我的OLEDBAccess数据库连接正确地计算了数据库中表中的行（项）数，但是当使用for循环读取其中一个字段的每个实例时，我得到一个错误，即当使用DatabaseReader获取表中第一个项的用户名（教师ID/学生ID）时，DatabaseReader（1）实例中没有数据。
错误：
System.Data.dll中出现“System.InvalidOperationException”类型的未处理异常
附加信息：行/列不存在数据。
（顺便说一句，我知道现在还没有密码验证）
Imports System.Data
Imports System.Data.OleDb
Public Class LoginForm
Const databaseLocation As String = "C:\Users\Patrick\Documents"
Private Sub OK_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles OKButton.Click
Dim enteredUsername As String = UsernameTextBox.Text
Dim enteredPassword As String = PasswordTextBox.Text
Login(enteredUsername, enteredPassword)
End Sub
Private Sub Cancel_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles CancelButton.Click
Me.Close()
End Sub
Sub Login(ByRef username As String, ByRef password As String)
Dim isUsernameVerified As Boolean
'This is the admin's username and password, and should not be shown to any other users
If username = "adminentry" And password = "iamanadmin" Then
MenuForm.Show()
Me.Close()
End If
isUsernameVerified = VerifyUsernameAndPassword(username, password)
If isUsernameVerified = True Then
MenuForm.Show()
Me.Close()
Else
LoginComboBox.Text = "Username or password not found"
End If
End Sub
Function VerifyUsernameAndPassword(ByRef usernameForVerification As String, ByRef passwordForVerification As String)
'This string tells the connection where the database is, and which provider to use
Const connectionString As String = ("Provider=Microsoft.ACE.OLEDB.12.0;Data Source=" & databaseLocation & "\ProjectDatabase.accdb")
Dim sqlQuery As String
Dim verified As Boolean
'database connection required in order to verify username and password inputs
Using connectionToDatabase As New OleDbConnection(connectionString)
Dim numberOfEntries As Integer
'Open the database connection
connectionToDatabase.Open()
If LoginComboBox.SelectedItem = "Staff login" Then
'sql code for counting number of staff ID's
sqlQuery = "SELECT COUNT(*) AS sqlQuery FROM Teacher_Table"
Else
'sql code for counting number of student ID's
sqlQuery = "SELECT COUNT(*) AS sqlQuery FROM Student_Table"
End If
Dim getNumberOfEntriesCommand As New OleDbCommand(sqlQuery, connectionToDatabase)
'executes counting sql code
numberOfEntries = getNumberOfEntriesCommand.ExecuteScalar
If LoginComboBox.SelectedItem = "Staff login" Then
'sql code for getting staff ID's
sqlQuery = "SELECT teacher_id FROM Teacher_Table"
Else
'sql code for getting student ID's
sqlQuery = "SELECT student_id FROM Student_Table"
End If
Dim loginVerificationCommand As New OleDbCommand(sqlQuery, connectionToDatabase)
'executes verification sql code
Dim databaseReader As OleDbDataReader = loginVerificationCommand.ExecuteReader()
For i = 1 To numberOfEntries
If databaseReader(i) = usernameForVerification Then
verified = True
End If
Next
End Using
If verified = True Then
Return True
Else
Return False
End If
End Function
End Class
最佳答案
对于数据读取器来说，这是错误的模型。数据读取器公开当前行，并允许您在行之间移动。databaseReader(i)将引用当前行中的字段i，而不是您的代码所认为的行i。
使用数据读取器的标准方法是
Do While databaseReader.Read
'Do something with the current row
If databaseReader(0) = usernameForVerification Then
verified = True
Exit Loop
End If
Loop
databaseReader.Close
（我没有使用vb.net，如果有语法错误，我深表歉意）
最好构建包含要搜索的值的查询。例如：
Select teacher_id From Teacher_Table Where teacher_id = ?
您需要了解参数绑定才能利用它。