问HtmlElement.GetAttribute("for")返回空字符串

EN

private string logLabelInputs(HtmlElementCollection collection) {
        StringBuilder sb = new StringBuilder();
        var forID = "";
        var labelValue = "";
        foreach (HtmlElement item in collection) {
            if (item.TagName.ToLower() == "label") {
                forID = item.GetAttribute("for");
                forID = forID == "" ? item.GetAttribute("FOR") : forID;
                labelValue = item.InnerText;
            }
            if (item.TagName.ToLower() == "input") {
                if(item.Name.Contains(forID)) {
                    sb.Append(labelValue + ": " + item.Name + "\r\n\r\n");
                }
            }
            if (item.All.Count > 0) {
                sb.Append(logLabelInputs(item.All));
            }               
        }
        return sb.ToString();
    }

private void button4_Click(object sender, EventArgs e) {
        HtmlElementCollection tagsCollection = webBrowser1.Document.GetElementsByTagName("form");
        textBox1.Text = logLabelInputs(tagsCollection);
    }