Tim Deschryver

Make your C# applications faster with LINQ joins

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I've experienced a handful of times that an application has slowed down sufficiently without a code change. So, what could (change to) make an application slower over time? Mostly, the answer to this question is data. To be more specific, the amount of data that an application has to process.

During development, we as developers, often develop against a small subset of the data. This has as outcome, that parts of an application seem fast during this stage. And when it reaches the production environment it still is fast. But after a while, when the application is in use, the data starts to accumulate and it might suddenly be the cause of performance issues.

From my experience, most of these performance problems are caused while combining two lists in order to manipulate the data. And don't get me wrong, it's an easy mistake.

The slow approach

As an example, let's take two lists, one with the customers, the second one with the customer preferences. We want to merge the two lists into a single list, representing the customer data with their preference.

How many times did you write or encounter the following code?

Another commonly approach is to write it with the LINQ Select method.

Well... then I got bad news for you because this isn't fast for larger lists.

The faster approach

But how should this be written then? A better approach with performance in mind is to use the LINQ Join method.

If you're more a fan of the LINQ query syntax, then the above snippet can be rewritten to:

Both of the LINQ examples don't only perform better, but they also make the intentions more clear in my opinion.

Why it does perform better

As pointed on out in a reddit thread, it isn't LINQ that's making this faster.

The cause of these improvements is found in the lookup of the Join method, not LINQ itself. To prove that LINQ does, in fact, make things actually a tiny bit slower can be tested with the following code.


You don't have to take my word for it, here's a comparison of all the methods that are described in this post. On my machine, this gives us the following result.

List size For-loop Foreach-loop LINQ Select LINQ method Join LINQ query Join Dictionary Prefilled Dictionary Manual iteration
1 00:00.0056705 00:00.0004749 00:00.0005044 00:00.0031932 00:00.0003097 00:00.0005084 00:00.0001750 .0057285
10 00:00.0055548 00:00.0004490 00:00.0005076 00:00.0034938 00:00.0002472 00:00.0004444 00:00.0001647 .0063443
100 00:00.0060491 00:00.0007347 00:00.0006980 00:00.0035554 00:00.0010058 00:00.0004902 00:00.0001806 .0079778
1 000 00:00.0216990 00:00.0170807 00:00.0169829 00:00.0041184 00:00.0010638 00:00.0006651 00:00.0002220 .0067956
10 000 00:00.7261891 00:00.6516171 00:00.7047633 00:00.0059576 00:00.0017884 00:00.0010040 00:00.0008011 .0092210
100 000 01:02.0488321 00:57.5521209 01:05.2631133 00:00.0451954 00:00.0366773 00:00.0091225 00:00.0079996 .0131450
200 000 04:09.4021135 04:21.7946002 04:25.9571240 00:00.0577996 00:00.0551096 00:00.0221926 00:00.0217287 .0202470
300 000 09:36.8749546 09:15.5743423 12:40.2008206 00:00.1337581 00:00.1380703 00:00.0269653 00:00.0286574 .0213501
400 000 17:14.1396757 19:35.5088802 22:50.3364594 00:00.1900785 00:00.1508965 00:00.0426907 00:00.0424060 .0376827
500 000 30:50.6746922 33:33.2734761 37:25.6146064 00:00.1524784 00:00.1470995 00:00.0586161 00:00.0571202 .0489719

A graph showing the benchmarks

Benchmarks are created with Simple microbenchmarking in C#

I expected the LINQ examples to be faster, but I was still surprised by the margins of these results.

To play with the solutions yourselves, here's a link to the GitHub repository with the examples included.


The reason why the numbers ramp up is that at its worst-case scenario, it has to traverse through the whole list until the predicate finds a match. The larger the list gets, the slower the code will run. We can express this algorithm with the Big O Notation as O(N), which describes a linear growth in proportion to the size of the list.

To do better we can make rewrite this to an O(1) algorithm, to make this lookup a single operation. Regardless of the size of the list.

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