未将对象引用设置为对象的实例VB.Net OleDbCommand

Question

每次我在数据库上搜索时，它总是说“对象引用未设置为对象的实例”。 请大家帮帮我。谢谢。

这里是我的代码搜索：

Private Sub BtnSearch_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles BtnSearch.Click

If TBSearch.Text = "" Then 
     MsgBox("Please enter keyword to search...", MsgBoxStyle.Information, "Keyword to search...")` 
     TBSearch.Focus() 
     Exit Sub 
    End If 
    Call OpenConnection() 
    With OleDa 
     Call Initialized() 
     .SelectCommand = New OleDb.OleDbCommand() <-------- Error appears here. 
     .SelectCommand.CommandText = "SELECT * FROM [HighSchool] WHERE [LastName] Like '%%" & TBSearch.Text & "%%' or [FirstName] Like '%%" & TBSearch.Text & "%%' or [MI] Like '%%" & TBSearch.Text & "%%'" & _ 
     "Or [Gender] Like '%%" & TBSearch.Text & "%%' or [Address] Like '%%" & TBSearch.Text & "%%' or [Birthday] Like '%%" & TBSearch.Text & "%%' or [RNumber] Like '%%" & TBSearch.Text & "%%'" & _ 
     "Or [ENumber] Like '%%" & TBSearch.Text & "%%' ORDER By LastName ASC" 
     .SelectCommand.Connection = OleCn 

     Call PopulateListView() 

     If Me.LV.Items.Count >= 1 Then 
      MsgBox(Me.LV.Items.Count & " Record(s) found for " & "(" & Me.TBSearch.Text & ")", MsgBoxStyle.OkOnly, "Record(s) found...") 
     Else 
      MsgBox("No record(s) found for " & "(" & Me.TBSearch.Text & ")" & " , please try again... ", MsgBoxStyle.Critical, "No record found...") 
      TBSearch.Focus() 
      TBSearch.SelectAll() 
     End If 
    End With 
    Call CloseConnection() 
End Sub 

这里是我的填充ListView控件代码：

Public Sub PopulateListView() Me.LV.Items.Clear() Dim OleDr As OleDb.OleDbDataReader OleDr = OleDa.SelectCommand.ExecuteReader()

Do While OleDr.Read() 
     Dim Item As New ListViewItem 
     Item.Text = IIf(OleDr.IsDBNull(0), "", OleDr.Item(0)) 
     For shtCntr = 1 To OleDr.FieldCount() - 1 
      If Not OleDr.IsDBNull(shtCntr) Then 
       Item.SubItems.Add(OleDr.Item("FirstName")) 
       Item.SubItems.Add(OleDr.Item("LastName")) 
       Item.SubItems.Add(OleDr.Item("MI")) 
       Item.SubItems.Add(OleDr.Item("Gender")) 
       Item.SubItems.Add(OleDr.Item("Address")) 
       Item.SubItems.Add(OleDr.Item("Birthday")) 
       Item.SubItems.Add(OleDr.Item("RNumber")) 
       Item.SubItems.Add(OleDr.Item("ENumber")) 
      Else 
       Item.SubItems.Add("") 
      End If 
     Next shtCntr 
     Me.LV.Items.Add(Item) 
    Loop 
End Sub 

Answer 1

好吧，让我们修正了一些东西。

1. 请勿使用Call。这只是VB 6（即90年代后期）的延期
2. 不要使用Exit Sub。改为使用Return。 （原因与＃1相同）

3. 使用参数化的SQL而不是字符串连接。您的代码就像是不会有人搜索“O'Mally”一样。

   Private Sub BtnSearch_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles BtnSearch.Click 
       If TBSearch.Text = "" Then 
        MsgBox("Please enter keyword to search...", MsgBoxStyle.Information, "Keyword to search...")` 
        TBSearch.Focus() 
        Exit Sub 
       End If 
       Call OpenConnection() 
       With OleDa 
        Call Initialized() 
        .SelectCommand = New OleDb.OleDbCommand() 
        .SelectCommand.CommandText = "SELECT * FROM [HighSchool] WHERE [LastName] Like @TBSearch or [FirstName] Like @TBSearch or [MI] Like @TBSearch Or [Gender] Like @TBSearch or [Address] Like @TBSearch or [Birthday] Like @TBSearch or [RNumber] Like @TBSearch Or [ENumber] Like @TBSearch ORDER By LastName ASC" 
        .SelectCommand.Connection = OleCn 
        .SelectCommand.Parameters.AddWithValue("@TBSearch", "%" & TBSearch.Text & "%") 
   
        Call PopulateListView() 
   
        If Me.LV.Items.Count >= 1 Then 
         MsgBox(Me.LV.Items.Count & " Record(s) found for " & "(" & Me.TBSearch.Text & ")", MsgBoxStyle.OkOnly, "Record(s) found...") 
        Else 
         MsgBox("No record(s) found for " & "(" & Me.TBSearch.Text & ")" & " , please try again... ", MsgBoxStyle.Critical, "No record found...") 
         TBSearch.Focus() 
         TBSearch.SelectAll() 
        End If 
       End With 
       Call CloseConnection() 
   End Sub 
   

至于你的错误，唯一可能的原因是OleDa是Nothing。