NullReferenceException - 在标记拼写测试时具有经典"Object reference not set to an instance of an object."

Question

最近，我一直在为我的大学做这个名为"拼写比赛项目"的课程。在这里，学生将参加从Access 2010数据库加载的测试，并将其输出到表单上。

我有一个算法来检查学生输入的拼写的准确性，它会根据条件给出2分、1分或0分。<——可以在'testword'私有函数中找到。

该算法将在一个名为"btnMarkIt"的过程中使用，本质上它只是取所有答案并计算总分20分。

代码如下:

Imports System.Data
Imports System.Data.OleDb
Public Class frmTakeTest
Dim TestConnection As New OleDbConnection
Dim DtasetTest As New DataSet
Dim DtaadpTest As New OleDbDataAdapter
Dim SqlCmdBldTest As New OleDbCommandBuilder(DtaadpTest)
Dim CurrentRowNo As Integer = -1
Dim MarkTest As Boolean = False
Dim TakeTestNow As Boolean = False
Dim ViewOnly As Boolean
Dim totalMarks As Integer
Private Sub frmTakeTest_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
    Dim DriveLetter As Char = Application.StartupPath.Substring(0, 1)
    TestConnection.ConnectionString = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=" & DriveLetter & ":Coursework - Computingprj_computer-courseJamie - CourseworkCourseworkDB.accdb"
    DtaadpTest.SelectCommand = New OleDbCommand
    DtaadpTest.SelectCommand.Connection = TestConnection
    DtaadpTest.SelectCommand.CommandText = "SELECT * FROM tbl_test"
    DtaadpTest.Fill(DtasetTest, "tblTakeTest")
    ViewOnly = True
    Protect()
End Sub
Private Sub DigitsOnly(ByRef Character As Char)
    'Validate character input: digit keys only
    If Char.IsDigit(Character) = False And Char.IsControl(Character) = False Then
        MessageBox.Show("Digits only.", "Validation Error", MessageBoxButtons.OK, MessageBoxIcon.Error)
        'Stop invalid character appearing in field
        Character = Nothing
    End If
End Sub
Private Sub DisplayAccount()
    'Purpose: Display a test when the user adds or edit a new record.
    If DtasetTest.Tables("tblTakeTest").Rows.Count > 0 Then
        txtDef1.Text = DtasetTest.Tables("tblTakeTest").Rows(CurrentRowNo).Item("Def1").ToString
        txtDef2.Text = DtasetTest.Tables("tblTakeTest").Rows(CurrentRowNo).Item("Def2").ToString
        txtDef3.Text = DtasetTest.Tables("tblTakeTest").Rows(CurrentRowNo).Item("Def3").ToString
        txtDef4.Text = DtasetTest.Tables("tblTakeTest").Rows(CurrentRowNo).Item("Def4").ToString
        txtDef5.Text = DtasetTest.Tables("tblTakeTest").Rows(CurrentRowNo).Item("Def5").ToString
        txtDef6.Text = DtasetTest.Tables("tblTakeTest").Rows(CurrentRowNo).Item("Def6").ToString
        txtDef7.Text = DtasetTest.Tables("tblTakeTest").Rows(CurrentRowNo).Item("Def7").ToString
        txtDef8.Text = DtasetTest.Tables("tblTakeTest").Rows(CurrentRowNo).Item("Def8").ToString
        txtDef9.Text = DtasetTest.Tables("tblTakeTest").Rows(CurrentRowNo).Item("Def9").ToString
        txtDef10.Text = DtasetTest.Tables("tblTakeTest").Rows(CurrentRowNo).Item("Def10").ToString
    End If
End Sub
Private Sub Protect()
    'Purpose: To enable/disable screen objects depending on whether ViewOnly is true or false
    If TakeTestNow = True Then
        txtDef1.ReadOnly = True
        txtDef2.ReadOnly = True
        txtDef3.ReadOnly = True
        txtDef4.ReadOnly = True
        txtDef5.ReadOnly = True
        txtDef6.ReadOnly = True
        txtDef7.ReadOnly = True
        txtDef8.ReadOnly = True
        txtDef9.ReadOnly = True
        txtDef10.ReadOnly = True
    Else
        txtDef1.ReadOnly = ViewOnly
        txtDef2.ReadOnly = ViewOnly
        txtDef3.ReadOnly = ViewOnly
        txtDef4.ReadOnly = ViewOnly
        txtDef5.ReadOnly = ViewOnly
        txtDef6.ReadOnly = ViewOnly
        txtDef7.ReadOnly = ViewOnly
        txtDef8.ReadOnly = ViewOnly
        txtDef9.ReadOnly = ViewOnly
        txtDef10.ReadOnly = ViewOnly
    End If
    txtAns1.ReadOnly = ViewOnly
    txtAns2.ReadOnly = ViewOnly
    txtAns3.ReadOnly = ViewOnly
    txtAns4.ReadOnly = ViewOnly
    txtAns5.ReadOnly = ViewOnly
    txtAns6.ReadOnly = ViewOnly
    txtAns7.ReadOnly = ViewOnly
    txtAns8.ReadOnly = ViewOnly
    txtAns9.ReadOnly = ViewOnly
    txtAns10.ReadOnly = ViewOnly
    txtSearch.ReadOnly = Not ViewOnly
    btnLoadTest.Enabled = ViewOnly
    btnMarkIt.Enabled = Not ViewOnly
    btnSubmit.Enabled = Not ViewOnly
    btnPrintPreview.Enabled = Not ViewOnly
    btnPrint.Enabled = Not ViewOnly
End Sub
Private Sub CloseToolStripMenuItem_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles CloseToolStripMenuItem.Click
    Dim Result = MessageBox.Show("Are you sure you want to quite? Any information entered will not be saved.", "Warning", MessageBoxButtons.YesNo, MessageBoxIcon.Warning)
