Microsoft.Office.Interop.Excel 为什么老是报错

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;

using System.Data.SqlClient;
using Microsoft.Office.Interop.Excel;
using System.Reflection;
using Microsoft.Office.Core;

程序老是说命名空间"Microsoft Office"中不存在类型或命名空间名称"Interop" 那我该怎么正确添加Microsoft.Office.Interop.Excel;

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