VB.net 空错误

Question

我在尝试处理 ArgumentNullExeption 发生时遇到问题

值不能为 Null

参数名称项目

到目前为止我尝试了以下方法

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim ArgumentNullException As New Boolean
        If (ListBox2.Items.Add(ListBox1.SelectedItem.ToString = "")) Then
           MessageBox.Show("please pick from list", "error")
        End If
        ListBox1.Items.Remove(ListBox1.SelectedItem)

 Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim ArgumentNullException As New Boolean
        If (ListBox2.Items.Add(ListBox1.SelectedItem Is Nothing)) Then
           MessageBox.Show("please pick from list", "error")
        End If
        ListBox1.Items.Remove(ListBox1.SelectedItem)


 Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim ArgumentNullException As New BooleanListBox2
        ListBox2.Items.Add(ListBox1.SelectedItem)
        If (ListBox1.SelectedItem) = ""
           MessageBox.Show("please pick from list", "error")
        ListBox1.Items.Remove(ListBox1.SelectedItem)

错误仍然出现，有人能帮忙吗

已更新：

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
            ListBox2.Items.Add(ListBox1.SelectedItem)
            If IsNothing (ListBox1.SelectedItem)
               MessageBox.Show("please pick from list", "error")
            ListBox1.Items.Remove(ListBox1.SelectedItem)

仍然报错

Answer 1

问题不太清楚，我试试看。

当您的应用程序具有 Listboxes 并使用 .SelectedItem 属性时，您应该始终使用 Try/Catch 或更好地检查用户是否选择了项目：

If IsNothing(ListBox1.SelectedItem) Then Exit Sub

您也可以使用

If ListBox1.SelectedItems.Count = 0 Then Exit Sub

您也可以使用 If/Else，但在我看来，将语句放在代码的第一行使其更具可读性。

使用此代码将使您的按钮在没有选定项目时不执行任何操作。否则你可以使用 multiline-If 放一个 MsgBox("Please pick an Item")，来警告用户：

If IsNothing(ListBox1.SelectedItem) Then
    MsgBox("Please pick an Item first.")
    Exit Sub
End If
// Your Code after that

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
  // Leave the Sub and Message the User, when there is no SelectedItem
  If IsNothing(ListBox1.SelectedItem) Then
      MsgBox("Please pick an Item first.")
      Exit Sub
  End If
  // Code will only reach here, when there is a SelectedItem
  ListBox2.Items.Add(ListBox1.SelectedItem)
  ListBox1.Items.Remove(ListBox1.SelectedItem)
End sub

VB.net 空错误

VB.net Null error

vb.net

null

argumentnullexception