Filter datagrid based on textbox input

Hello..

I'm trying to filter data in a datagrid depending on what the user enters in a text box which contains an order id. I'm using WPF. I understand the following works:

       private void btnFindByOrderID_Click(object sender, RoutedEventArgs e)
        {
            if (txtFind.Text != string.Empty)
            {
                source.Filter = filter;
            }
            else
            {
                MessageBox.Show("Find text box is empty!");
                txtFind.Focus();
            }
        }
        private bool filter(object item)
        {
            int val = int.Parse(txtFind.Text);
            Order o = (item as Order);
            if (o.OrderID == val)
            {
                return true;
            }
            return false;
        }

No problem with the code above, however, I don't want all the code in the same page. I will like the code in a class from a different project, which of course is referenced in the main project. I'm trying to separate all the data from the main page. hence, I though this would work, however, as I was debugging, it seems that is sending all the records, not only the one record that is supposed to. Here is my code in separate projects:

CODE IN MY MAIN PAGE:

     private void btnFindByOrderID_Click(object sender, RoutedEventArgs e)
        {
            if (txtFind.Text != string.Empty)
            {
                 source.Filter = App.StoreDb.filterByOrderID;
            }
            else
            {
                MessageBox.Show("Find text box is empty!");
                txtFind.Focus();
            }
        }

CODE IN ANOTHER PROJECT CALLED NorthwindData, CLASS StoreDB:

      public bool filterByOrderID(object item)
        {
            Order order = item as Order;
            int ordertofind = order.OrderID;
            if (order.OrderID == ordertofind)
            {
                return true;
            }
            return false;
        }

Any help will be appreciated...

Guisselle