public override void ItemAdded(SPItemEventProperties properties)
        {
            SPWeb web = properties.OpenWeb();
            bool allowUpdates = web.AllowUnsafeUpdates; //store original value
            this.EventFiringEnabled = false;
 
            try
            {
                web.AllowUnsafeUpdates = true;
                /*获取列表*/
                SPList list = web.Lists[properties.ListId];
                var highestvalue = 0;
                var objQuery = new SPQuery
                                   {
                                       Query ="<OrderBy><FieldRef Name='" + 栏名称 
                                       +"' Ascending='False' /></OrderBy><RowLimit>1</RowLimit>",
                                       Folder = list.RootFolder
                                   };
 
                SPListItemCollection colItems = list.GetItems(objQuery);
                if (colItems.Count > 0)
                {
                    highestvalue = int.Parse(colItems[0][ColumnName].ToString());
                }
 
                var currItem = properties.ListItem;
                currItem[ColumnName] = highestvalue + 1;
                currItem.SystemUpdate(false);
                 
            }
            catch (Exception ex)
            {
                Trace.WriteLine(ex.Message);
            }
            finally
            {
                this.EventFiringEnabled = true;
                web.AllowUnsafeUpdates = allowUpdates; //恢复原值
            }
        }