Why does enumerating an empty array does not allocate on the heap?

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Consider the following benchmark:

[MemoryDiagnoser]
public class EnumerableBenchmark
{
    private IEnumerable<string> _emptyArray = new string[0];
    private IEnumerable<string> _notEmptyArray = new string[1];

    [Benchmark]
    public IEnumerator<string> ArrayEmpty()
    {
        return _emptyArray.GetEnumerator();
    }

    [Benchmark]
    public IEnumerator<string> ArrayNotEmpty()
    {
        return _notEmptyArray.GetEnumerator();
    }
}

BenchmarkDotNet reports the following results on .net framework 4.8 and .net core 3.1:

// * Summary *

BenchmarkDotNet=v0.12.1, OS=Windows 10.0.19041.329 (2004/?/20H1)
Intel Core i7-9750H CPU 2.60GHz, 1 CPU, 12 logical and 6 physical cores
.NET Core SDK=3.1.301
  [Host]     : .NET Core 3.1.5 (CoreCLR 4.700.20.26901, CoreFX 4.700.20.27001), X64 RyuJIT
  DefaultJob : .NET Core 3.1.5 (CoreCLR 4.700.20.26901, CoreFX 4.700.20.27001), X64 RyuJIT


|        Method |     Mean |     Error |    StdDev |  Gen 0 | Gen 1 | Gen 2 | Allocated |
|-------------- |---------:|----------:|----------:|-------:|------:|------:|----------:|
|    ArrayEmpty | 3.692 ns | 0.1044 ns | 0.0872 ns |      - |     - |     - |         - |
| ArrayNotEmpty | 7.235 ns | 0.2177 ns | 0.3051 ns | 0.0051 |     - |     - |      32 B |

From the result, it seems that GetEnumerator causes a heap allocation when the array is not empty, but not when the array is empty. I've rewritten the benchmark in many different ways but always got the same result, so I don't think BenchmarkDotNet is wrong.

My logical conclusion was that empty arrays have a cached enumerator. However, this code seems to contradict that theory:

var emptyArray = new string[0];

var enum1 = emptyArray.GetEnumerator();
var enum2 = emptyArray.GetEnumerator();

Console.WriteLine("Equals: " + object.ReferenceEquals(enum1, enum2));
Console.WriteLine(enum1.GetType().Name + " - " + enum1.GetType().IsValueType);

Which displays:

Equals: False
SZArrayEnumerator - False

I'm really scratching my head on this one. Does somebody know what's going on?

enumerator arrays c#
1 Answers

Your hypothesis is correct. In the presented benchmark, a cached version of the enumerator is used. Here is the decompiled code:

internal IEnumerator<T> GetEnumerator<T>()
{
  T[] array = Unsafe.As<T[]>((object) this);
  return array.Length != 0
      ? (IEnumerator<T>) new SZGenericArrayEnumerator<T>(array)
      : (IEnumerator<T>) SZGenericArrayEnumerator<T>.Empty;
}

However, when you tried to check your hypothesis, you changed the code. In the benchmark, _emptyArray is IEnumerable<string>, but in the code snippet, it's string[]. Here is the decompiled code for the string[].GetEnumerator:

public IEnumerator GetEnumerator()
{
  int lowerBound = this.GetLowerBound(0);
  return this.Rank == 1 && lowerBound == 0
      ? (IEnumerator) new SZArrayEnumerator(this)
      : (IEnumerator) new ArrayEnumerator(this, lowerBound, this.Length);
}

Let's try to change the snippet and cast the array to IEnumerable<string>:

IEnumerable<string> emptyArray = new string[0];

var enum1 = emptyArray.GetEnumerator();
var enum2 = emptyArray.GetEnumerator();

Console.WriteLine("Equals: " + object.ReferenceEquals(enum1, enum2));
Console.WriteLine(enum1.GetType().Name + " - " + enum1.GetType().IsValueType);

Here is the updated output that correctly verifies the hypothesis about the cached enumerator:

Equals: True
SZGenericArrayEnumerator`1 - False
user_1848420
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