IMessageFilter implementation in Excel VSTO Add-in

ThisAddin.cs

using System;
using System.Windows.Forms;
using System.Runtime.InteropServices;

namespace ExcelOleFilterAddIn
{
    public partial class ThisAddIn : ExcelOleFilterAddIn.IMessageFilter
    {
        [DllImport("ole32.dll")]
        static extern int CoRegisterMessageFilter(IMessageFilter lpMessageFilter, out IMessageFilter lplpMessageFilter);

        private IMessageFilter oldMessageFilter;

        private void RegisterFilter()
        {
            CoRegisterMessageFilter(this, out oldMessageFilter);
        }

        int IMessageFilter.HandleInComingCall(uint dwCallType, IntPtr htaskCaller, uint dwTickCount, INTERFACEINFO[] lpInterfaceInfo)
        {
            MessageBox.Show("The caller is: " + htaskCaller.ToString());
            return 1;
        }

        int IMessageFilter.RetryRejectedCall(IntPtr htaskCallee, uint dwTickCount, uint dwRejectType)
        {
            return 1;
        }

        int IMessageFilter.MessagePending(IntPtr htaskCallee, uint dwTickCount, uint dwPendingType)
        {
            return 1;
        }

        private void ThisAddIn_Startup(object sender, System.EventArgs e)
        {
            RegisterFilter();
        }

        private void ThisAddIn_Shutdown(object sender, System.EventArgs e)
        {
        }

        #region VSTO generated code

        /// <summary>
        /// Required method for Designer support - do not modify
        /// the contents of this method with the code editor.
        /// </summary>
        private void InternalStartup()
        {
            this.Startup += new System.EventHandler(ThisAddIn_Startup);
            this.Shutdown += new System.EventHandler(ThisAddIn_Shutdown);
        }
        #endregion
    }
}