Is there a built-in way to convert IEnumerator to IEnumerable

Is there a built-in way to convert IEnumerator<T> to IEnumerable<T>?

The easiest way of converting I can think of is via the yield statement

public static IEnumerable<T> ToIEnumerable<T>(this IEnumerator<T> enumerator) {
  while ( enumerator.MoveNext() ) {
    yield return enumerator.Current;
  }
}

compared to the list version this has the advantage of not enumerating the entire list before returning an IEnumerable. using the yield statement you'd only iterate over the items you need, whereas using the list version, you'd first iterate over all items in the list and then all the items you need.

for a little more fun you could change it to

public static IEnumerable<K> Select<K,T>(this IEnumerator<T> e, 
                                         Func<K,T> selector) {
      while ( e.MoveNext() ) {
        yield return selector(e.Current);
      }
    }

you'd then be able to use linq on your enumerator like:

IEnumerator<T> enumerator;
var someList = from item in enumerator
               select new classThatTakesTInConstructor(item);

You could use the following which will kinda work.

public class FakeEnumerable<T> : IEnumerable<T> {
  private IEnumerator<T> m_enumerator;
  public FakeEnumerable(IEnumerator<T> e) {
    m_enumerator = e;
  }
  public IEnumerator<T> GetEnumerator() { 
    return m_enumerator;
  }
  // Rest omitted 
}

This will get you into trouble though when people expect successive calls to GetEnumerator to return different enumerators vs. the same one. But if it's a one time only use in a very constrained scenario, this could unblock you.

I do suggest though you try and not do this because I think eventually it will come back to haunt you.

A safer option is along the lines Jonathan suggested. You can expend the enumerator and create a List<T> of the remaining items.

public static List<T> SaveRest<T>(this IEnumerator<T> e) {
  var list = new List<T>();
  while ( e.MoveNext() ) {
    list.Add(e.Current);
  }
  return list;
}

EnumeratorEnumerable<T>

A threadsafe, resettable adaptor from IEnumerator<T> to IEnumerable<T>

I use Enumerator parameters like in C++ forward_iterator concept.

I agree that this can lead to confusion as too many people will indeed assume Enumerators are /like/ Enumerables, but they are not.

However, the confusion is fed by the fact that IEnumerator contains the Reset method. Here is my idea of the most correct implementation. It leverages the implementation of IEnumerator.Reset()

A major difference between an Enumerable and and Enumerator is, that an Enumerable might be able to create several Enumerators simultaneously. This implementation puts a whole lot of work into making sure that this never happens for the EnumeratorEnumerable<T> type. There are two EnumeratorEnumerableModes:

• Blocking (meaning that a second caller will simply wait till the first enumeration is completed)
• NonBlocking (meaning that a second (concurrent) request for an enumerator simply throws an exception)

Note 1: 74 lines are implementation, 79 lines are testing code :)

Note 2: I didn't refer to any unit testing framework for SO convenience

using System;
using System.Diagnostics;
using System.Linq;
using System.Collections;
using System.Collections.Generic;
using System.Threading;

namespace EnumeratorTests
{
	public enum EnumeratorEnumerableMode
	{
		NonBlocking,
		Blocking,
	}

	public sealed class EnumeratorEnumerable<T> : IEnumerable<T>
	{
		#region LockingEnumWrapper

		public sealed class LockingEnumWrapper : IEnumerator<T>
		{
			private static readonly HashSet<IEnumerator<T>> BusyTable = new HashSet<IEnumerator<T>>();
			private readonly IEnumerator<T> _wrap;

			internal LockingEnumWrapper(IEnumerator<T> wrap, EnumeratorEnumerableMode allowBlocking) 
			{
				_wrap = wrap;

				if (allowBlocking == EnumeratorEnumerableMode.Blocking)
					Monitor.Enter(_wrap);
				else if (!Monitor.TryEnter(_wrap))
					throw new InvalidOperationException("Thread conflict accessing busy Enumerator") {Source = "LockingEnumWrapper"};

				lock (BusyTable)
				{
					if (BusyTable.Contains(_wrap))
						throw new LockRecursionException("Self lock (deadlock) conflict accessing busy Enumerator") { Source = "LockingEnumWrapper" };
					BusyTable.Add(_wrap);
				}

