不能设置iHTMLVEndoB2::FROM元素

我正试图在InternetExplorer扩展中生成合成的JavaScript事件,并且在获取FromElement属性时遇到问题。以下是我的代码摘录:

MsHtml.IHTMLDocument4 doc4 = ... // the document object
Object o = null;
MsHtml.IHTMLEventObj2 eObj = 
    (MsHtml.IHTMLEventObj2)doc4.CreateEventObject(ref o);

// string that specifies the from element, e.g. "document.getElementById('id1')":
string locator = ... 
object from = doc4.Script.GetType().InvokeMember("eval", 
                                                 BindingFlags.InvokeMethod, 
                                                 null, 
                                                 doc4.Script, 
                                                 new object[] { locator });

// from now holds a ref to an object that implements the IHTMLElement interface
eObj.fromElement = from;
IHTMLElement el = eObj.fromElement;
// el == null

我在这里做错什么了?fromeElement应该等于from,但似乎没有设置。