c#office开发:Microsoft.office.Tools.Word.Controls.Button出现错误

我正在阅读一篇关于如何使用控制块在word中操作文档的教程
问题在于声明：

private Microsoft.Office.Tools.Word.Controls.Button button = null;

正在给出错误。

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Xml.Linq;
using Word = Microsoft.Office.Interop.Word;
using Office = Microsoft.Office.Core;
using Microsoft.Office.Tools.Word;
namespace SecondWordAddIns {
    public partial class ThisAddIn {
        private Microsoft.Office.Tools.Word.Controls.Button button = null; // error here
        private RichTextContentControl richTextControl = null;
        private void ThisAddIn_Startup(object sender, System.EventArgs e) { }
        private void ThisAddIn_Shutdown(object sender, System.EventArgs e) { }
        internal void toggleButton() { }
        #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
    }
}

错误消息：

错误1命名空间"Microsoft.Office.Tools.Word"中不存在类型或命名空间名称"Controls"（是否缺少程序集引用？）C:''Users''root''Documents''Visual Studio 2010''Projects''WordAddIn2''WordAddIn%2''ThisAddIn.cs 14 45 WordAddIn2