Will this do what you need:

Friend Sub MasKit(ByVal cols As DataColumnCollection)
        For Each Ctrl In Me.Controls
            If (TypeOf Ctrl Is IControlMaskit) Then
                CType(Ctrl, IControlMaskit).MaskIt(cols)
            End If
        Next
    End Sub

   Friend Sub MaskIt(ByVal cols As DataColumnCollection) Implements IControlMaskit.MaskIt
        If Me.Mask <> "" Then
            Exit Sub
        End If

        Dim B As Binding = Me.DataBindings.Item(0)

        Dim O As Object = B.DataSource
        If Not (TypeOf O Is DataTable) Then
            O = CType(O, DataView).Table
        End If

        Dim c As DataColumn = CType(O, DataTable).Columns(B.BindingMemberInfo.BindingField)

        Select Case c.DataType
            Case GetType(System.String)
                ' Possible error since MaxLength can be negative.....
                Me.Mask = ">" & New String("A", c.MaxLength)
            Case GetType(System.DateTime)
                Me.Mask = ">##/AAA/##"
        End Select
    End Sub

If not then at least it's a bit shorter (s)

>

>    Private Sub frmXXX_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
>        Me.MasKit(Me.XXXDataset.XXX.Columns)
>    End Sub
>

>
>so, cols is really Me.XXXDataset.XXX.Columns is a DataColumnCollection, belonging to a datatable ... I think.
>
>in my baseform I have :
>

>    Friend Sub MasKit(ByVal cols As DataColumnCollection)
>        Dim ctrl2 As IControlMaskit = Nothing
>        For Each Ctrl As Control In Me.Controls
>            Dim lApplicable As Boolean = True
>            Try
>                ctrl2 = Ctrl
>            Catch 
>                lApplicable = False
>            End Try
>            If lApplicable Then
>                ctrl2.MaskIt(cols)
>            End If
>        Next
>    End Sub
>

>
>and at the txtBase level (inferring from your wise advice ...)
>
>

>    Friend Sub MaskIt(ByVal cols As DataColumnCollection) Implements IControlMaskit.MaskIt
>        If Me.Mask <> "" Then
>            Exit Sub
>        End If
>
>        Dim B As Binding = Me.DataBindings.Item(0)
>        Dim t As DataTable = CType(B.DataSource, DataTable)
>        Dim c As DataColumn = Nothing
>
>        If t Is Nothing Then
>            Dim t1 As DataView = B.DataSource
>            c = t1.Columns(B.BindingMemberInfo.BindingField)
>        Else
>            c = t.Columns(Me.DataBindings.Item(0).BindingMemberInfo.BindingField)
>        End If
>
>        ' Dim o As DataColumn = cols(Me.DataBindings.Item(0).BindingMemberInfo.BindingField)
>        Dim cT As String = c.DataType.ToString
>        If cT = "System.String" Then
>            Me.Mask = ">" & New String("A", c.MaxLength)
>            Exit Sub
>        End If
>        If cT = "System.DateTime" Then
>            Me.Mask = ">##/AAA/##"
>            Exit Sub
>        End If
>        MsgBox(Me.Name & " cannot be masked yet (" & cT & ")")
>    End Sub
>

>
>Now this won't compile because "columns is not a member of 'system.data.dataview', and leaving out the dataview section does not work. (I don't think I need the dataview section). When I debug, B.Datasource does not seem to be set (in fact it returns "nothing", your "null" I guess?)
>
>I get the feeling from what I read in my frmXXX.designer.vb that the datasource of the "binding" is a dataset, whereas you seemed to assume that it should be a datatable or a dataview.
>
>That is precisely the problem I guess, because the form generates a xxxdataset that and binds the controls to a datatable that belongs the xxxDataset and is called XXX and referred to as xxxDataset.xxx. And it's the latter that I would like get a hold of from my base class textbox.