如何在使用SharePoint Web服务时仅返回列表的某些列

SharePoint Web Service有一个具有以下签名的方法：

 public System.Xml.XmlNode GetListItems(string listName, string viewName, System.Xml.XmlNode query, System.Xml.XmlNode viewFields, string rowLimit, System.Xml.XmlNode queryOptions, string webID) {
            object[] results = this.Invoke("GetListItems", new object[] {
                        listName,
                        viewName,
                        query,
                        viewFields,
                        rowLimit,
                        queryOptions,
                        webID});
            return ((System.Xml.XmlNode)(results[0]));
        }

我只想返回列表中的"ID"answers"Title"列，例如"ProductNames"，而不是返回所有列。

如何使用此Web方法实现只返回某些列？

我使用以下方法返回所有列：

public XmlNode GetListItems(string listName, XmlElement camlQuery)
        {
            sp.Lists listService = this.getSPListService();
            XmlDocument xmlDoc = new XmlDocument();
            XmlNode ndViewFields = xmlDoc.CreateNode(XmlNodeType.Element, "ViewFields", "");
            XmlElement queryOptions = xmlDoc.CreateElement("QueryOptions");
            XmlElement foldersEl = xmlDoc.CreateElement("Folder");
            if (camlQuery == null) // override default view filters
            {
                camlQuery = xmlDoc.CreateElement("Query");
                camlQuery.InnerXml = "<Where><Gt><FieldRef Name='ID'/><Value Type='Number'>0</Value></Gt></Where>";
            }
            queryOptions.AppendChild(foldersEl);
            return listService.GetListItems(listName, null, camlQuery, ndViewFields, int.MaxValue.ToString(), queryOptions, null);
        }

所以，现在我想写一个新的方法，比如：

public XmlNode GetListItems(string listName, XmlElement camlQueryForFilteringRows, bool includeAllColumns, string[] columnNamesToInclude)         
{
}

并用称之为

var listItems = GetListItems("ProductNames", null, new [] {"ID", "Title"});

我尝试将以下结构添加到ViewFields变量中，但它仍然返回所有列：

<ViewFields>
   <FieldRef Name="ID" />
   <FieldRef Name="Title" />
</ViewFields>

谢谢，

XmlNode ndViewFields = xmlDoc.CreateNode(XmlNodeType.Element, "ViewFields", "");
XmlElement queryOptions = xmlDoc.CreateElement("QueryOptions");
if (!includeAllColumns)
{
    ndViewFields.InnerXml =
        string.Join(string.Empty,
            columnNamesToInclude
            .Select(t1 => string.Format("<FieldRef Name='"{0}'" />", t1))
            .ToArray());
    queryOptions.InnerXml = "<IncludeMandatoryColumns>FALSE</IncludeMandatoryColumns>";
}