处理Sharepoint中的SPListItem时，有时需要获得这个对象的"Created By"(创建者)，并且希望是SPUser类型，而不是string，这样可以进而得到该用户的权限、ID等等。
"Person or Group"(用户或组)对应的类型是SPFieldUser，怎么把SPFieldUser转成SPUser呢？
网上找了些资料，发现要获得SPUser需要借助SPFieldUserValue，借鉴了一些代码，自己试了试，写了一个通用的方法：
         SPUser GetSPUserFromSPListItemByFieldName(SPListItem spItem, string fieldName)
        {
            string userName = spItem[fieldName].ToString();
            SPFieldUser _user = (SPFieldUser)spItem.Fields[fieldName];
            SPFieldUserValue userValue = http://www.mamicode.com/(SPFieldUserValue)_user.GetFieldValue(userName); >            return userValue.User;
        }
//SharePoint中的"用户或用户组"栏,当选择了"允许多重选择"后,用对象模型SPListItemCollection["栏名"]获得到的是SPFieldUserValueCollection的对象:
SPFieldUserValueCollection users = SPListItem["栏名"] as SPFieldUserValueCollection;
如果将SPListItemCollection["栏名"]输入字符串的话,是"用户ID;#用户Name",
 一　为栏赋值有这几种方式,
SPListItem["栏名"] = SPUser实例或者SPGroup实例,但是不能赋SPUserCollection;
SPListItem["栏名"] = "用户ID;#用户Name;#用户ID;#用户Name......";
SPListItem["栏名"] = SPFieldUserValueCollection实例;(当栏设置允许多重选择为否时,赋的值都是集合中的第一个)
SPListItem["栏名"] = SPFieldUserValue实例;
 二　获取栏的值
当栏设置为"允许多重选择"为否时,不管用SPFieldUserValueCollection userValues=item["栏名"] as SPFieldUserValueCollection;还是SPFieldUserValue userValue = http://www.mamicode.com/item["栏名"] as SPFieldUserValue;得到的值都是Null;
如果想判断是否选择了"允许多重选择",可以先查看SPFieldUser字段的AllowMutipleValues属性.不过真麻烦.
实际上在该字段在"允许多重选择"为否时,字段类型是string,为是时,字段类型是SPFieldUserValueCollection.
          protected void btnOK_Click(object sender, EventArgs e)
        {
            ArrayList list = PeopleEditor1.ResolvedEntities;
            string msgTo = "";
            //获取id和显示名称
            foreach (Microsoft.SharePoint.WebControls.PickerEntity p in list)
            {
                string userId = p.EntityData["SPUserID"].ToString();
                msgTo += userId + "#";
            }
            //获取帐号
            ArrayList selectedAccoutList = PeopleEditor1.Accounts;
            string selectedAccouts2 = PeopleEditor1.CommaSeparatedAccounts;
            SPSecurity.RunWithElevatedPrivileges(delegate()
            {
                if (msgTo != "")
                {
                    for (int i = 0; msgTo.Length > 1; i++)
                    {
                        string lassmsgto = string.Empty;
                        lassmsgto = msgTo.Substring(msgTo.IndexOf("#") + 1);
                        msgTo = msgTo.Substring(0, msgTo.IndexOf("#"));
                        SPSite site = new SPSite(SPContext.Current.Site.Url);
                        SPWeb web = site.RootWeb;
                        .AllowUnsafeUpdates = true;
                        web.AllowUnsafeUpdates = true;
                        SPListItem item = web.Lists["Messageboard"].Items.Add();
                        item["ToNameID"] = msgTo;
                        item["Content"] = this.txtContent.Text.Trim();
                        item["Title"] = this.txtTitle.Text.Trim();
                        SPFieldUserValue userValue = http://www.mamicode.com/new SPFieldUserValue(web,msgTo); >                        item["ToName"] = userValue;
                        item.Update();
                        web.AllowUnsafeUpdates = false;
                        site.AllowUnsafeUpdates = false;
                        msgTo = lassmsgto;
                    }
                }
            });
            GridView1Show();
        }