Are you using Dictionaries correctly?

A while ago I wrote a small comparison between Dictionary, HashSet, SortedList and SortedDictionary when performing a lookup operation.

I have noticed however that many developers don’t use the most efficient method for checking if an item exists and retrieving it.

Most of the times, you’ll see something like this:

if (myDictionary.ContainsKey(key))
{
    var value = myDictionary[key];
    // do something with value
}

…which is fine and does its job. But if this code is in performance-sensitive paths, there’s a better and more concise way:

if (myDictionary.TryGetValue(key, out var value)) 
{
  // do something with value
}

“No big deal”, you might say. Well, this benchmark says otherwise:

The calls to TryGetValue are almost 2x faster! But why is that?

The reason is pretty simple actually: calling ContainsKey and then accessing the value by key is performing the lookup twice.

As you can see, I have also included a comparison between .NET 8 and 9. The difference is not abyssal, but we can still see an improvement.

As usual, the code for the benchmark is available on GitHub.

Enjoy!