Question

我在VB.Net中收到以下错误。

“对象引用未设置为对象的实例”

它突出显示For循环结束时的“下一步”。

任何帮助都会很棒。

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

尝试替换此代码：

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

使用此代码：

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

您接下来要参考哪个？

在你的第二个中，定义那个是I.这可能无法解决问题，但这绝对是一个更好的实践。

您的数据是否可能构成一个'null'字符（chr（0））？

如果Mid到达字符串的末尾，则返回null，但看起来不会发生这种情况。

尽管如此，您可能希望使用String.Substring而不是mid。这是一个用字符串对象找到的函数。

Answer 3

我会猜测这是在LoginButton_Click里面的“For Each I In Username”循环导致你出现问题吗？

我猜这个循环因为变量“I”的类型似乎没有被声明，所以它默认是Object类型，匹配错误“Object reference not set to a object of object”。

Answer 4

Sub ReadUsers（）使用本地定义的变量用于用户名，索引和密码。从Sub ReadUsers（）中删除这些行。
```
Dim Username() As String
Dim Password() As String
Dim Index As Integer
```
在班级。

一个。将此导入添加到文件的顶部：
```
    Imports System.Collections.Generic
```
B中。将String数组定义更改为List（of String）
```
    Dim Username As List(Of String)
```
℃。然后你不再需要Redim了。只是：
```
    Username.add(encrypt(Feild(0)))
```

循环计数而不是项目：

    For i as integer = 0 to Username.length - 1


            If UserNameTextBox.Text = Username(i) Then 
            ...

        Next

最后，这是你的代码：

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

Visual Basic - 未将对象引用设置为对象的实例

4 个答案: