Question

我正在获取NullReferenceException：

faxnum = Customer.ContactLink.Contact.DefaultFaxLink.Phone.PhoneNumber

null ref在DefaultFaxLink上。由于没有传真号码，DefaultFaxLink未初始化，我知道如果是，我不会在分配时收到错误。

所以，我的问题是，有没有一种方法可以捕获异常而无需测试每个对象以查看它是否无效？

我只想处理一个语句的整个右手部分，这样如果任何部分什么都没有，我只是不给左边的var赋值。

除此之外，我可以在基础对象上使用反射来评估每个成员及其子成员并分配一个空值吗？

Answer 1

您可以使用Try-Catch块进行NullReferenceException

Public Class Customer
    Public ContactLink As ContactLink
End Class

Public Class ContactLink
    Public Contact As Contact
End Class

Public Class Contact
    Public DefaultFaxLink As FaxLink
End Class

Public Class FaxLink
    Public Phone As Phone
End Class

Public Class Phone
    Public PhoneNumber As String
End Class

Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load

    Dim objCustomer As New Customer
    objCustomer.ContactLink = New ContactLink
    objCustomer.ContactLink.Contact = New Contact
    objCustomer.ContactLink.Contact.DefaultFaxLink = New FaxLink

    Dim PhoneNumber As String = ""

    Try
        PhoneNumber = objCustomer.ContactLink.Contact.DefaultFaxLink.Phone.PhoneNumber
    Catch ex As NullReferenceException
        PhoneNumber = ""
    Catch ex As Exception
        MsgBox(ex.Message)
    End Try

    If Not String.IsNullOrEmpty(PhoneNumber) Then
        MsgBox("Fax number is..." & PhoneNumber)
    Else
        MsgBox("No fax number!")
    End If

End Sub

Answer 2

写一个函数。

Public Class Customer

    Public Function GetFaxNumberSafe() As String

        If Me.ContactLink IsNot Nothing AndAlso
           Me.ContactLink.Contact IsNot Nothing AndAlso
           Me.ContactLink.Contact.DefaultFaxLink IsNot Nothing AndAlso
           Me.ContactLink.Contact.DefaultFaxLink.Phone IsNot Nothing Then

            Return Customer.ContactLink.Contact.DefaultFaxLink.Phone.PhoneNumber

        Else

            Return Nothing

        End If

    End Function

End Class

您还可以将对象设置为对访问进行延迟加载实例化，以便始终拥有对象引用。

Public Class Customer

    Private _contactLink As New Lazy(Of ContactLink)()

    Public ReadOnly Property ContactLink As ContactLink
        Get
            Return _contactLink.Value
        End Get
    End Property

End Class

没有任何东西和反射测试对象

2 个答案: