Function GetValue(ByVal o As Object) As Object
  If o is nothing OrElse o Is System.DBNull.Value Then
    Return cobj(" ")
  End If
Return o
End Function

Dim dt As New System.Data.DataTable
Dim r As System.Data.datarow
dt.Columns.Add("Column")
r = dt.NewRow
r("Column") = System.DBNull.Value 
dt.Rows.Add(r)
r = dt.Rows(0)	
MessageBox.Show  (me.GetValue(r("Column")).ToString())