Return an empty IEnumerator

c# ienumerator

I have an interface that, among other things, implements a "public IEnumerator GetEnumerator()" method, so I can use the interface in a foreach statement.

I implement this interface in several classes and in one of them, I want to return an empty IEnumerator. Right now I do this the following way:

public IEnumerator GetEnumerator()
{
    ArrayList arr = new ArrayList();
    return arr.GetEnumerator();
}

However I consider this an ugly hack, and I can't help but think that there is a better way of returning an empty IEnumerator. Is there?

This is simple in C# 2:

public IEnumerator GetEnumerator()
{
    yield break;
}

You need the yield break statement to force the compiler to treat it as an iterator block.

This will be less efficient than a "custom" empty iterator, but it's simpler code...

There is an extra function in the framework:

public static class Enumerable
{
    public static IEnumerable<TResult> Empty<TResult>();
}

Using this you can write:

var emptyEnumerable = Enumerable.Empty<int>();
var emptyEnumerator = Enumerable.Empty<int>().GetEnumerator();

You could implement a dummy class that implements IEnumerator, and return an instance of it:

class DummyEnumerator : IEnumerator
{
    public object Current
    {
        get
        {
            throw new InvalidOperationException();
        }
    }

    public bool MoveNext()
    {
        return false;
    }

    public void Reset()
    {
    }
}

I was curious and went a bit further. I made a test that checks how efficient the methods are comparing yield break, Enumerable.Emtpy and custom class.

You can check it out on dotnetfiddle https://dotnetfiddle.net/p5ZkUN or use the code below.

The result of one of the many dotnetfiddle runs using 190 000 iterations was:

Yield break: 00:00:00.0012208

Enumerable.Empty(): 00:00:00.0007815

EmptyEnumerator instance: 00:00:00.0010226

using System;
using System.Diagnostics;
using System.Collections;
using System.Linq;
                    
public class Program
{
    private const int Iterations = 190000;
    public static void Main()
    {
        var sw = new Stopwatch();
        
        IEnumerator enumerator1 = YieldBreak();
        sw.Start();
        for (int i = 0; i < Iterations; i++)
        {
            while(enumerator1.MoveNext())
            {
                throw new InvalidOperationException("Should not occur");
            }           
        }
        sw.Stop();
        
        Console.WriteLine("Yield break: {0}", sw.Elapsed);
        
        GC.Collect();
        
        IEnumerator enumerator2 = Enumerable.Empty<object>().GetEnumerator();
        sw.Restart();
        for (int i = 0; i < Iterations; i++)
        {
            while(enumerator2.MoveNext())
            {
                throw new InvalidOperationException("Should not occur");
            }           
        }
        sw.Stop();
        
        Console.WriteLine("Enumerable.Empty<T>(): {0}", sw.Elapsed);
        
        GC.Collect();
        
        var enumerator3 = new EmptyEnumerator();
        sw.Restart();
        for (int i = 0; i < Iterations; i++)
        {
            while(enumerator3.MoveNext())
            {
                throw new InvalidOperationException("Should not occur");
            }           
        }
        sw.Stop();
        
        Console.WriteLine("EmptyEnumerator instance: {0}", sw.Elapsed);
    }
    
    public static IEnumerator YieldBreak()
    {
        yield break;
    }
    
    private class EmptyEnumerator : IEnumerator
    {
        //public static readonly EmptyEnumerator Instance = new EmptyEnumerator();
        
        public bool MoveNext()
        {
            return false;
        }
        
        public void Reset()
        {
        }
        
        public object Current { get { return null; } }
    }
}

Return an empty IEnumerator

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