C#’s foreach ruined my afternoon

How I stumbled into a breaking change between C# 4.0 & 5.0

Forest Fire” by CIFOR is licensed under CC BY-NC-ND 2.0

The other afternoon I ran into some nightmarish debugging with the following code:

(I know, I know, I wish I could be using TPL but in this case I couldn’t)

On my local machine, the code above ran and gave me my expected console output of 123 (your results may vary depending on what order the threads run in).

When I ran this code on my server however, the output was 333.

<begin pulling out hair>

Long story short, after a couple hours of investigation I figured out that the way a foreach loop works under the hood in C# ≥ 5.0, which is what I run on my local machine, works differently than a foreach loop in C# < 5.0, which is what I had on my server.

From the C# 4.0 spec, a foreach loop is really a while loop behind the scenes, meaning the code above really translates into something like this:

The important thing to note in the above code is that int v gets declared outside of the while loop.

In the C# 5.0 spec, that int v gets declared inside the loop (causing it to get recreated with every iteration):

Because my local machine and server were running different versions of .NET, the same exact code was producing totally different results.

Eventually I found Eric Lippert’s article about the matter. Since I’m still fairly new to the world of .NET, I wasn’t around for the big debate that took place in his comment’s section regarding which should be the correct implementation. However, it is interesting to note that the C# devs decided to switch the logic on how the foreach loop operates so late in the game.

My eventual workaround for the .NET 3.5/C# 4.0 server was to assign the int to a newly created variable inside the foreach:

As frustrating it may be to debug problems like this, it is nice to learn a little bit more of the language’s history and idiosyncrasies.

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XmlReader vs XmlDocument Performance

Running C# Tests for Code Performance

Recently I have been working on a project where I needed to parse XML files that were between 5mb and 20mb in size. Performance was critical for the project, so I wanted to make sure that I would parse these files as quickly as possible.

The two C# classes that I know of for parsing XML are XmlReader and XmlDocument. Based on my understanding of the two classes, XmlReader should perform faster in my scenario because it reads through an XML document only once, never storing more than the current node in memory. On the contrary, XmlDocument stores the whole XML file in memory which has some performance overhead.

Not knowing for certain which method I should use, I decided to write a quick performance test to measure the actual results of these two classes.

The Data

In my project, I knew what data I needed to extract from the XML up front so I decided to configure test in a way that mimics that requirement. If my project required me to run recursive logic in the XML document, needing a piece of information further down in the XML in order to know what pieces of information to pull earlier on from the XML, I would have set up an entirely different test.

For my test, I decided to use the Photography Stack Exchange user data dump as our sample file since it mimics the structure and file size of one my actual project’s data. The Stack Exchange data dumps are great sample data sets because they involve real-world data and are released under a Creative Commons license.

The Test

The C# code for my test can be found in its entirety on GitHub.

In my test I created two methods to extract the same exact data from the XML; one of the methods used XmlReader and the other XmlDocument.

The first test uses XmlReader. The XmlReader object only stores a single node in memory at a time, so in order to read through the whole document we need to usewhile(reader.Read()) in order to loop all of the nodes. Inside of the loop, we check if each node is an element that we are looking for and if so then parse out the necessary data:

On the other hand, the code for XmlDocument is much simpler: we load the whole XML file into memory and then write a LINQ query to find the elements of interest:

After writing these two methods and confirming that they are returning the same exact results it was time to pit them against each other. I wrote a method to run each of my two XML parsing methods above 50 times and to take the average elapsed run time of each to eliminate any outlier data:

Results and Conclusions

Cutting to the chase, XmlReader performed faster in my test:

Performance test results.

Now, is this ~.14 seconds of speed difference significant? In my case, it is, because I will be parsing many more elements and many more files dozens of times a day. After doing the math, I estimate I will save 45–60 seconds of parsing time for each set of XML files, which is huge in an almost-real-time system.

Would I have come to the same conclusion if blazing fast speed was not one of my requirements? No, I would probably go the XmlDocument route because the code is much cleaner and therefore easier to maintain.

And if my XML files were 50mb, 500mb, or 5gb in size? I would probably still use XmlReader at that point because trying to store 5gb of data in memory will not be pretty.

What about a scenario where I need to go backwards in my XML document — this might be a case where I would use XmlDocument because it is more convenient to go backwards and forwards with that class. However, a hybrid approach might be my best option if the data allows it: if I can use XmlReader to get through the bulk of my content quickly and then load just certain child trees of elements into XmlDocument for easier backwards/forwards traversal, then that would seem like an ideal scenario.

In short, XmlReader was faster than XmlDocumet for me in my scenario. The only way I could come to this conclusion though was by running some real world tests and measuring the performance data.

So should you use XmlReader or XmlDocument in your next project? The answer is it depends.

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