Question

所以我收到错误“对象引用未设置为对象的实例”。当我运行我的程序。我正在尝试使用多态来产生我的结果，但这个错误介于我和我的结局之间。有人能帮助我理解这个吗？

Module PublicationTester

    Sub Main()

        Dim publications(4) As Publication

        Dim book1 As New Book("Booktitle1", "Publisherb1", "100.00", "Myself",
                              "b1b1b1b1", "11 / 11 / 11")
        Dim book2 As New Book("Booktitle2", "Publisherb2", "100.00", "Myself",
                              "b2b2b2b2", "11 / 11 / 11")
        Dim mag1 As New Magazine("Magtitle1", "Publisherm1", "100.00", "Myself",
                              "issue1", "1")
        Dim mag2 As New Magazine("Magtitle2", "Publisherm2", "100.00", "Myself",
                              "issue2", "2")

        publications(1) = book1
        publications(2) = book2
        publications(3) = mag1
        publications(4) = mag2

        For Each publication In publications
            ***Console.WriteLine(publication.ToString())***
        Next

        'Console.WriteLine(publications(1))
        'Console.WriteLine(publications(2))
        'Console.WriteLine(publications(3))
        'Console.WriteLine(publications(4))
        'Console.Read()

    End Sub

End Module




Public MustInherit Class Publication
    Private titleValue As String
    Private publisherValue As String
    Private priceValue As Decimal

    Public Sub New(ByVal title As String, ByVal publisher As String,
                   ByVal price As String)
        titleValue = title
        publisherValue = publisher
        priceValue = price

    End Sub

    Public Property titleVal() As String
        Get
            Return titleValue

        End Get
        Set(title As String)
            titleValue = title
        End Set
    End Property

    Public Property publisherVal() As String
        Get
            Return publisherValue
        End Get

        Set(publisher As String)
            publisherValue = publisher
        End Set
    End Property

    Public Property priceVal() As String
        Get
            Return priceValue
        End Get

        Set(price As String)
            priceValue = price
        End Set
    End Property

    Public Overrides Function ToString() As String
        Return "Title: " & titleValue & vbCrLf &
           "Publisher: " & publisherValue & vbCrLf &
           "Price: " & priceValue & vbCrLf

    End Function

End Class






Public Class Book
    Inherits Publication
    Private authorValue As String
    Private ISBNValue As String
    Private publishDateValue As String

    Public Sub New(ByVal title As String, ByVal publisher As String,
                   ByVal price As String, author As String, ISBN As String,
                   publishDate As String)
        MyBase.New(title, publisher, price)
        authorValue = author
        ISBNValue = ISBN
        publishDateValue = publishDate

    End Sub

    Public Property authorVal() As String
        Get
            Return authorValue
        End Get

        Set(author As String)
            authorValue = author
        End Set
    End Property

    Public Property ISBNVal() As String
        Get
            Return ISBNValue
        End Get

        Set(ISBN As String)
            ISBNValue = ISBN
        End Set
    End Property

    Public Property publishDateVal() As String
        Get
            Return publishDateValue
        End Get

        Set(publishDate As String)
                publishDateValue = publishDate
        End Set
    End Property

    Public Overrides Function ToString() As String
        Return MyBase.ToString() &
            "Author: " & authorValue & vbCrLf &
           "ISBN: " & ISBNValue & vbCrLf &
           "Publish Date: " & publishDateValue & vbCrLf
    End Function

End Class






Public Class Magazine
    Inherits Publication
    Private editorValue As String
    Private issueValue As String
    Private frequencyValue As Decimal

    Public Sub New(ByVal title As String, ByVal publisher As String,
                   ByVal price As String, editor As String,
                   issue As String, frequency As String)
        MyBase.New(title, publisher, price)
        editorValue = editor
        issueValue = issue
        frequencyValue = frequency

    End Sub

    Public Property editorVal() As String
        Get
            Return editorValue
        End Get

        Set(edit As String)
            editorValue = edit
        End Set
    End Property

    Public Property issueVal() As String
        Get
            Return issueValue
        End Get

        Set(issue As String)
                issueValue = issue
        End Set
    End Property

    Public Property frequencyVal() As String
        Get
            Return frequencyValue
        End Get

        Set(frequency As String)
            frequencyValue = frequency
        End Set
    End Property

    Public Overrides Function ToString() As String
        Return MyBase.ToString() &
            "Editor: " & editorValue & vbCrLf &
           "Issue: " & issueValue & vbCrLf &
           "Frequency: " & frequencyValue & vbCrLf
    End Function

End Class

Answer 1

数组在VB中基于零。在VB中，您不指定数组大小，而是指定上部索引。因此，您的数组包含五个元素。你没有初始化publications(0)。

Const N As Integer = 4
Dim publications(N - 1) As Publication '[0 .. N-1] = [0 .. 3]

...

publications(0) = book1
publications(1) = book2
publications(2) = mag1
publications(3) = mag2

....

什么是：对象引用未设置为对象的实例

1 个答案: