c# HTML敏捷包与IF语句

我有这段代码，我需要运行一个if语句来设置变量的值。问题是，当第一个条件为空时，它就失败了。有人能告诉我我做错了什么吗?

IF语句在foreach循环中，每次迭代向列表中添加一个值。

非常感谢!

 string result = string.Empty;
        HttpWebRequest request = (HttpWebRequest)WebRequest.Create(url);
        request.Method = "GET";
        using (var stream = request.GetResponse().GetResponseStream())
        using (var reader = new StreamReader(stream, Encoding.UTF8))
        {
            result = reader.ReadToEnd();
        }
        HtmlAgilityPack.HtmlDocument doc = new HtmlAgilityPack.HtmlDocument();
        doc.Load(new StringReader(result));
        HtmlNode root = doc.DocumentNode;
        string itemdesc = doc.DocumentNode.SelectSingleNode("//h1[@class='producttitle']").InnerText;
        HtmlNodeCollection nodes = doc.DocumentNode.SelectNodes("//div[@class='resultsset']/table/tbody[@class='result']/tr");
        List<string> sellers = new List<string>();
        List<string> prices = new List<string>();
        foreach (HtmlNode node in nodes)
        {
            string seller = string.Empty;
                if(node.SelectSingleNode(".//ul[@class='sellerInformation']/img").GetAttributeValue("alt", string.Empty) != null)
                {
                    seller = node.SelectSingleNode(".//ul[@class='sellerInformation']/img").GetAttributeValue("alt", string.Empty);
                }
                else if (node.SelectSingleNode(".//ul[@class='sellerInformation']/a/img").GetAttributeValue("alt", string.Empty) != null)
                {
                    seller = node.SelectSingleNode(".//ul[@class='sellerInformation']/a/img").GetAttributeValue("alt", string.Empty);
                }
                else
                {
                    seller = node.SelectSingleNode(".//ul[@class='sellerInformation']/li/div/span/a/b").InnerText;
                }

            sellers.Add(seller);
            string price = node.SelectSingleNode(".//span[@class='price']").InnerText;
            prices.Add(price);
        }