Visual Basic-对象引用未设置为对象的实例

Question

I'm receiving the following error in VB.Net.

"Object reference not set to an instance of an object"

It highlights "Next" at the end of the For Loop.

Any help would be great.

Imports System.IO
Public Class LoginForm
    Dim Username() As String
    Dim Password() As String
    Dim Index As Integer

    Public Function encrypt(ByVal data As String) As String
        Dim answer As String = ""
        Dim I As Integer
        data = RTrim(data)
        If Mid(data, 1, 1) <> Chr(0) Then
            For I = 1 To Len(data)
                answer = answer + Chr(Asc(Mid(data, I, 1)) Xor 23) 
               ' Xor 23 is a simple encription cipher, a string can be 
               ' encrypted or de-encrypted by the value following the Xor 
               'i.e. "23" '
            Next I
        End If
        encrypt = answer
    End Function

    Private Sub LoginButton_Click(ByVal sender As System.Object, _
                                    ByVal e As System.EventArgs) _
                                Handles LoginButton.Click

        For Each I In Username

            If UserNameTextBox.Text = Username(Index) Then
                UserAdd.Show()
                Me.Hide()
                If PasswordTextBox.Text = Password(Index) Then
                    MessageBox.Show("Correct Password")
                Else
                    MessageBox.Show("Invalid Password, Sorry")
                End If
            Else : MessageBox.Show("Invalid Username, Sorry")
            End If    
        Next    
    End Sub

    Public Sub ReadUsers()
        Dim CurrentFileReader As StreamReader
        Dim FileName, Line As String
        Dim Delimiter As Char = ","
        Dim Feild() As String
        Dim Username() As String
        Dim Password() As String
        Dim Index As Integer

        FileName = "C:\Computing\Projects\Login\Users.txt"  'location of 
                                                            'user file 
        CurrentFileReader = New StreamReader(FileName)

        Do Until CurrentFileReader.EndOfStream

            Line = CurrentFileReader.ReadLine
            If Line = Nothing Then
                Exit Do
            End If

            ReDim Preserve Username(Index)
            ReDim Preserve Password(Index)

            Feild = Line.Split(Delimiter)

            Username(Index) = encrypt(Feild(0))
            Password(Index) = encrypt(Feild(1))        
        Loop
    End Sub        

    Private Sub LoginForm_Load(ByVal sender As Object, _
                                ByVal e As System.EventArgs) _
                            Handles Me.Load
        Call ReadUsers()
    End Sub
End Class

Answer 1

Try replacing this code:

For Each I In Username 

            If UserNameTextBox.Text = Username(Index) Then 
                UserAdd.Show() 
                Me.Hide() 
                If PasswordTextBox.Text = Password(Index) Then 
                    MessageBox.Show("Correct Password") 
                Else 
                    MessageBox.Show("Invalid Password, Sorry") 
                End If 
            Else : MessageBox.Show("Invalid Username, Sorry") 
            End If 

Next 

with this code:

For Each I In Username 

      if Username(i) is not null then

            If UserNameTextBox.Text = Username(Index) Then 
                UserAdd.Show() 
                Me.Hide() 
                If PasswordTextBox.Text = Password(Index) Then 
                    MessageBox.Show("Correct Password") 
                Else 
                    MessageBox.Show("Invalid Password, Sorry") 
                End If 
            Else : MessageBox.Show("Invalid Username, Sorry") 
            End If 
      else
            ....handle empty string
      end if

        Next 

Answer 2

Which next are yout refering to?

In your second for, define that is I. That may not solve the problem, but this is definitly a better practive.

Is it possible that your data constains a 'null' character (chr(0))?

Mid will return null if it reaches the end of the string , but it doesn't look like this would happen to you.

Nevertheless, you might want to use String.Substring instead of mid. It's a function found with the string object.

Answer 3

I'll take a guess that it's the "For Each I In Username" loop inside LoginButton_Click that's causing you the problem?

I'm guessing at this loop as the type of variable "I" does not appear to be declared, so it would be type Object by default, matching the error "Object reference not set to an instance of an object".

Answer 4

1. Sub ReadUsers(), uses the locally defined variables for Username, Index and Password. Remove these lines from Sub ReadUsers().

    Dim Username() As String Dim Password() As String Dim Index As Integer 

2. At your class level.

   A. Add this Imports to the top of the file:

     Imports System.Collections.Generic 

   B. Change your String array definitions to List(of String)

     Dim Username As List(Of String) 

   C. Then you no longer need to Redim. Just:

     Username.add(encrypt(Feild(0))) 

3. Loop on the count instead of item:

     For i as integer = 0 to Username.length - 1 If UserNameTextBox.Text = Username(i) Then ... Next 

And finally, here's your code:

Imports System.IO
Imports System.Collections.Generic
Public Class LoginForm

    ' At the Class level Dim is equivalent to Private
    Private Username As List(Of String)
    Private Password As List(Of String)
    Private Index As Integer

    Public Function encrypt(ByVal data As String) As String
        Dim answer As String = ""
        Dim I As Integer
        data = RTrim(data)
        If Mid(data, 1, 1) <> Chr(0) Then
            For I = 1 To Len(data)
                answer = answer + Chr(Asc(Mid(data, I, 1)) Xor 23)
                ' Xor 23 is a simple encription cipher, a string can be 
                ' encrypted or de-encrypted by the value following the Xor 
                'i.e. "23" '
            Next I
        End If
        encrypt = answer
    End Function

    Private Sub LoginButton_Click(ByVal sender As System.Object, _
                                  ByVal e As System.EventArgs) _
                                  Handles LoginButton.Click

        For i As Integer = 0 To Username.length - 1

            If UserNameTextBox.Text = Username(i) Then
                UserAdd.Show()
                Me.Hide()
                If PasswordTextBox.Text = Password(i) Then
                    MessageBox.Show("Correct Password")
                Else
                    MessageBox.Show("Invalid Password, Sorry")
                End If
            Else : MessageBox.Show("Invalid Username, Sorry")
            End If
        Next
    End Sub

    Public Sub ReadUsers()
        Dim CurrentFileReader As StreamReader
        Dim FileName, Line As String
        Dim Delimiter As Char = ","
        Dim Feild() As String


        FileName = "C:\Computing\Projects\Login\Users.txt"  'location of 
        'user file 
        CurrentFileReader = New StreamReader(FileName)

        Do Until CurrentFileReader.EndOfStream

            Line = CurrentFileReader.ReadLine
            If Line = Nothing Then
                Exit Do
            End If

            Feild = Line.Split(Delimiter)

            Username.Add(encrypt(Feild(0)))
            Password.add(encrypt(Feild(1)))
        Loop
    End Sub

    Private Sub LoginForm_Load(ByVal sender As Object, _
                                ByVal e As System.EventArgs) _
                            Handles Me.Load
        Call ReadUsers()
    End Sub
    End Class