Return singleton enumerators from IEnumerable.GetEnumerator for empty collections

[stephentoub]

Change the IEnumerable<T>.GetEnumerator() implementations on our core collection types to special-case Count==0 in order to return a single enumerator instead of allocating one a new each time. This saves an allocation when enumerating these collections via the interface in exchange for an extra length check as part of GetEnumerator.

This changes List<>, Queue<>, Stack<>, LinkedList<>, PriorityQueue<,>, SortedDictionary<,>, SortedList<,>, SortedSet<>, HashSet<>, Dictionary<,>, and ArraySegment<>.

Fixes #59596

Relates to #81523, as this means that code like:

var list = new List<int>();
IEnumerator<int> e = list.GetEnumerator();
list.Add(42);
e.MoveNext();

which would previously have thrown an exception on the MoveNext call will no longer throw an exception.

Method	Toolchain	Mean	Error	StdDev	Median	Ratio	Allocated	Alloc Ratio
SumEmpty	\main\corerun.exe	14.920 ns	0.3240 ns	0.6396 ns	14.642 ns	1.00	40 B	1.00
SumEmpty	\pr\corerun.exe	5.496 ns	0.0202 ns	0.0157 ns	5.491 ns	0.36	-	0.00
SumOne	\main\corerun.exe	20.182 ns	0.1604 ns	0.1500 ns	20.160 ns	1.00	40 B	1.00
SumOne	\pr\corerun.exe	20.068 ns	0.0642 ns	0.0501 ns	20.066 ns	0.99	40 B	1.00

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;
using System.Collections.Generic;

[MemoryDiagnoser(false)]
public partial class Program
{
    static void Main(string[] args) => BenchmarkSwitcher.FromAssembly(typeof(Program).Assembly).Run(args);

    private IEnumerable<int> _empty = new List<int>();
    private IEnumerable<int> _one = new List<int>() { 42 };

    [Benchmark]
    public int SumEmpty()
    {
        int sum = 0;
        foreach (int i in _empty) sum += i;
        return sum;
    }

    [Benchmark]
    public int SumOne()
    {
        int sum = 0;
        foreach (int i in _one) sum += i;
        return sum;
    }
}

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Issue Details

---

Change the IEnumerable<T>.GetEnumerator() implementations on our core collection types to special-case Count==0 in order to return a single enumerator instead of allocating one a new each time. This saves an allocation when enumerating these collections via the interface in exchange for an extra length check as part of GetEnumerator.

This changes List<>, Queue<>, Stack<>, LinkedList<>, PriorityQueue<,>, SortedDictionary<,>, SortedList<,>, SortedSet<>, HashSet<>, Dictionary<,>, and ArraySegment<>.

Fixes #59596

Relates to #81523, as this means that code like:

var list = new List<int>();
IEnumerator<int> e = list.GetEnumerator();
list.Add(42);
e.MoveNext();

which would previously have thrown an exception on the MoveNext call will no longer throw an exception.

Method	Toolchain	Mean	Error	StdDev	Median	Ratio	Allocated	Alloc Ratio
SumEmpty	\main\corerun.exe	14.920 ns	0.3240 ns	0.6396 ns	14.642 ns	1.00	40 B	1.00
SumEmpty	\pr\corerun.exe	5.496 ns	0.0202 ns	0.0157 ns	5.491 ns	0.36	-	0.00
SumOne	\main\corerun.exe	20.182 ns	0.1604 ns	0.1500 ns	20.160 ns	1.00	40 B	1.00
SumOne	\pr\corerun.exe	20.068 ns	0.0642 ns	0.0501 ns	20.066 ns	0.99	40 B	1.00

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;
using System.Collections.Generic;

[MemoryDiagnoser(false)]
public partial class Program
{
    static void Main(string[] args) => BenchmarkSwitcher.FromAssembly(typeof(Program).Assembly).Run(args);

    private IEnumerable<int> _empty = new List<int>();
    private IEnumerable<int> _one = new List<int>() { 42 };

    [Benchmark]
    public int SumEmpty()
    {
        int sum = 0;
        foreach (int i in _empty) sum += i;
        return sum;
    }

    [Benchmark]
    public int SumOne()
    {
        int sum = 0;
        foreach (int i in _one) sum += i;
        return sum;
    }
}

Author:	stephentoub
Assignees:	stephentoub
Labels:	area-System.Collections, tenet-performance
Milestone:	8.0.0

[eiriktsarpalis]

Thanks