check boxes must be selected to continue asp net

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VB

ASP

I am working on a survey and a have a chechkbox list, I need at the and to prevent that tthe chechkboxes are not left unchechked.

in the asp net code I have chosed as <asp:customvalidator>

Protected Sub Button1_Click(ByVal sender As Object, ByVal e As System.EventArgs)
        For Each item As DataListItem In DataList1.Items
            If item.ItemType = ListItemType.Item Or item.ItemType = ListItemType.AlternatingItem Then
                Dim questionid As Integer
                Dim choiceid As Integer = 0
                Dim choicetext As String = ""
                questionid = CType(item.FindControl("Label3"), Label).Text
                Dim anstype As HiddenField = item.FindControl("HiddenField1")
                Select Case anstype.Value
                    Case "S"
                        Dim rbl As RadioButtonList = item.FindControl("RadioButtonList1")
                        If rbl.SelectedValue = "" Then
                            choiceid = 0
                        Else
                            choiceid = rbl.SelectedValue
                        End If
                        SaveAnswer(questionid, choiceid, "")
                    Case "M"
                        Dim cbl As CheckBoxList = item.FindControl("CheckBoxList1")
                        For i As Integer = 0 To cbl.Items.Count - 1
                            If cbl.Items(i).Selected Then
                                choiceid = cbl.Items(i).Value

                            Else
                                'MsgBox.show("Ve molime dopolnete gi site prasanja")
                            End If
                            SaveAnswer(questionid, choiceid, "")
                        Next
                    Case "T"
                        Dim txt As TextBox = item.FindControl("TextBox1")
                        choicetext = txt.Text
                        SaveAnswer(questionid, 0, choicetext)
                End Select
            End If
        Next
        DataList1.Visible = False
        Label5.Text = "Ви благодариме за учеството во анкетата!"
    End Sub