'在Microsoft.VisualBasic.dll'錯誤消息中發生未處理的類型'System.InvalidCastException'異常？

Question

我已經做了一個測驗，應該將學生姓名和分數保存到訪問數據庫中的正確表格中。有3張桌子。我試圖連接和代碼數據庫，但我不斷收到以下錯誤信息：

「」類型的未處理的異常出現在Microsoft.VisualBasic.dll中 其他信息：從字符串轉換「INSERT到Class（[StudentScor」鍵入‘雙師型’是無效的。

我試圖找到這個錯誤的解決方案不過，我不完全理解。這裏是我的保存按鈕，應該保存代碼名稱和分數到正確的表格：

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轉型正在進行。 Textbox.Text已經是字符串類型。您不希望將分數轉換爲整數，因爲您將連接到一個字符串。另外，習慣String.Format函數，因爲它有助於代碼的可讀性，並幫助您找到錯誤指定的DB列名。不要爲可讀性害怕的空白：

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 

現在，你必須尋找到SQL注入糾正，你應該使用using語句來初始化連接和Command對象。