我正在做我的家庭作业。我的任务是创建一个将用户选择的文件读入内存的应用程序。在这种情况下，它正在读取包含国家和匹配缩写的数组对象结构。文件正确加载，当我使用应用程序按国家或缩写搜索时，它会在文件中找到匹配的国家或缩写，并按应有的方式显示。但是，当我错误地输入信息并且找不到匹配项时，它应该转到 If Not Found 代码块并显示一个消息框。相反，它崩溃并抛出一个异常，上面写着：
你调用的对象是空的。
我不明白发生了什么。如果可以，请查看我的代码并提供帮助。谢谢你。
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