				// always implicit Reset
				_wrap.Reset();
			}
			
			#region Implementation of IDisposable and IEnumerator

			public void Dispose()
			{
				lock (BusyTable)
					BusyTable.Remove(_wrap);

				Monitor.Exit(_wrap);
			}
			public bool MoveNext()      { return _wrap.MoveNext(); }
			public void Reset()         { _wrap.Reset(); }
			public T Current            { get { return _wrap.Current; } }
			object IEnumerator.Current  { get { return Current; } }

			#endregion
		}

		#endregion

		private readonly IEnumerator<T> _enumerator;
		private readonly EnumeratorEnumerableMode _allowBlocking;

		public EnumeratorEnumerable(IEnumerator<T> e, EnumeratorEnumerableMode allowBlocking)
		{
			_enumerator = e;
			_allowBlocking = allowBlocking;
		}

		private LockRecursionPolicy a;
		public IEnumerator<T> GetEnumerator()
		{
			return new LockingEnumWrapper(_enumerator, _allowBlocking);
		}

		IEnumerator IEnumerable.GetEnumerator()
		{
			return GetEnumerator();
		}
	}

	class TestClass
	{
		private static readonly string World = "hello world\n";

		public static void Main(string[] args)
		{
			var master = World.GetEnumerator();
			var nonblocking = new EnumeratorEnumerable<char>(master, EnumeratorEnumerableMode.NonBlocking);
			var blocking    = new EnumeratorEnumerable<char>(master, EnumeratorEnumerableMode.Blocking);

			foreach (var c in nonblocking)  Console.Write(c); // OK (implicit Reset())
			foreach (var c in blocking)     Console.Write(c); // OK (implicit Reset())
			foreach (var c in nonblocking)  Console.Write(c); // OK (implicit Reset())
			foreach (var c in blocking)     Console.Write(c); // OK (implicit Reset())

			try
			{
				var willRaiseException = from c1 in nonblocking from c2 in nonblocking select new {c1, c2};
				Console.WriteLine("Cartesian product: {0}", willRaiseException.Count()); // RAISE
			}
			catch (Exception e) { Console.WriteLine(e); }

			foreach (var c in nonblocking)  Console.Write(c); // OK (implicit Reset())
			foreach (var c in blocking)     Console.Write(c); // OK (implicit Reset())

			try
			{
				var willSelfLock = from c1 in blocking from c2 in blocking select new { c1, c2 };
				Console.WriteLine("Cartesian product: {0}", willSelfLock.Count()); // LOCK
			}
			catch (Exception e) { Console.WriteLine(e); }

			// should not externally throw (exceptions on other threads reported to console)
			if (ThreadConflictCombinations(blocking, nonblocking))
				throw new InvalidOperationException("Should have thrown an exception on background thread");
			if (ThreadConflictCombinations(nonblocking, nonblocking))
				throw new InvalidOperationException("Should have thrown an exception on background thread");

			if (ThreadConflictCombinations(nonblocking, blocking))
				Console.WriteLine("Background thread timed out");
			if (ThreadConflictCombinations(blocking, blocking))
				Console.WriteLine("Background thread timed out");

			Debug.Assert(true); // Must be reached
		}

		private static bool ThreadConflictCombinations(IEnumerable<char> main, IEnumerable<char> other)
		{
			try
			{
				using (main.GetEnumerator())
				{
					var bg = new Thread(o =>
						{
							try { other.GetEnumerator(); }
							catch (Exception e) { Report(e); }
						}) { Name = "background" };
					bg.Start();

					bool timedOut = !bg.Join(1000); // observe the thread waiting a full second for a lock (or throw the exception for nonblocking)

					if (timedOut)
						bg.Abort();

					return timedOut;
				}
			} catch
			{
				throw new InvalidProgramException("Cannot be reached");
			}
		}

		static private readonly object ConsoleSynch = new Object();
		private static void Report(Exception e)
		{
			lock (ConsoleSynch)
				Console.WriteLine("Thread:{0}\tException:{1}", Thread.CurrentThread.Name, e);
		}
	}
}

Note 3: I think the implementation of the thread locking (especially around BusyTable) is quite ugly; However, I didn't want to resort to ReaderWriterLock(LockRecursionPolicy.NoRecursion) and didn't want to assume .Net 4.0 for SpinLock

Nope, IEnumerator<> and IEnumerable<> are different beasts entirely.

