Question

这是结构的定义

Structure Ct

    Public name As String

    Structure Pt
        Public identity As String
    End Structure

    Public Pty() As Pt

End Structure


Public Cty() As Main.Ct

这是在名为main的模块中声明的。

然后运行位于另一个类的子例程中的这段代码

Dim i As Integer = 1
    For Each item As String In cataList.Items
        'cataList is a listbox
        Cty(i).name = item
        i += 1
    Next

抛出nullReferenceException。

缺少什么？我需要结构是全球性的。

Answer 1

cataList.Items由listviewitems而不是字符串组成。这可能会导致问题。另外，Cty在声明时没有成员。

试试这个：

ReDim Cty(cataList.Items.Count-1)

For Each item As ListViewItem In cataList.Items
    'cataList is a listbox
    Cty(i).name = item.Text
    i += 1
Next

您也可以使用列表（使用.add）而不是数组来避免重新开始。

Answer 2

您的数组已声明但未实例化，因为链接的dupe描述。但是，一系列结构并不是最有效的方法。

Friend Class Ct                     ' cries out for a meaningful name        

    Public Property Name As String
    Private _identities As New List(of String)    ' ie "Pty"

    ' a ctor to create with the name prop
    Public Sub New(n As String)
        Name = n
    End Sub

    Public Sub AddIdentity(id As String)
         _identities.Add(id)
    End Sub

    ' get one
    Public Function GetIdentity(index As Integer) As String
         Return _identities(index)
    End Sub

    ' get all
    Public Function Identities As String()
        Return _identities.ToArray
    End If

    ' and so on for Count, Clear...  
End Class

然后列出这些事情：

' as (elegantly) described in the NRE link, NEW creates an instance:
Friend Cty As New List(Of Ct)

然后从ListBox填充List：

For Each s As String In cataList.Items
    Cty.Add(New CT(s))             ' create CT with name and add to list
Next

您使用列表和收藏集的次数越多，您就会越了解它们的灵活性和强大程度。例如，可能根本不需要手动填充ListBox。使用DataSource属性将列表绑定到列表框;这样更有效，因为你映射而不是将数据复制到UI控件：

' tell the listbox which property on your class (Type) to display
cataList.DisplayMember = "Name"
cataList.DataSource = Cty

对于您需要/需要复制数据的情况：

For n As Integer = 0 to Cty.Count-1
    cataList.Items.Add(Cty(n).Name)
Next

或者：

For Each item As Ct In cty          
    cataList.Items.Add(item.Name)
Next

全局结构数组给出NullReferenceException

2 个答案: