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How to use linq2Xml without the possibility of exception?

I wrote this simple linq-to-xml query, and it seems that the null exception cannot be thrown using linq syntax. Am I using this incorrectly? What should be the correct (and short) Linq2Xml syntax?

Linq2Xml request

var userData = queryUserResponseData.Elements("user") .Single(u => u.Element("username").Value == userName);

XML

 <data> <user> <username>User1</username> <userid>123</userid> </user> <user> <username>User2</username> <userid>456</userid> </user> <user> <userid>999</userid> </user> </data>
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5 answers

XElement and XAttribute have some explicit conversion operators to convert their value to specific types. They are so useful that they return null when an element or attribute is missing.

 var userData = queryUserResponseData.Elements("user").Single(u => (string)u.Element("username") == userName);
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From your comment on Ahmad Mahad’s answer:

The problem is actually in u.Element("username").Value , where Resharper notifies me of a possible null exception

It looks like you can worry about a potential rather than a real problem. You know that your data will mean that you will always return 1 result, however ReSharper does not have access to your data, so it emphasizes the fact that if there were no results, it throws an exception with a null reference.

You can do one of three things:

  • Ignore the warning and do nothing.

  • Recode this account to exclude the possibility of exclusion (see other answers).

  • Wrap Linq in try {} catch {} so that if the "unthinkable" happens, your program will not crash.

Only you can decide what you want to do.

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Using Single means that you expect exactly 1 result. When more results are returned, Single throws an exception. You can use First to get the first element or Last for the last. For several elements, you will need to iterate over the results and access each of them individually.

If the matching result does not exist, you can use SingleOrDefault to return null or the default value of the type used.

Is queryUserResponseData XElement or XDocument? If it's an XDocument, you need to access the XML root first, for example:

 var userData = queryUserResponseData.Root.Elements("user") .Single(u => u.Element("username").Value == userName);

Also, searching for User1 or User2 in your example will work. However, if you were looking for User3 that does not exist, Single will throw an exception. In this case, you should use SingleOrDefault:

 var userData = queryUserResponseData.Elements("user") .SingleOrDefault(u => u.Element("username").Value == "User3");
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According to your comment on Ahmad's answer, I assume that you will get a NullReferenceException when the element does not have a node. You can fix it like this:

 var userData = doc.Elements("user") .Single(u => u.Element("username") != null && u.Element("username").Value == userName);

But if this requires a node username in DTD or XSD or you are sure that all elements will have a node username, you can simply ignore the ReSharper warning.

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 var userData = queryUserResponseData.Elements("user") .Select(u => u.Element("username")) .Where(uNode => uNode != null) .Single(uName => uName.Value == userName);
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