How to read a processing instruction from XML string using C# .NET?

Question

I would like to read a processing instruction from some small well-formed XML chunk, based on this post: How to read processing instruction from an XML file using .NET 3.5

However, it doesn't work. I get the error, that the pi object is null. This is what I do:

XmlDocument doc = new XmlDocument();
doc.LoadXml("<root>Text <?pi 1?>blabla</root>");
Console.WriteLine(doc.ChildNodes[0].Name); // output: root

XmlProcessingInstruction pi = doc.OfType<XmlProcessingInstruction>().Where(x => x.Name == "pi").FirstOrDefault();
Console.WriteLine(pi.Value);

Parsing the XML works. When I get the error (System-NullReferenceException) in Visual Studio, I get it for line "Console.WriteLine(pi.Value);".

Where the error? How do I get/read the processing instruction?

Answer 1

You have two issues in your code.

The first is that it fails on Console.WriteLine(doc.ChildNodes[1].Name);, "root" is the first node, not the second (zero-based). But this is probably a typo.

As to the issue in question:

Enumerable.OfType() is using GetEnumerator() of XmlDocument class which is implemented by it's parent XmlNode.GetEnumerator(). The documentation states:

An IEnumerator object that can be used to iterate through the child nodes in the current node.

But you are trying to run OfType on doc, who has only a single child: root.

If you change your code to run on root's children instead, it will work as expected:

XmlProcessingInstruction pi = doc.ChildNodes[0].OfType<XmlProcessingInstruction>().Where(x => x.Name == "pi").FirstOrDefault();