尝试从表单、列表添加到另一个表单和列表时，帮助解决 NULL 错误。

Question

Public Sub AddPrintBook() Select Case ShoppingCart.lstCart.Items.Add(lstPrintBooks.SelectedIndex) '"""""Getting a NULL error here. """""Copy

        Case 0
            strSelectedBook = strYourWay
            decSubTotal += decYourWay
            decShippingTotal += decShippingTotal
        Case 1
            strSelectedBook = strHistoryScotland
            decSubTotal += decHistoryScotland
            decShippingTotal += decShippingTotal
        Case 2
            strSelectedBook += decCalculus
            decSubTotal += decHistoryScotland
            decShippingTotal += decShippingTotal
        Case 3
            strSelectedBook = strRelax
            decSubTotal += decRelax
            decShippingTotal += decShippingTotal

    End Select

End Sub

Note：此问题总结整理于:Help with NULL error, when trying to add from a form, a list, to another form and list.

Answer 1

请看下面的例子： 我在 form1 上有 listbox1，在 form2 上有 listbox1。 表格 1：

Public Class Form1  
    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load  
        Dim frm2 As Form2 = New Form2(Me)  
        frm2.Show()  
    End Sub  
End Class  

表格2：

Public Class Form2  
    Private ReadOnly frm1 As Form1  
    Public Sub New(ByVal form1 As Form1)  
        frm1 = form1  
        InitializeComponent()  
    End Sub  
    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click  
        ListBox1.Items.Add(frm1.ListBox1.SelectedIndex)  
    End Sub  
End Class  

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