WPF DataGrid自定义模板里面的数据怎么取 急！！！

通过 + - 对数量进行修改，不知道怎么取数量这个值，这个值是不绑定的

绑定
XG是你自己创建类里面的成员变量

xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:local="clr-namespace:cs"
mc:Ignorable="d"
Title="MainWindow" Height="350" Width="525" Loaded="Window_Loaded" >

</Grid>

namespace cs
{
///
/// MainWindow.xaml 的交互逻辑
///
public class Quantity
{
public string count { set; get; }

}
public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
    }
    List<Quantity> Qu;
    Quantity quantity;
    private void Window_Loaded(object sender, RoutedEventArgs e)
    {
        Qu = new List<Quantity>();
        quantity = new Quantity();
        quantity.count = "0";
        Qu.Add(quantity);
        dataGrid.ItemsSource = Qu;
    }

    private void Button_Click(object sender, RoutedEventArgs e)
    {
       // List<Quantity> Qu1 = new List<Quantity>();
       // Quantity  quantity = new Quantity();
        string data = Qu[dataGrid.SelectedIndex].count;
        int indata = Convert.ToInt32(data);
        indata = indata + 1;
        quantity.count = Convert.ToString(indata);
        dataGrid.ItemsSource = null;
        Qu.Add(quantity);
        dataGrid.ItemsSource = Qu;
    }


}

}