Question

我做了一个测验，该测验应该将学生姓名和分数保存到访问数据库中的正确表中。有3个表。我试图连接和编码数据库，但我不断收到以下错误消息：

未处理的类型＆＃39; System.InvalidCastException＆＃39;发生在Microsoft.VisualBasic.dll中附加信息：从字符串＆＃34; INSERT INTO Class1转换（[StudentScor＆＃34;键入＆＃39; Double＆＃39;无效。

我试图找到这个错误的解决方案，但是我并不完全理解。这是我的保存按钮代码，它应该将名称和分数保存到正确的表中：

Private Sub btnSave_Click(sender As Object, e As EventArgs) Handles btnSave.Click
    StudentClass = cbSelectClass.SelectedItem
    Provider = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source="
    DataFile = IO.Path.Combine(Application.StartupPath, "StudentScores.accdb")
    ConnString = Provider & DataFile
    If StudentClass = "Class1" Then
        Strng = "INSERT INTO Class1 ([StudentScores], [Score]) Values('" + CType(txtName.Text, String) + CType(Score, Integer)
    ElseIf StudentClass = "Class2" Then
        Strng = "INSERT INTO Class2 ([Student Scores], [Score]) Values('" + CType(txtName.Text, String) + CType(Score, Integer)
    ElseIf StudentClass = "Class3" Then
        Strng = "INSERT INTO Class3([StudentScores], [Score]) Values('" + CType(txtName.Text, String) + CType(Score, Integer)
    End If
    MyConnection.ConnectionString = ConnString
    Dim cmd As OleDbCommand = New OleDbCommand(Strng, MyConnection)
    MyConnection.Open()
    Try
        cmd.ExecuteNonQuery()
        cmd.Dispose()
        MyConnection.Close()
        txtName.Clear()
    Catch ex As Exception
        MsgBox(ex.Message)
        cmd.Dispose()
        MyConnection.Close()
    End Try
    Me.Close()
End Sub

Answer 1

你有很多不必要的CType继续进行。 Textbox.Text已经是string类型。您不希望将Score转换为Integer，因为您正在连接到字符串。此外，习惯了String.Format函数，因为它将帮助您编写可读性并帮助您找到错误命名的DB列名称。不要害怕空白区域的可读性：

Private Sub btnSave_Click(sender As Object, e As EventArgs) Handles btnSave.Click

    Const SQL_INSERT As String = "INSERT INTO {0} (StudentScores, Scores) VALUES ('{1}', {2})"

    Provider = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source="
    DataFile = IO.Path.Combine(Application.StartupPath, "StudentScores.accdb")
    ConnString = Provider & DataFile

    Dim sql As String = String.Format(SQL_INSERT, cbSelectClass.SelectedItem, txtName.Text.Trim, Score.ToString)

    MyConnection.ConnectionString = ConnString
    Dim cmd As OleDbCommand = New OleDbCommand(sql, MyConnection)
    MyConnection.Open()
    Try
        cmd.ExecuteNonQuery()
        cmd.Dispose()
        MyConnection.Close()
        txtName.Clear()
    Catch ex As Exception
        MsgBox(ex.Message)
        cmd.Dispose()
        MyConnection.Close()
    End Try
    Me.Close()
End Sub

现在，您必须查看SQL注入以更正它，并且您应该使用Using语句来初始化Connection和Command对象。

＆＃39;未处理的类型＆＃39; System.InvalidCastException＆＃39;发生在Microsoft.VisualBasic.dll＆＃39;错误信息？

1 个答案: