datagrid cells

i have  a datagrid in silverlight   and i fill it
with data . the problem is    i want to display cell row 1 col1 in a
textbox but i cant get access to individual cells?
 
    Public Sub methodHandler_filldatacompleted(ByVal sender As Object, ByVal e As ServiceReference1.filldataCompletedEventArgs)
        Dim a As New List(Of datagrid.ServiceReference1.subject)
        Dim data2 As New List(Of Class1)

        dg1.ItemsSource = e.Result
        '   a = CType(dg1.ItemsSource, datagrid.ServiceReference1.subject)
        'tb.Text = a.Count


    End Sub


Public Class Class1
    Dim m_id As String
    D ...