VB.NET異常問題

Question

我正在做我的功課。我的任務是創建一個應用程序來讀取用戶選擇到內存中的文件。在這種情況下，它正在讀取包含國家和它的匹配縮寫的數組對象的結構。文件加載正確，當我使用應用程序按國家或縮寫進行搜索時，它會在文件中找到匹配的國家或縮寫，並按照它的顯示方式進行顯示。但是，如果我輸入的信息不正確，而且找不到匹配項，則應將其發送到If Not Found代碼塊並顯示一個消息框。相反，它崩潰並拋出一個異常，說：

對象引用未設置爲對象的實例。

我不明白髮生了什麼事。請仔細查看我的代碼，如果可以，請協助。謝謝。

Option Strict On 

'import a file 
Imports System.IO 

Public Class frmCountry 

'Module Structure 
Structure Countries 
    Dim Names As String 
    Dim Abbreviation As String 
End Structure 

'module array 
Dim Country(257) As Countries 
'file reader 
Private sr As StreamReader 

'search button click event 
Private Sub btnSearch_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnSearch.Click 
    'event level variables 
    Dim Found As Boolean 
    Dim Counter As Integer 

    'looks for entry match 
    If rdoAbbrev.Checked = True Then 
     Do Until Found Or Counter > 256 
      Found = Country(Counter).Abbreviation.ToUpper = txtAbbrev.Text.ToUpper 
      If Found Then 
       txtCountry.Text = Country(Counter).Names 
      Else 
       Counter += 1 
      End If 
     Loop 
    Else 
     Do Until Found Or Counter > 256 
      Found = Country(Counter).Names.ToUpper = txtCountry.Text.ToUpper 
      If Found Then 
       txtAbbrev.Text = Country(Counter).Abbreviation 
      Else 
       Counter += 1 
      End If 
     Loop 

    End If 
    'match not found response 
    If Not Found Then 
     MessageBox.Show("This is not a valid entry.", "NO MATCH FOUND", MessageBoxButtons.OK) 
     If rdoAbbrev.Checked = True Then 
      txtAbbrev.Text = "" 
      txtAbbrev.Focus() 
     Else 
      txtCountry.Text = "" 
      txtCountry.Focus() 

     End If 
    End If 
    'reset variables 
    Counter = 0 
    Found = False 
End Sub 
'exit button click event 
Private Sub btnExit_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnExit.Click 
    Me.Close() 
End Sub 
'clear button click event 
Private Sub btnClear_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnClear.Click 
    rdoAbbrev.Checked = False 
    rdoCountry.Checked = False 
    txtAbbrev.Text = "" 
    txtAbbrev.ReadOnly = True 
    txtCountry.Text = "" 
    txtCountry.ReadOnly = True 
    lblAbbrev.Visible = False 
    lblName.Visible = False 
    lblTitle.Focus() 

End Sub 
'radio button selected event 
Private Sub rdoCountry_CheckedChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles rdoCountry.CheckedChanged, rdoAbbrev.CheckedChanged 
    Dim ButtonSelected As RadioButton 

    ButtonSelected = CType(sender, RadioButton) 

    Select Case ButtonSelected.Name 
     Case "rdoAbbrev" 
      lblName.Text = "Matching Name: " 
      txtCountry.Text = "" 
      lblName.Visible = True 
      txtCountry.ReadOnly = True 
      lblAbbrev.Text = "Enter Abbreviation: " 
      lblAbbrev.Visible = True 
      txtAbbrev.ReadOnly = False 
      txtAbbrev.Focus() 

     Case Else 
      lblName.Text = "Enter Name: " 
      lblName.Visible = True 
      txtCountry.ReadOnly = False 
      txtCountry.Focus() 
      lblAbbrev.Text = "Matching Abbreviation: " 
      lblAbbrev.Visible = True 
      txtAbbrev.ReadOnly = True 
      txtAbbrev.Text = "" 
    End Select 


End Sub 
'user uses browse button to select file 
Private Sub btnBrowse_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnBrowse.Click 
    Dim DialogueResponse As DialogResult 
    Dim IndexInteger As Integer 

    If sr IsNot Nothing Then 
     sr.Close() 
    End If 

    With OpenFileDialog1 
     .InitialDirectory = Directory.GetCurrentDirectory 
     .FileName = "Country.txt" 
     .Title = "Select File or Directory for File" 
     DialogueResponse = .ShowDialog 
    End With 

    If DialogueResponse <> DialogResult.Cancel Then 
     sr = New StreamReader(OpenFileDialog1.FileName) 
    End If 

    Do Until sr.Peek = -1 
     If IndexInteger <= 256 Then 
      Country(IndexInteger).Abbreviation = sr.ReadLine 
      Country(IndexInteger).Names = sr.ReadLine 
      IndexInteger += 1 
     Else 
      Exit Do 
     End If 
    Loop 
    sr.Close() 
End Sub 
End Class 

Answer 1

顯然是拋出異常，因爲數組和循環被設置爲搜索比包含的文件更多的對象。

此問題已得到解決

-solved-