In my latest project I learned some thinks about the performance of Linq when you often query in-memory repositories. Linq is very easy to use and with that you can very quick implement the methods that you need for an repository. But sometimes it’s useful to think a little bit more how often a method will be called. In my latest project I had the situation to call the GetById method for more than 20.000 times. And my in-memory repository had more than 700 items. In my first attempt I have used the SingleOrDefault method to return the correct item. But this was not fast enough. My function has needed more than 2 seconds in sum. My second attempt was, to convert the items into a Dictionary with the id as key and use this dictionary in the GetById function. And this was so much faster. It needs just 220 ms for all calls and this was the performance that I needed.
To generate a dictionary very quick, you can use an extension method from the System.Linq namespace. With the following code you can do this very easy
The same experience I made with the query for any items with the same key. The Linq function SelectMany was in summary to slow. In this situation you can’t use a dictionary because the key isn’t unique. But Linq has also an Extension method to solve this problem. With the ToLookup function you can generate a very fast lookup table.