLink In allMessageIds
            messages.Add(ctx.InboxMessages.FirstOrDefault(Function(o) o.InboxMessageId = i.InboxMessageId))
        Next

    End Using

    If Not IsNothing(messages) Then
        Return messages.Count()
    End If

    Return 0

End Function

尝试执行message.Add

时会生成异常

Answer 1

您收到此错误是因为ctx.InboxMessages.FirstOrDefault(...)返回null。

您应首先进行空检查，例如：

For Each i As InboxLink In allMessageIds
    Dim firstMsg = ctx.InboxMessages.FirstOrDefault(...)
    If Not IsNothing(firstMsg) Then
        messages.Add(firstMsg)
    End If
Next

Answer 2

您没有实例化您的消息变量，因此当您尝试添加到它时，它不会因为消息仍然是Nothing。将第一行更改为

Dim messages As New List(Of InboxMessage)

话虽如此，您可能需要考虑将整个操作视为基于集合的操作。如果你想要做的就是过滤它们并计算过滤后的结果，你就不需要给物体加水。从您提供的代码中，您应该能够将其简化为：

Using ctx As New UFCWEntities.UFCWEntities
   return ctx.InboxLinks.Count(function(o) o.MemberId = mId)
End Using

实体框架NullReferenceException

2 个答案: