做法：

1，先添加引用COM，找 Excel

2，using Excel = Microsoft.Office.Interop.Excel;

3，

代码

        private Excel.Application m_objExcel = null;
        private Excel.Workbooks m_objBooks = null;
        private Excel._Workbook m_objBook = null;
        private Excel.Sheets m_objSheets = null;
        private Excel._Worksheet m_objSheet = null;
        private Excel.Range m_objRange = null;

        private object m_objOpt = System.Reflection.Missing.Value;

        private void button3_Click(object sender, EventArgs e)
        {
            m_objExcel = new Excel.Application();
            m_objExcel.Visible = false;
            m_objExcel.DisplayAlerts = false;

            if (m_objExcel.Version != "11.0")
            {
                MessageBox.Show("您的 Excel 版本不是 11.0 （Office 2003），操作可能会出现问题。");
                m_objExcel.Quit();
                return;
            }

            m_objBooks = (Excel.Workbooks)m_objExcel.Workbooks;

            m_objBook = m_objBooks.Open(@"E:\WebSite2\Book2.xls", m_objOpt, m_objOpt, m_objOpt, m_objOpt, m_objOpt, m_objOpt, m_objOpt, m_objOpt, m_objOpt, m_objOpt, m_objOpt, m_objOpt, m_objOpt, m_objOpt);

            m_objSheets = (Excel.Sheets)m_objBook.Worksheets;
            m_objSheet = (Excel._Worksheet)(m_objSheets.get_Item(1));
            m_objRange = m_objSheet.get_Range("B3", m_objOpt);
            m_objRange.Borders[Microsoft.Office.Interop.Excel.XlBordersIndex.xlDiagonalDown].LineStyle = Excel.XlLineStyle.xlLineStyleNone;
            m_objRange.Borders[Microsoft.Office.Interop.Excel.XlBordersIndex.xlDiagonalUp].LineStyle = Excel.XlLineStyle.xlContinuous;
            m_objRange.Borders[Microsoft.Office.Interop.Excel.XlBordersIndex.xlDiagonalUp].ColorIndex = Excel.XlColorIndex.xlColorIndexAutomatic;
            m_objRange.Borders[Microsoft.Office.Interop.Excel.XlBordersIndex.xlEdgeLeft].LineStyle = Excel.XlLineStyle.xlLineStyleNone;
            m_objRange.Borders[Microsoft.Office.Interop.Excel.XlBordersIndex.xlEdgeTop].LineStyle = Excel.XlLineStyle.xlLineStyleNone;
            m_objRange.Borders[Microsoft.Office.Interop.Excel.XlBordersIndex.xlEdgeBottom].LineStyle = Excel.XlLineStyle.xlLineStyleNone;
            m_objRange.Borders[Microsoft.Office.Interop.Excel.XlBordersIndex.xlEdgeRight].LineStyle = Excel.XlLineStyle.xlLineStyleNone;
            m_objBook.Save();

            m_objBook.Close(false, m_objOpt, m_objOpt);
            m_objExcel.Quit();

            this.DisposeExcel();
        }

 /// <summary>
        /// 释放所引用的COM对象。注意：这个过程一定要执行。
        /// </summary>
        public void DisposeExcel()
        {

            ReleaseObj(m_objSheets);
            ReleaseObj(m_objBook);
            ReleaseObj(m_objBooks);
            ReleaseObj(m_objExcel);
            System.GC.Collect();
            System.GC.WaitForPendingFinalizers();
        }
        private void ReleaseObj(object o)
        {
            try
            {
                System.Runtime.InteropServices.Marshal.ReleaseComObject(o);
            }
            catch { }
            finally { o = null; }
        }