As Jason Watts said -- no, not directly.

If you really want to, you could loop through the IEnumerator<T>, putting the items into a List<T>, and return that, but I'm guessing that's not what you're looking to do.

The basic reason you can't go that direction (IEnumerator<T> to a IEnumerable<T>) is that IEnumerable<T> represents a set that can be enumerated, but IEnumerator<T> is a specific enumeratation over a set of items -- you can't turn the specific instance back into the thing that created it.

static class Helper
{
  public static List<T> SaveRest<T>(this IEnumerator<T> enumerator)
  {
    var list = new List<T>();
    while (enumerator.MoveNext())
    {
      list.Add(enumerator.Current);
    }
    return list;
  }
  public static ArrayList SaveRest(this IEnumerator enumerator)
  {
    var list = new ArrayList();
    while (enumerator.MoveNext())
    {
      list.Add(enumerator.Current);
    }
    return list;
  }
}

This is a variant I have written... The specific is a little different. I wanted to do a MoveNext() on an IEnumerable<T>, check the result, and then roll everything in a new IEnumerator<T> that was "complete" (so that included even the element of the IEnumerable<T> I had already extracted)

// Simple IEnumerable<T> that "uses" an IEnumerator<T> that has
// already received a MoveNext(). "eats" the first MoveNext() 
// received, then continues normally. For shortness, both IEnumerable<T>
// and IEnumerator<T> are implemented by the same class. Note that if a
// second call to GetEnumerator() is done, the "real" IEnumerator<T> will
// be returned, not this proxy implementation.
public class EnumerableFromStartedEnumerator<T> : IEnumerable<T>, IEnumerator<T>
{
    public readonly IEnumerator<T> Enumerator;

    public readonly IEnumerable<T> Enumerable;

    // Received by creator. Return value of MoveNext() done by caller
    protected bool FirstMoveNextSuccessful { get; set; }

    // The Enumerator can be "used" only once, then a new enumerator
    // can be requested by Enumerable.GetEnumerator() 
    // (default = false)
    protected bool Used { get; set; }

    // The first MoveNext() has been already done (default = false)
    protected bool DoneMoveNext { get; set; }

    public EnumerableFromStartedEnumerator(IEnumerator<T> enumerator, bool firstMoveNextSuccessful, IEnumerable<T> enumerable)
    {
        Enumerator = enumerator;
        FirstMoveNextSuccessful = firstMoveNextSuccessful;
        Enumerable = enumerable;
    }

    public IEnumerator<T> GetEnumerator()
    {
        if (Used)
        {
            return Enumerable.GetEnumerator();
        }

        Used = true;
        return this;
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
        return GetEnumerator();
    }

    public T Current
    {
        get
        {
            // There are various school of though on what should
            // happens if called before the first MoveNext() or
            // after a MoveNext() returns false. We follow the 
            // "return default(TInner)" school of thought for the
            // before first MoveNext() and the "whatever the 
            // Enumerator wants" for the after a MoveNext() returns
            // false
            if (!DoneMoveNext)
            {
                return default(T);
            }

            return Enumerator.Current;
        }
    }

    public void Dispose()
    {
        Enumerator.Dispose();
    }

    object IEnumerator.Current
    {
        get
        {
            return Current;
        }
    }

    public bool MoveNext()
    {
        if (!DoneMoveNext)
        {
            DoneMoveNext = true;
            return FirstMoveNextSuccessful;
        }

        return Enumerator.MoveNext();
    }

    public void Reset()
    {
        // This will 99% throw :-) Not our problem.
        Enumerator.Reset();

        // So it is improbable we will arrive here
        DoneMoveNext = true;
    }
}

Use:

var enumerable = someCollection<T>;

var enumerator = enumerable.GetEnumerator();
bool res = enumerator.MoveNext();
// do whatever you want with res/enumerator.Current

var enumerable2 = new EnumerableFromStartedEnumerator<T>(enumerator, res, enumerable);

Now, the first GetEnumerator() that will be requested to enumerable2 will be given through the enumerator enumerator. From the second onward the enumerable.GetEnumerator() will be used.

