Question

我正在尝试创建一个可以充当“购物清单”的程序。我已经完成了所有代码并进行了设置，但是当我尝试运行程序时，它将无法运行。我的代码中有0个错误，它也不会调试。我遇到这个问题有两个程序。这是两个程序的代码计划1：

Public Class Form1
    Dim Item As String = txtAdd.Text
    Private Sub btnAdd_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnAdd.Click
        Item = Me.txtAdd.Text
        Me.lstItems.Items.Add(Item)
    End Sub
    Private Sub btnMoveToList_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnMoveToList.Click
        Me.lstItems.Items.Remove(Item)
        Me.lstList.Items.Add(Item)
    End Sub
Private Sub btnMoveToItems_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnMoveToItems.Click
    Me.lstList.Items.Remove(Item)
    Me.lstItems.Items.Add(Item)
End Sub
Private Sub btnDelete_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnDelete.Click
    Me.lstItems.Items.Remove(Item)
End Sub
End Class

计划2：

Public Class Form1
    Dim total As Double
    Dim Cost As Double = Me.txtCost.Text
    Dim Number As Integer = Me.txtNumber.Text


    Private Sub btnCalculate_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnCalculate.Click
        total = (Cost * Number)

        If Int32.TryParse(Me.txtCost.Text, Me.txtNumber.Text) Then
            MessageBox.Show("The total cost of your items is:" & total)
        Else
            MessageBox.Show("Enter correct numeric values")
        End If
    End Sub

    Private Sub ExitToolStripMenuItem_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ExitToolStripMenuItem.Click
        Application.Exit()
    End Sub
End Class

我收到一个弹出窗口，显示“创建表单时出错。有关详细信息，请参阅Exception.InnerException。错误是：对象引用未设置为对象的实例。”

Answer 1

Dim Item As String = txtAdd.Text

不会上课; txtAdd在此时未提及您的文本框。您可以在New中初始化它：

Private Item As String

Public Sub New()
    Me.InitializeComponent()
    Me.Item = txtAdd.Text
End Sub

你的第二个表格有同样的问题，但是看到你没有使用那里的两个字段，你不需要它们。如果它们应该反映输入，请改用属性，例如

Public ReadOnly Property Cost() As String
    Get
        Return Me.txtCost.Text
    End Get
End Property

Visual Basic问题

1 个答案: