跳过字符串，如果无效

Question

我正在编写一个调用API，接收XML并显示跟踪的程序，同时显示了使用跟踪信息打开Web浏览器。字符串将为null，如何显示错误而不是抛出null参考异常？

//ONLINE PORTAL TRACKING
string ConNo = Con.Text;

XmlDocument doc1 = new XmlDocument();
doc1.Load(url1 + ConNo + url2 + APIKEY);
XmlElement root = doc1.DocumentElement;
XmlNodeList nodes = root.SelectNodes("/Response/Detail/Data");

foreach (XmlNode node in nodes)
{
    string ConsignmentNumber = node["ConNo"].InnerText;
    string tracking_url = node["tracking_url"].InnerText;
    string Lifts = node["Lifts"].InnerText;
    string TrackingID = node["TrackingID"].InnerText;
    string JobStatus = node["Status"].InnerText;
    string Barcodes = node["Barcodes"].InnerText;
    string Weight = node["Weight"].InnerText;
    string DateMan = node["DateEntered"].InnerText;

    this.ConsignmentNumber.Text = ConsignmentNumber;
    this.Lifts.Text = Lifts;
    // this.TrackingURL.Text = tracking_url;
    this.TrackingID.Text = TrackingID;
    this.JobStatus.Text = JobStatus;
    this.Barcodes.Text = Barcodes;
    this.Weight.Text = Weight;
    this.DelMan.Text = DateMan;

    System.Diagnostics.Process.Start(PortalURL1 + TrackingID);

Answer 1

如果发生错误，您可以尝试在XML响应中获取一些信息。这样的事情：

XmlNodeList nodes = root.SelectNodes("/Response/Detail/Data");
if (nodes == null)
{
    // Look the root content to see if contains a node with the error. 
    // In that case, do a SelectSingleNode. Here is an example with a 
    // sample path (is not real, I don't know anything about the XML 
    // error response schema)
    XmlNode node = root.SelectSingleNode("/Response/Detail/Error");
    var error = node?.InnerText;
    if (string.IsNullOrEmpty(error))
    {
        // If for any reason we can't get any error...
        error = $"Unknown error on query: {ConNo}";
    }
}
else
{
    foreach (XmlNode node in nodes)
    {
        // ...
    }
}