Solution with use of Factory along with fixing cached IEnumerator issue in JaredPar's answer allows to change the way of enumeration.

Consider a simple example: we want custom List<T> wrapper that allow to enumerate in reverse order along with default enumeration. List<T> already implements IEnumerator for default enumeration, we only need to create IEnumerator that enumerates in reverse order. (We won't use List<T>.AsEnumerable().Reverse() because it enumerates the list twice)

public enum EnumerationType {
	Default = 0,
	Reverse
}

public class CustomList<T> : IEnumerable<T> {
	private readonly List<T> list;

	public CustomList(IEnumerable<T> list) => this.list = new List<T>(list);
	
	IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
	
	//Default IEnumerable method will return default enumerator factory
	public IEnumerator<T> GetEnumerator() 
		=> GetEnumerable(EnumerationType.Default).GetEnumerator();

	public IEnumerable<T> GetEnumerable(EnumerationType enumerationType)
		=> enumerationType switch {
			EnumerationType.Default => new DefaultEnumeratorFactory(list),
			EnumerationType.Reverse => new ReverseEnumeratorFactory(list)
		};
	
	//Simple implementation of reverse list enumerator
	private class ReverseEnumerator : IEnumerator<T> {
		private readonly List<T> list;
		private int index;

		internal ReverseEnumerator(List<T> list) {
			this.list = list;
			index = list.Count-1;
			Current = default;
		}

		public void Dispose() { }

		public bool MoveNext() {
			if(index >= 0) {
				Current = list[index];
				index--;
				return true;
			}
			Current = default;
			return false;
		}

		public T Current { get; private set; }

		object IEnumerator.Current => Current;

		void IEnumerator.Reset() {
			index = list.Count - 1;
			Current = default;
		}
	}
	
	private abstract class EnumeratorFactory : IEnumerable<T> {
		protected readonly List<T> List;
		protected EnumeratorFactory(List<T> list) => List = list;
		
		IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
		
		public abstract IEnumerator<T> GetEnumerator();
	}


	private class DefaultEnumeratorFactory : EnumeratorFactory {
		
		public DefaultEnumeratorFactory(List<T> list) : base(list) { }
		
        //Default enumerator is already implemented in List<T>
		public override IEnumerator<T> GetEnumerator() => List.GetEnumerator();
	}


	private class ReverseEnumeratorFactory : EnumeratorFactory {

		public ReverseEnumeratorFactory(List<T> list) : base(list) { }

		public override IEnumerator<T> GetEnumerator() => new ReverseEnumerator(List);
	}
	
}

The other answers here are ... strange. IEnumerable<T> has just one method, GetEnumerator(). And an IEnumerable<T> must implement IEnumerable, which also has just one method, GetEnumerator() (the difference being that one is generic on T and the other is not). So it should be clear how to turn an IEnumerator<T> into an IEnumerable<T>:

    // using modern expression-body syntax
    public class IEnumeratorToIEnumerable<T> : IEnumerable<T>
    {
        private readonly IEnumerator<T> Enumerator;

        public IEnumeratorToIEnumerable(IEnumerator<T> enumerator) =>
            Enumerator = enumerator;

        public IEnumerator<T> GetEnumerator() => Enumerator;
        IEnumerator IEnumerable.GetEnumerator() => Enumerator;
    }

    foreach (var foo in new IEnumeratorToIEnumerable<Foo>(fooEnumerator))
        DoSomethingWith(foo);

    // and you can also do:

    var fooEnumerable = new IEnumeratorToIEnumerable<Foo>(fooEnumerator);

    foreach (var foo in fooEnumerable)
        DoSomethingWith(foo);

    // Some IEnumerators automatically repeat after MoveNext() returns false,
    // in which case this is a no-op, but generally it's required.
    fooEnumerator.Reset();

    foreach (var foo in fooEnumerable)
        DoSomethingElseWith(foo);

However, none of this should be needed because it's unusual to have an IEnumerator<T> that doesn't come with an IEnumerable<T> that returns an instance of it from its GetEnumerator method. If you're writing your own IEnumerator<T>, you should certainly provide the IEnumerable<T>. And really it's the other way around ... an IEnumerator<T> is intended to be a private class that iterates over instances of a public class that implements IEnumerable<T>.