c# – 委托,我不明白的事件约定

using System;

public class PriceChangedEventArgs : EventArgs
{
  public readonly decimal LastPrice;
  public readonly decimal NewPrice;

  public PriceChangedEventArgs (decimal lastPrice,decimal newPrice)
  {
    LastPrice = lastPrice; NewPrice = newPrice;
  }
}

public class Stock
{
  string symbol;
  decimal price;

  public Stock (string symbol) {this.symbol = symbol;}

  public event EventHandler<PriceChangedEventArgs> PriceChanged;

  ****protected virtual void OnPriceChanged (PriceChangedEventArgs e)
  {
    if (PriceChanged != null) PriceChanged (this,e);
  }****

  public decimal Price
  {
    get { return price; }
    set
    {
      if (price == value) return;
      OnPriceChanged (new PriceChangedEventArgs (price,value));
      price = value;
    }  
  }
}

class Test
{
  static void Main()
  {
    Stock stock = new Stock ("THPW");
    stock.Price = 27.10M;
    // register with the PriceChanged event    
    stock.PriceChanged += stock_PriceChanged;
    stock.Price = 31.59M;
  }

  static void stock_PriceChanged (object sender,PriceChangedEventArgs e)
  {
    if ((e.NewPrice - e.LastPrice) / e.LastPrice > 0.1M)
      Console.WriteLine ("Alert,10% stock price increase!");
  }
}

我不明白,为什么使用这个惯例…

****protected virtual void OnPriceChanged (PriceChangedEventArgs e)
  {
    if (PriceChanged != null) PriceChanged (this,e);
  }****

为什么我需要这种方法,为什么我要给它“this”参数？！？我不能直接用测试类中的方法PriceChanged附加该类中的事件并跳过该方法吗？