XAML:

        <DataGrid AutoGenerateColumns="False" Height="275" HorizontalAlignment="Left" Margin="337,51,0,0" Name="dataGridProducts" VerticalAlignment="Top" Width="403" Foreground="#FF803E3E">
            <DataGrid.Columns>
                <DataGridTextColumn Header="ID" Width="30" />
                <DataGridTextColumn Header="Nazwa" Width="200" />
                <DataGridTextColumn Header="Kod" Width="120" />
                <DataGridTextColumn Header="Cena" Width="100" />
            </DataGrid.Columns>
        </DataGrid>


Code behind:

        private void categoryListBox_SelectionChanged(object sender, SelectionChangedEventArgs e)
        {

            SqlConnection SqlConn = new SqlConnect().ConnectToSql();
            using (SqlConn)
            {
                SqlConn.Open();
                SqlCommand fillCategories = new SqlCommand("...", SqlConn);
                SqlDataReader rdr1;
                rdr1 = fillCategories.ExecuteReader();
                while (rdr1.Read())
                {
                    Products p = new Products(rdr1.GetInt32(rdr1.GetOrdinal("ID")), rdr1.GetString(rdr1.GetOrdinal("ProductName")), rdr1.GetString(rdr1.GetOrdinal("Barcode")), rdr1.GetString(rdr1.GetOrdinal("Price")));
                    completeProductList.Add(p);
                }
            }
            dataGridProducts.ItemsSource = completeProductList;

        }