从c# Visual Studio中的应用程序中抓取选定的ID

我有一段代码，当提交按钮被点击时执行

XmlDocument xml = new XmlDocument();
xml.Load("menu.xml");
XmlNode Quantity = xml.DocumentElement.SelectSingleNode("/MenuInfo/Meal/Quantity");
int qty = Convert.ToInt32(Quantity.InnerText);
// Deducting 1 from orginal quantity, you can use variable instead of quantity 1
Quantity.InnerText = (qty - 1).ToString();
//Finally you can retrieve modified xml from using doc.InnerXml
string modifiedXml = xml.InnerXml;
xml.Save("menu.xml");
MessageBox.Show("The order has been placed.");
Main nf = new Main();
nf.ShowDialog();
this.Close();

这段代码的作用是，当用户单击应用程序上的submit时，quantity元素将在"menu.xml"文件中减去1。然而，我的问题是，它只做第一餐。我目前在外部"menu.xml"文件中有四种不同的膳食，我希望该程序能够识别点击了哪一餐并从该餐中减去数量。例如，如果用户点击了第1餐，那么数量应该减去1，如果用户点击了第2餐，那么数量应该从该餐中减去。我知道需要做什么，即抓住id，但我不知道如何做到这一点，有人能帮助我吗?希望你能理解。

<?xml version="1.0" encoding="utf-8"?>
    <MenuInfo>
        <Meal>
          <MealID>1</MealID>
          <Food>Meal 1 (Fish and Chips)</Food>
          <Price>£4.99</Price>
          <Time>25 minutes</Time>
          <Quantity>12</Quantity>
        </Meal>
        <Meal>
          <MealID>2</MealID>
          <Food>Meal 2 (Chicken and Chips)</Food>
          <Price>£3.99</Price>
          <Time>25 minutes</Time>
          <Quantity>12</Quantity>
        </Meal>
        <Meal>
          <MealID>3</MealID>
          <Food>Meal 3 (Saussage and Chips)</Food>
          <Price>£2.99</Price>
          <Time>25 minutes</Time>
          <Quantity>12</Quantity>
        </Meal>
        <Meal>
          <MealID>4</MealID>
          <Food>Meal 4 (Burger and Chips)</Food>
          <Price>£5.99</Price>
          <Time>25 minutes</Time>
          <Quantity>12</Quantity>
        </Meal>
    </MenuInfo>

xml.DocumentElement.SelectSingleNode("//Meal[MealID = '4']/..//Quantity")