Having trouble with XmlDocument C#

Question

I'm having trouble getting my program to read this XML file properly, it will need to write to it also but not yet. Just note that this is only a little bit of the code

XmlDocument InstalledList = new XmlDocument();
InstalledList.Load(AppsInstalledFileNamePath);

//Sets the PackageNode to the correct part of the XmlDocument
XmlNodeList PackagesNode = InstalledList.GetElementsByTagName("installed");

foreach (XmlNode InstalledListNodes in PackagesNode)
{
    //If the title is the same as what the user typed, continue on
    if (InstalledListNodes.Attributes["title"].InnerText.Equals(packagename) == true)
    {
        BatchProcessFileName = InstalledListNodes.Attributes["uninstallername"].InnerText;
        Console.WriteLine("Filename OK");

I also took the try statement out so I wouldn't have to add the catch

Below is the XML File that it is trying to read (and later write)

<?xml version="1.0" encoding="utf-8" ?>
<packages>
  <installed>
  <sampleapp title="sampleapp" id="00001" uninstallername="sampleapp.bat" installdate="11/15/09"></sampleapp>
  <sampleapp2 title="sampleapp2" id="00002" uninstallername="sampleapp2.bat" installdate="11/16/09"></sampleapp2>
  </installed>
  <uninstalled>

 </uninstalled>
</packages>

The code runs, but it has a NullReference Exception at

InstalledListNodes.Attributes["title"].InnerText.Equals(packagename) == true

Answer 1

FYI and contrary to popular believe: InnerText is never null for attributes or elements. That means, that you don't have to check for InnerText being null at all. Empty elements and attributes have an empty string for InnerText:

XmlDocument docx = new XmlDocument();
docx.LoadXml("<root test='' />");
Debug.WriteLine("Empty: " + docx.FirstChild.InnerText);
Debug.WriteLine("Empty: " + docx.FirstChild.Attributes["test"].InnerText);

However, the attribute itself can return null if it doesn't exist. And it is useless, as was already pointed out by jrista, to use InnerText unless you really have to. Stick to Value instead.

---

Solving your issue

Many have already commented on that. You have:

XmlNodeList PackagesNode = InstalledList.GetElementsByTagName("installed");
foreach (XmlNode InstalledListNodes in PackagesNode)
{
    if (InstalledListNodes.Attributes["title"].InnerText.Equals(packagename) == true)
    ....

with the XML you showed, this will never work, as <installed> does not have attributes. Try:

XmlNodeList PackagesNode = InstalledList.GetElementsByTagName("installed");
foreach (XmlNode InstalledListNodes in PackagesNode)
{
    XmlNode someNode = InstalledListNodes.FirstChild;
    if (someNode.Attributes["title"].InnerText.Equals(packagename) == true)
    ....

which will not (yet) give the effect you want, but someNode now points to a node that actually holds the title attribute, showing you how to get rid of this error.

On to an easier solution: SelectNodes

After removing your error, I'd like to show you another way: XPath. This type of tasks is really much easier by using XPath. Here is my take at your problem (not tested):

// assuming <packages> is root:
XmlNodeList applicationNodes = InstalledList.SelectNodes("/packages/installed/*");

foreach (XmlNode applicationNode in applicationNodes)
{
     if (applicationNode.Attributes["title"].Value.Equals(packagename) == true)
     { 
     .... etc

Important note: what others have said about checking the return values of node steps is still very important. If anything of your input data is not there, your code will fail hard. Just always check every step, or use more XPath to make your life easier.

Update: the FYI
Update: added solution
Update: added alternative solution (couldn't resist)

Answer 2

Seems a small typo;

Note you wrote Attributes["unintallername"] and it should be Attributes["uninstallername"]

Answer 3

Well, I can see three different failure points in the following line:

InstalledListNodes.Attributes["title"].InnerText.Equals(packagename) == true
   ^--Null?                 ^-- Null?    ^--Null?

I would break that long chain up into more discrete elements, and check for null on each one to narrow it down:

   if (InstalledListNodes != null)
   {
       var attribute = InstalledListNodes.Attributes["title"]; 
       if (attribute != null)
       {
           if (attribute.InnerText.Equals(packagename) == true)
           {
              // ...
           }
       }
   }

That said, I would use the .Value property rather than the .InnerText property when retrieving attribute values. InnerText should be used when retrieving text content from within element open and close tags.