เขียน sub เพื่อเคลียร์ค่าใน Control Combobox VB.net

Private Sub LookupIssueTypeBy(ByVal str_JobtypeCode As String)
        If ds.Tables.Contains("ds_IssueType") = True Then
            ds.Tables("ds_IssueType").Clear()
        End If
     
        strSQL = ""
        strSQL = "select issueTypecode,issueTypeDesc from tblissueType where jobtypeID='" & str_JobtypeCode & "' or jobtypeID='00'  order by issueTypeCode ASC"
        da = New SqlDataAdapter(strSQL, Cn)
     
        da.Fill(ds, "ds_Issuetype")
     
        If ds.Tables("ds_Issuetype").Rows.Count <> 0 Then
            With cmbIssueType
                .DataSource = ds.Tables("ds_Issuetype")
                .ValueMember = "IssueTypecode"
                .DisplayMember = "IssueTypedesc"
                .SelectedIndex = 0
            End With
        End If
    End Sub
    Private Sub LookupIssueSystemBy(ByVal str_IssuetypeCode As String)

        If ds.Tables.Contains("ds_IssueSystem") = True Then
            ds.Tables("ds_IssueSystem").Clear()
        End If
      
        strSQL = ""
        strSQL = "select sysCode,sysDesc from tblissueSystem where issuetypeCode='" & str_IssuetypeCode & "' or issuetypeCode='00' order by sysCode ASC"
        da = New SqlDataAdapter(strSQL, Cn)
    
        If ds.Tables("ds_IssueSystem").Rows.Count <> 0 Then
            With cmbissueSystem
                .DataSource = ds.Tables("ds_IssueSystem")
                .ValueMember = "syscode"
                .DisplayMember = "sysdesc"
                .SelectedIndex = 0
            End With
        End If
    End Sub
    Private Sub LookupIssueTopicBy(ByVal str_sysCode As String)

        If ds.Tables.Contains("ds_IssueTopic") = True Then
            ds.Tables("ds_IssueTopic").Clear()
        End If
      
        strSQL = ""
        strSQL = "select TopicCode,TopicDesc from tblissueTopic where sysCode='" & str_sysCode & "' or sysCode='00'  order by topicCode ASC"
        da = New SqlDataAdapter(strSQL, Cn)
      
        If ds.Tables("ds_IssueTopic").Rows.Count <> 0 Then
            With cmbIssueTopic
                .DataSource = ds.Tables("ds_IssueTopic")
                .ValueMember = "Topiccode"
                .DisplayMember = "Topicdesc"
                .SelectedIndex = 0
            End With
        End If
    End Sub

    Private Sub cmbIssueType_SelectedIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles cmbIssueType.SelectedIndexChanged
    
        LookupIssueSystemBy(cmbIssueType.SelectedValue.ToString)
    End Sub

    Private Sub cmbJobtype_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cmbJobtype.SelectedIndexChanged
        LookupIssueTypeBy(cmbJobtype.SelectedValue.ToString)

    End Sub

    Private Sub cmbissueSystem_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cmbissueSystem.SelectedIndexChanged
   
            LookupIssueTopicBy(cmbissueSystem.SelectedValue.ToString)

    End Sub