ArraySegment<T> iteration performance in C#
Introduction
ArraySegment<T>
is older and somewhat similar to Span<T>
.
ArraySegment<T>
has a lot less usage limitations than Span<T>
. Both are value types but Span<T>
cannot be used as a generics type and can only be used as a field type inside a ref struct
. With any other struct
, or class
, you’ll have to use a Memory<T>
field type. To enumerate a Memory<T>
, you have to call its Span
property, which creates a new instance of Span<T>
. This may be a big performance hit if, for example, the type containing this field has a method that accesses just one item. It takes time to create the Span<T>
instance to then simply access a memory location.
Span<T>
supports arrays and other types of contiguous memory allocations. ArraySegment<T>
only supports arrays.
Span<T>
doesn’t implement IEnumerable<T>
so the LINQ operations defined in System.Linq
cannot be used. You’ll have to use NetFabric.Hyperlinq or SpanLinq libraries. ArraySegment<T>
implements IEnumerable<T>
so it can be used with any LINQ implementation.
So, how do these fairs in terms of performance?
Let’s use BenchmarkDotNet to run the following benchmarks:
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using System;
using System.Linq;
using BenchmarkDotNet.Attributes;
namespace ArrayIteration;
public class ArraySegmentBenchmarks
{
protected ArraySegment<int> arraySegment;
[Params(100)]
public int Offset { get; set; }
[Params(10, 1_000)]
public int Count { get; set; }
[Params(2_000)]
public int Length { get; set; }
[GlobalSetup]
public void GlobalSetup()
{
var array = Enumerable.Range(0, Length).ToArray();
arraySegment = new ArraySegment<int>(array, Offset, Count);
}
[Benchmark(Baseline = true)]
public int For()
{
var sum = 0;
for (var index = 0; index < arraySegment.Count; index++)
sum += arraySegment[index];
return sum;
}
[Benchmark]
public int ForEach()
{
var sum = 0;
foreach (var item in arraySegment)
sum += item;
return sum;
}
[Benchmark]
public int Linq()
=> arraySegment
.Sum();
[Benchmark]
public int Array_For()
{
var array = arraySegment.Array!;
var start = arraySegment.Offset;
var end = start + arraySegment.Count;
var sum = 0;
for (var index = start; index < end; index++)
sum += array[index];
return sum;
}
[Benchmark]
public int AsSpan_ForEach()
{
var sum = 0;
foreach (var item in arraySegment.AsSpan())
sum += item;
return sum;
}
[Benchmark]
public int Array_Linq()
=> arraySegment.Array!
.Skip(Offset)
.Take(Count)
.Sum();
}
These benchmarks use an array of 2.000 integers. It creates a ArraySegment<int>
with an Offset
of 100 and two different Count
values, 10 and 1.000 items.
The For()
benchmarks uses a for
loop to iterate the ArraySegment<T>
. It uses its indexer to get the item. The ForEach()
benchmark uses a foreach
loop to iterate the ArraySegment<T>
.
The Linq()
benchmark uses the Sum()
provided by the System.Linq
namespace.
The Array_For()
benchmark uses a for
loop on the inner array of ArraySegment<T>
. The inner array is available through the Array
property, and the segment properties are available through the Offset
and Count
properties.
The Span_ForEach()
benchmark uses AsSpan()
and a foreach
loop to iterate the inner array of ArraySegment<T>
. As you can see in its source, AsSpan()
uses the Array
, Offset
, and Count
properties to create a Span<T>
equivalent to the ArraySegment<T>
.
The Array_Linq()
benchmark uses on the inner array the Skip()
, Take()
, and Count()
operations provided by the System.Linq
namespace.
Here are the benchmarking results using a configuration to test on .NET 6, .NET 7 and .NET 8 (all “modern” .NET versions):
ForEach()
is around 1.3x slower than For()
for the shorter collection. Unlike arrays and Span<T>
, the C# compiler doesn’t treat ArraySegment<T>
as a special case. foreach
does not use the indexer. It allocates an instance of an enumerator and uses it. Although the enumerator is a value-type, it’s still slower than using the indexer.
Both the indexer and the Current
property of the enumerator check bounds twice. One explicitly in the code and another implicitly when indexing the array. The indexer does it in a more efficient way with only one comparison but Current
does it with two comparisons.
Linq()
is around 10x slower than For()
, but 3x to 5x slower when on .NET 8. Adding to the double bounds checking in the Current
property, it boxes the enumerator, making it a reference-type.
NOTE: Check my other article “Performance of value-type vs reference-type enumerators” to understand why having a value-type enumerator is an advantage.
The Array_For()
and AsSpan_ForEach()
have similar performance. These are 1.6x to 2x faster than For()
. These are equivalent and faster because the array is copied to a local variable so the JIT compiler can remove bounds checking.
NOTE: Check my other article “Array iteration performance in C#” to understand how the JIT compiler can remove bounds checking.
The Array_Linq()
is 8x to 20x slower than For()
because more enumerators are used. Notice that 96 bytes are allocated against the 72 bytes allocated by the Linq()
benchmark. Skip()
and Take()
are optimized to create only one enumerator when used together but still adds this extra enumerator.
Conclusions
Using the indexer or the enumerator of a ArraySegment<T>
is slow. Given that it’s possible to access its inner array, it’s just like having the array and the slice bounds in a value type. Using it this way, allows to work around a the Memory<T>
indexing one element performance limitation.
Avoid the use of LINQ with ArraySegment<T>
when possible.
Favor the use of Span<T>
and Memory<T>
when possible.