C# Tip: Initialize lists size to improve performance

Lists have an inner capacity. Every time you add more items than the current Capacity, you add performance overhead. How to prevent it?

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Some collections, like List<T>, have a predefined initial size.

Every time you add a new item to the collection, there are two scenarios:

1. the collection has free space, allocated but not yet populated, so adding an item is immediate;
2. the collection is already full: internally, .NET resizes the collection, so that the next time you add a new item, we fall back to option #1.

Clearly, the second approach has an impact on the overall performance. Can we prove it?

Here’s a benchmark that you can run using BenchmarkDotNet:

[Params(2, 100, 1000, 10000, 100_000)]
public int Size;

[Benchmark]
public void SizeDefined()
{
    int itemsCount = Size;

    List<int> set = new List<int>(itemsCount);
    foreach (var i in Enumerable.Range(0, itemsCount))
    {
        set.Add(i);
    }
}

[Benchmark]
public void SizeNotDefined()
{
    int itemsCount = Size;

    List<int> set = new List<int>();
    foreach (var i in Enumerable.Range(0, itemsCount))
    {
        set.Add(i);
    }
}

Those two methods are almost identical: the only difference is that in one method we specify the initial size of the list: new List<int>(itemsCount).

Have a look at the result of the benchmark run with .NET 7:

Notice that, in general, they execute in a similar amount of time; for instance when running the same method with 100000 items, we have the same magnitude of time execution: 1,416,074.69 ns vs 1,705,672.83 ns.

The huge difference is with the allocated space: 400,300 B vs 1,049,485 B. Almost 2.5 times better!

Ok, it works. Next question: How can we check a List capacity?

We’ve just learned that capacity impacts the performance of a List.

How can you try it live? Easy: have a look at the Capacity property!

List<int> myList = new List<int>();

foreach (var element in Enumerable.Range(0,50))
{
    myList.Add(element);
    Console.WriteLine($"Items count: {myList.Count} - List capacity: {myList.Capacity}");
}

If you run this method, you’ll see this output:

Items count: 1 - List capacity: 4
Items count: 2 - List capacity: 4
Items count: 3 - List capacity: 4
Items count: 4 - List capacity: 4
Items count: 5 - List capacity: 8
Items count: 6 - List capacity: 8
Items count: 7 - List capacity: 8
Items count: 8 - List capacity: 8
Items count: 9 - List capacity: 16
Items count: 10 - List capacity: 16
Items count: 11 - List capacity: 16
Items count: 12 - List capacity: 16
Items count: 13 - List capacity: 16
Items count: 14 - List capacity: 16
Items count: 15 - List capacity: 16
Items count: 16 - List capacity: 16
Items count: 17 - List capacity: 32
Items count: 18 - List capacity: 32
Items count: 19 - List capacity: 32
Items count: 20 - List capacity: 32
Items count: 21 - List capacity: 32
Items count: 22 - List capacity: 32
Items count: 23 - List capacity: 32
Items count: 24 - List capacity: 32
Items count: 25 - List capacity: 32
Items count: 26 - List capacity: 32
Items count: 27 - List capacity: 32
Items count: 28 - List capacity: 32
Items count: 29 - List capacity: 32
Items count: 30 - List capacity: 32
Items count: 31 - List capacity: 32
Items count: 32 - List capacity: 32
Items count: 33 - List capacity: 64
Items count: 34 - List capacity: 64
Items count: 35 - List capacity: 64
Items count: 36 - List capacity: 64
Items count: 37 - List capacity: 64
Items count: 38 - List capacity: 64
Items count: 39 - List capacity: 64
Items count: 40 - List capacity: 64
Items count: 41 - List capacity: 64
Items count: 42 - List capacity: 64
Items count: 43 - List capacity: 64
Items count: 44 - List capacity: 64
Items count: 45 - List capacity: 64
Items count: 46 - List capacity: 64
Items count: 47 - List capacity: 64
Items count: 48 - List capacity: 64
Items count: 49 - List capacity: 64
Items count: 50 - List capacity: 64

So, as you can see, List capacity is doubled every time the current capacity is not enough.

Further readings

To populate the lists in our Benchmarks we used Enumerable.Range. Do you know how it works? Have a look at this C# tip:

🔗 C# Tip: LINQ’s Enumerable.Range to generate a sequence of consecutive numbers

This article first appeared on Code4IT 🐧

Wrapping up

In this article, we’ve learned that just a minimal change can impact our application performance.

We simply used a different constructor, but the difference is astounding. Clearly, this trick works only if already know the final length of the list (or, at least, an estimation). The more precise, the better!

I hope you enjoyed this article! Let’s keep in touch on Twitter or on LinkedIn, if you want! 🤜🤛

Happy coding!

🐧