如何在sharepoint列表中添加多个查找值。在sharepoint中添加项目时

我的任务有多个relateddocId，例如RelatedDoc=“3,5,2,6”，现在我需要将multiplevalue添加到任务列表中名为related Document的sharepoint查找字段中。如何添加这个

参考见下面我的代码

   private static void CreateItem(SPWeb web, SPList TaskList, string FolderURL, string ItemName, string RelatedDoc)
    {
        var ParentURL = string.Empty;
        if (!TaskList.ParentWebUrl.Equals("/"))
        {
            ParentURL = TaskList.ParentWebUrl;
        }
        SPListItem _taskList = TaskList.AddItem(ParentURL + FolderURL, SPFileSystemObjectType.File, null);
        _taskList["Title"] = ItemName;
        string DocName = "4,6,3,6";//Document ref id.
        SPFieldLookupValue lookupvalue = new SPFieldLookupValue();

        if (DocName != "")
            lookupvalue = new SPFieldLookupValue(RelatedDoc, DocName);
        _taskList["EYRelatedSharedDocument"] = lookupvalue;
        _taskList.Update();

    }

SPFieldLookupValueCollection documents = new SPFieldLookupValueCollection();

foreach ( ... )
{
    documents.Add(new SPFieldLookupValue(documentId, documentTitle));
}

_taskList["EYRelatedSharedDocument"] = documents;
_taskList.Update();