Gérer une exception

Bonjour, comment gérer une exception dans un code ayant plusieurs if a l'interieur
Voici le code:

Dim requetrx As String = " SELECT [CIRCUIT], [ERLG_TRAFIC], [BLCG_TABLERLG], '' as [NBR_CIRCUIT] " & _
" FROM [DIMENSIONNEMENT].[dbo].[ERLANG_TABLE]"
Dim grillestokcirct As New DataTable
grillestokcirct = SourceRecherche(requetrx, "DIMENSIONNEMENT")

For Each _row As DataRow In grillestokcirct.Rows


Dim CIRCUIT As String = _row.Item("CIRCUIT").ToString
Dim ERLG_TRAFIC As String = _row.Item("ERLG_TRAFIC").ToString
Dim BLCG_TABLERLG As String = _row.Item("BLCG_TABLERLG").ToString
Dim NBR_CIRCUIT As String = _row.Item("NBR_CIRCUIT").ToString
Dim valblocg1 As Double
Dim valblocg2 As Double
Dim valtraf1 As Double
Dim valtraf2 As Double
Dim valcircuit As Integer
Dim valgos As Double
Dim valtrafmoy As Double
Dim valtrafdim As Double
valgos = Me.Txtbgos.Text
valtrafmoy = Me.Txtbtrafessai.Text


For i As Integer = 0 To 807

valblocg1 = grillestokcirct.Rows(i).Item("BLCG_TABLERLG")
Try
If valgos = CDbl(valblocg1) Then

valblocg2 = grillestokcirct.Rows(i + 1).Item("BLCG_TABLERLG")
valtraf1 = grillestokcirct.Rows(i).Item("ERLG_TRAFIC")
valtraf2 = grillestokcirct.Rows(i + 1).Item("ERLG_TRAFIC")

If CDbl(valtraf2) >= valtrafmoy And CDbl(valtraf1) < valtrafmoy And valgos = CDbl(valblocg2) Then
valtrafdim = valtraf2
valcircuit = grillestokcirct.Rows(i + 1).Item("CIRCUIT")
Dim lignajoute As String() = {valcircuit, valtrafmoy, valtraf2}
Me.Dtgvnbrcircuit2.Rows.Add(lignajoute)

ElseIf CDbl(valtraf2) > valtrafmoy And CDbl(valtraf1) = valtrafmoy Then

valtrafdim = valtraf1
valcircuit = grillestokcirct.Rows(i).Item("CIRCUIT")
Dim lignajoute As String() = {valcircuit, valtrafmoy, valtraf1}
Me.Dtgvnbrcircuit2.Rows.Add(lignajoute)

End If

End If


Catch ex As Exception
MessageBox.Show(ex.Message)
End Try

Next