Best way to get InnerXml of an XElement?

Question

I wanted to see which of these suggested solutions performed best, so I ran some comparative tests. Out of interest, I also compared the LINQ methods to the plain old System.Xml method suggested by Greg. The variation was interesting and not what I expected, with the slowest methods being more than 3 times slower than the fastest.

The results ordered by fastest to slowest:

1. CreateReader - Instance Hunter (0.113 seconds)
2. Plain old System.Xml - Greg Hurlman (0.134 seconds)
3. Aggregate with string concatenation - Mike Powell (0.324 seconds)
4. StringBuilder - Vin (0.333 seconds)
5. String.Join on array - Terry (0.360 seconds)
6. String.Concat on array - Marcin Kosieradzki (0.364)

---

Method

I used a single XML document with 20 identical nodes (called 'hint'):

<hint>
  <strong>Thinking of using a fake address?</strong>
  <br />
  Please don't. If we can't verify your address we might just
  have to reject your application.
</hint>

The numbers shown as seconds above are the result of extracting the "inner XML" of the 20 nodes, 1000 times in a row, and taking the average (mean) of 5 runs. I didn't include the time it took to load and parse the XML into an XmlDocument (for the System.Xml method) or XDocument (for all the others).

The LINQ algorithms I used were: (C# - all take an XElement "parent" and return the inner XML string)

CreateReader:

var reader = parent.CreateReader();
reader.MoveToContent();

return reader.ReadInnerXml();

Aggregate with string concatenation:

return parent.Nodes().Aggregate("", (b, node) => b += node.ToString());

StringBuilder:

StringBuilder sb = new StringBuilder();

foreach(var node in parent.Nodes()) {
    sb.Append(node.ToString());
}

return sb.ToString();

String.Join on array:

return String.Join("", parent.Nodes().Select(x => x.ToString()).ToArray());

String.Concat on array:

return String.Concat(parent.Nodes().Select(x => x.ToString()).ToArray());

I haven't shown the "Plain old System.Xml" algorithm here as it's just calling .InnerXml on nodes.

---

Conclusion

If performance is important (e.g. lots of XML, parsed frequently), I'd use Daniel's CreateReader method every time. If you're just doing a few queries, you might want to use Mike's more concise Aggregate method.

If you're using XML on large elements with lots of nodes (maybe 100's), you'd probably start to see the benefit of using StringBuilder over the Aggregate method, but not over CreateReader. I don't think the Join and Concat methods would ever be more efficient in these conditions because of the penalty of converting a large list to a large array (even obvious here with smaller lists).

I think this is a much better method (in VB, shouldn't be hard to translate):

Given an XElement x:

Dim xReader = x.CreateReader
xReader.MoveToContent
xReader.ReadInnerXml

How about using this "extension" method on XElement? worked for me !

public static string InnerXml(this XElement element)
{
    StringBuilder innerXml = new StringBuilder();

    foreach (XNode node in element.Nodes())
    {
        // append node's xml string to innerXml
        innerXml.Append(node.ToString());
    }

    return innerXml.ToString();
}

OR use a little bit of Linq

public static string InnerXml(this XElement element)
{
    StringBuilder innerXml = new StringBuilder();
    doc.Nodes().ToList().ForEach( node => innerXml.Append(node.ToString()));

    return innerXml.ToString();
}

Note: The code above has to use element.Nodes() as opposed to element.Elements(). Very important thing to remember the difference between the two. element.Nodes() gives you everything like XText, XAttribute etc, but XElement only an Element.

With all due credit to those who discovered and proved the best approach (thanks!), here it is wrapped up in an extension method:

public static string InnerXml(this XNode node) {
    using (var reader = node.CreateReader()) {
        reader.MoveToContent();
        return reader.ReadInnerXml();
    }
}

Keep it simple and efficient:

String.Concat(node.Nodes().Select(x => x.ToString()).ToArray())

• Aggregate is memory and performance inefficient when concatenating strings
• Using Join("", sth) is using two times bigger string array than Concat... And looks quite strange in code.
• Using += looks very odd, but apparently is not much worse than using '+' - probably would be optimized to the same code, becase assignment result is unused and might be safely removed by compiler.
• StringBuilder is so imperative - and everybody knows that unnecessary "state" sucks.