Visio AddAdvice抛出异常

当元素被添加到图中时，我试图处理一个事件，但是AddAdvice()抛出一个未处理的COM异常:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using Visio = Microsoft.Office.Interop.Visio;
namespace VisioAddAdviceWinForms
{
    public partial class Form1 : Form
    {
        private EventSink eventSink = null;
        public Form1()
        {
            InitializeComponent();
            this.eventSink = new EventSink();
            unchecked
            {
                axDrawingControl1.Window.EventList.AddAdvise(((short)Visio.VisEventCodes.visEvtAdd + (short)Visio.VisEventCodes.visEvtShape), this.eventSink, "", "");
            }
        }
    }
    public class EventSink : Visio.IVisEventProc
    {
        object Visio.IVisEventProc.VisEventProc(
                short eventCode,
                object source,
                int eventID,
                int eventSeqNum,
                object subject,
                object moreInfo)
        {
            Visio.IVApplication app = null;
            Visio.IVDocument doc = null;
            Visio.IVShape shape;
            try
            {
                if (source is Visio.IVApplication)
                {
                    app = (Visio.Application)source;
                }
                else if (source is Visio.IVDocument)
                {
                    doc = (Visio.Document)source;
                }
                switch (eventCode)
                {
                    case unchecked((short)Visio.VisEventCodes.visEvtAdd) +
                    (short)Visio.VisEventCodes.visEvtShape:
                        shape = (Visio.Shape)subject;
                        MessageBox.Show("added");
                        break;
                    case (short)Visio.VisEventCodes.visEvtApp +
                    (short)Visio.VisEventCodes.visEvtNonePending:
                        MessageBox.Show("pending");
                        break;
                    case (short)Visio.VisEventCodes.visEvtDel + (short)
                    Visio.VisEventCodes.visEvtShape:
                        shape = (Visio.Shape)subject;
                        MessageBox.Show("deleted");
                        break;
                    default:
                        break;
                }
            }
            catch (Exception err)
            {
                MessageBox.Show("Exception in IVisEventProc.VisEventProc: "
                        + err.Message);
            }
            return null;
        }
    }
}