sharepoint 2010 FieldLookup 的更新方法

网上有，但是是错误的，稍微改一下，就可以了

第一个方法是要更新的item，其中 _fieldname 为要更新的lookup字段名字，_lookupValue为要更新的值。

public void SetFieldValueLookup(SPListItem _item, string _fieldname, string _lookupValue)
        {
            if (_item != null)
            {
                SPFieldLookup field = _item.Fields.GetFieldByInternalName(_fieldname) as SPFieldLookup;
                _item[_fieldname] = GetLookupValue(_item.Web, field, _lookupValue);
                _item.Update();
            }
            else
            {
                _item[_fieldname] =null;
            }
        }

        public SPFieldLookupValue GetLookupValue(SPWeb _web, SPFieldLookup _field, string _lookupvalue)
        {
            string queryFormat = @"<Where><Eq><FieldRef Name='{0}' /><Value Type='Text'>{1}</Value></Eq></Where>";
            string queryText = string.Format(queryFormat,  _field.LookupField, _lookupvalue);
            SPList lookupList = _web.Lists[new Guid(_field.LookupList)];
            SPQuery query = new SPQuery();
            query.Query = queryText;
            SPListItemCollection lookupItems = lookupList.GetItems(query);

            if(lookupItems.Count > 0)
            {
                int lookupId = Convert.ToInt32(lookupItems[0][SPBuiltInFieldId.ID]);
                return new SPFieldLookupValue(lookupId, _lookupvalue);
            }
            else
            {
                return null;
            }

        }

用这个方法就可以了._field.LookupField是关联另一个item的字段