protected override void OnPreInit(EventArgs e)
    {
        base.Master.Init += new EventHandler(Masters_Init);
    }

    void Masters_Init(object sender, EventArgs e)
    {
    //code
    }

public override void OnInit(EventArgs e)
{
  // code before base oninit
  base.OnInit(e);
  // code after base oninit
}

public class Page1 : Page
{
  public Page1 : base() {
    PreInit += Page_PreInit;
  }
  void Page_PreInit(object sender, EventArgs e)
  {
    Master.Init += Master_Init;
  }
  void Master_Init(object sender, EventArgs e)
  {
    //code
  }
}

protected override void OnPreInit(EventArgs e)
{
  base.OnPreInit(e);
  base.Master.Init += new EventHandler(Masters_Init);
}