思路为：全选中datagrid表格中的内容，直接拷贝出来。
在导出按钮的Click事件中写入以下代码：

 DataGrid dataGrid = paramArray as DataGrid;
            string fileName = System.AppDomain.CurrentDomain.BaseDirectory+"开关量" +DateTime.Now.ToString("yyyy-MM-dd HHmmss")+".csv";     
                string strFormat = fileName;
                dataGrid.SelectAllCells();
                dataGrid.ClipboardCopyMode = DataGridClipboardCopyMode.IncludeHeader;
                ApplicationCommands.Copy.Execute(null, dataGrid);
                dataGrid.UnselectAllCells();
                string result = (string)System.Windows.Clipboard.GetData(System.Windows.DataFormats.CommaSeparatedValue);

                File.AppendAllText(strFormat, result, UnicodeEncoding.UTF8); Clipboard.Clear();//清空粘贴板
                MessageBox.Show("导出成功");
                           //注释部分是导出后提示是否打开查看功能
            //    var messageBoxResult = System.Windows.MessageBox.Show("导出成功，是否查看报告？", "提示",
            //           MessageBoxButton.YesNo, MessageBoxImage.Information, System.Windows.MessageBoxResult.No, System.Windows.MessageBoxOptions.ServiceNotification);
            //if (messageBoxResult == MessageBoxResult.Yes)
            //{
            //    Process.Start(fileName);
            //}