确保或验证学生只能使用我的投票系统投票一次。 -VB.Net

Question

我希望有一个投票系统，注册学生只能投票一次。我有这个代码，我使用注册的Firstname和Surname作为他们的登录访问。你能帮助我们，我怎么可能让投票的学生已经损害了他/她两次投票的权利？谢谢。

    Private Sub MetroButton2_Click(sender As Object, e As EventArgs) Handles MetroButton2.Click
    Dim sql As String

    If FirstName2MetroTextBox.Text = "" Then
        MsgBox("Type in your First Name.")
    ElseIf SurnameMetroTextBox.Text = "" Then
        MsgBox("Type in your Surname.")
    Else

        Try
            If FirstName2MetroTextBox.Text = "" And SurnameMetroTextBox.Text = "" Then
                MsgBox("First Name and Surname doesn't match.")
            Else
                sql = "select * from student_registrants where Firstname = '" & FirstName2MetroTextBox.Text & "' and Surname = '" & SurnameMetroTextBox.Text & "'"
                With cmd
                    .Connection = conn
                    .CommandText = sql
                End With
                Timer1.Start()






                da.Dispose()
            End If

        Catch ex As Exception
            MsgBox(ex.Message)
        End Try
        conn.Clone()

    End If
End Sub

它最终将登录我的投票表格。在他/她选择了他/她想要的候选人之后，我制作了这段代码。

    Private Sub TileItem26_ItemClick(sender As Object, e As DevExpress.XtraEditors.TileItemEventArgs) Handles ConfirmTileItem.ItemClick
    If PresidentMetroLabel.Text = "" Then
        MsgBox("Choose your next President.")
    ElseIf VicePresidentMetroLabel.Text = "" Then
        MsgBox("Choose your next Vice President.")
    ElseIf SecretaryMetroLabel.Text = "" Then
        MsgBox("Choose your next Secretary.")
    ElseIf AuditorMetroLabel.Text = "" Then
        MsgBox("Choose your next Auditor.")
    ElseIf Senator1MetroLabel.Text = "" Then
        MsgBox("Choose your next Senator(s).")
    ElseIf Senator2MetroLabel.Text = "" Then
        MsgBox("Choose your next Senator(s).")
    ElseIf Senator3MetroLabel.Text = "" Then
        MsgBox("Choose your next Senator(s).")
    ElseIf Senator4MetroLabel.Text = "" Then
        MsgBox("Choose your next Senator(s).")

    Else
        Try
            conn.ConnectionString = ("server=localhost;user id=root;password=;database=university_election")
            conn.Open()
            With cmd

                .Connection = conn
                .CommandText = "UPDATE `election_tally_presidents` SET Votes = Votes + 1 WHERE President_ID = '" & presidentint & "'; UPDATE `election_tally_vicepresidents` SET Votes = Votes + 1 WHERE VicePresident_ID = '" & vicepresidentint & "'; UPDATE `election_tally_secretary` SET Votes = Votes + 1 WHERE Secretary_ID = '" & secretaryint & "'; UPDATE `election_tally_auditor` SET Votes = Votes + 1 WHERE Auditor_ID = '" & auditorint & "'; UPDATE `election_tally_senators` SET Votes = Votes + 1 WHERE Senator_ID = '" & senator1int & "'; UPDATE `election_tally_senators` SET Votes = Votes + 1 WHERE Senator_ID = '" & senator2int & "'; UPDATE `election_tally_senators` SET Votes = Votes + 1 WHERE Senator_ID = '" & senator3int & "'; UPDATE `election_tally_senators` SET Votes = Votes + 1 WHERE Senator_ID = '" & senator4int & "';"
                result = cmd.ExecuteNonQuery
                Timer1.Start()
                MetroProgressBar1.Show()
                MetroLabel1.Visible = True



            End With
        Catch ex As Exception
            MsgBox("Cannot register to the database: " & ex.Message)
        Finally
            conn.Dispose()

        End Try



    End If
End Sub

那么最符合我系统的代码是什么？谢谢。