    If Result = Windows.Forms.DialogResult.Yes Then
        Me.Close()
    Else
        'Do Nothing
    End If
End Sub
Private Sub btnLoadTest_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnLoadTest.Click
    'Purpose: Load an existing test from the database and output it to the user
    If txtSearch.Text = Nothing Then
        MessageBox.Show("Please enter an ID number.", "Search Error", MessageBoxButtons.OK, MessageBoxIcon.Error)
    Else
        If IsNumeric(txtSearch.Text) = True Then
            DtaadpTest.SelectCommand.CommandText = "SELECT * FROM tbl_test WHERE TestID = " & txtSearch.Text
        End If
        DtasetTest.Tables("tblTakeTest").Clear()
        DtaadpTest.Fill(DtasetTest, "tblTakeTest")
        If DtasetTest.Tables("tblTakeTest").Rows.Count = 0 Then
            MessageBox.Show("Test not found.", "Search Error", MessageBoxButtons.OK, MessageBoxIcon.Error)
            ViewOnly = True
        Else
            Dim Result = MessageBox.Show("Test found! Would you like to take it now?", "Question", MessageBoxButtons.YesNo, MessageBoxIcon.Question)
            If Result = Windows.Forms.DialogResult.Yes Then
                CurrentRowNo = 0
                TakeTestNow = True
                DisplayAccount()
                ViewOnly = False
                Protect()
            Else
                'Do Nothing
            End If
        End If
    End If
End Sub
Private Sub txtSearch_KeyPress(ByVal sender As Object, ByVal e As System.Windows.Forms.KeyPressEventArgs) Handles txtSearch.KeyPress
    DigitsOnly(e.KeyChar)
End Sub
Private Sub btnMarkIt_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnMarkIt.Click
    Dim marks(9) As Integer
    For i = 0 To 9
        marks(i) = testWord(Controls("txtAns" & i + 1).Text, DtasetTest.Tables("tblTakeTest").Rows(0).Item("Ans" & i + 1))
    Next i
    Dim msgText As String = ""
    For i = 0 To 9
        msgText += DtasetTest.Tables("tblTakeTest").Rows(CurrentRowNo).Item("Ans" & i + 1) & " " & marks(i) & " points" & vbNewLine
    Next
    MsgBox(msgText)
    totalMarks = CInt(marks(0) + marks(1) + marks(2) + marks(3) + marks(4) + marks(5) + marks(6) + marks(7) + marks(8) + marks(9))
    txtTotalMark.Text = totalMarks
End Sub
Private Function testWord(ByVal inputWord As String, ByVal actualWord As String)
    Dim lengthScore, accuracyScore, accuracyTally As Integer
    inputWord = inputWord.ToLower
    actualWord = actualWord.ToLower
    'Length
    If inputWord.Length = actualWord.Length Then
        lengthScore = 2
    ElseIf inputWord.Length = actualWord.Length + 1 Or inputWord.Length = actualWord.Length - 1 Then
        lengthScore = 1
    Else
        lengthScore = 0
    End If
    'Accuracy
    Dim inputArray() As Char = inputWord.ToCharArray
    Dim actualArray() As Char = actualWord.ToCharArray
    Dim found As Boolean
    For i = 0 To inputArray.Length - 1
        found = False
        If actualArray.Length > i Then
            If inputArray(i) = actualArray(i) Then
                accuracyTally = accuracyTally + 2
                found = True
            End If
        End If
        If found = False And i > 0 And i <= (actualArray.Length) Then
            If inputArray(i) = actualArray(i - 1) Then
                accuracyTally = accuracyTally + 1
                found = True
            End If
        End If
        If found = False And i < (actualArray.Length - 1) Then
            If inputArray(i) = actualArray(i + 1) Then
                accuracyTally = accuracyTally + 1
                found = True
            End If
        End If
    Next i
    'Add up
    Dim accMax As Integer = inputWord.Length * 2
    Dim accPerc As Integer
    If accuracyTally > 0 Then
        accPerc = CInt((accuracyTally / accMax) * 100)
    Else
        accPerc = 0
    End If
    If accPerc = 100 Then
        accuracyScore = 2
    ElseIf accPerc > 70 Then
        accuracyScore = 1
    Else
        accuracyScore = 0
    End If
    If lengthScore = 2 And accuracyScore = 2 Then
        Return 2
    ElseIf lengthScore > 0 And accuracyScore > 0 Then
        Return 1
    Else
        Return 0
    End If
End Function
End Class

*注意:错误发生在'btnMarkIt'过程中:

For i = 0 To 9
    marks(i) = testWord(Controls("txtAns" & i + 1).Text, DtasetTest.Tables("tblTakeTest").Rows(0).Item("Ans" & i + 1))
Next i

Answer 1

请尝试:

For i as Integer = 0 To 9
    marks(i) = testWord(Controls("txtAns" & i + 1).Text, DtasetTest.Tables("tblTakeTest").Rows(0).Item("Ans" & i + 1))
Next i