namespace WordVSTOTest
{
    public partial class ThisAddIn
    {
        private AddinUtilities addinUtilities;
        protected override object RequestComAddInAutomationService()
        {
            if (addinUtilities == null)
            {
                addinUtilities = new AddinUtilities();
            }
            return addinUtilities;
        }

    }
}

namespace WordVSTOTest
{
    [ComVisible(true)]
    [InterfaceType(ComInterfaceType.InterfaceIsDual)]
    public interface IAddinUtilities
    {
        void test();
    }

    [ComVisible(true)]
    [ClassInterface(ClassInterfaceType.None)]
    public class AddinUtilities : StandardOleMarshalObject, IAddinUtilities
    {
        void test()
        {
            MessageBox.Show("Hello World");
        }
    }
}

public Word.Application app;
        public IAddinUtilities utils;
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            app = new Word.Application();
            string str = app.Name;
            
            app.Visible = false;
            object addinName = "WordVSTOTest";
            COMAddIn addin = app.COMAddIns.Item(ref addinName);
            
            utils = (IAddinUtilities)addin.Object;
            utils.test();

            //如果没有这个Quit的话，任务管理器的WINWORD.exe不会退出，越来越多。
            ((Word._Application)app).Quit(); 
            System.Runtime.InteropServices.Marshal.FinalReleaseComObject(app);
        }