Get the first item of an IEnumerable and return the rest as IEnumerable, iterating through only once

Question

I've got an enumerable that contains responses from a service call that come in gradually.

• I can't do ToList on the enumerable as that would block until all responses are received instead of listing them as they come.
• I also can't iterate twice as that would trigger another service call.

How to get the first element in the enumerable and return the continuation of the enumerable? I can't use an iterator method as I get a compilation error:

Iterators cannot have ref, in or out parameters.

I've tried this code:

public IEnumerable<object> GetFirstAndRemainder(IEnumerable<object> enumerable, out object first)
{
      first = enumerable.Take(1).FirstOrDefault();
      return enumerable.Skip(1);  // Second interation - unexceptable
}


// This one has a compilation error: Iterators cannot have ref, in or out parameters
public IEnumerable<object> GetFirstAndRemainder2(IEnumerable<object> enumerable, out object first)
{
    var enumerator = enumerable.GetEnumerator();
    enumerator.MoveNext();
    first = enumerator.Current;
    while (enumerator.MoveNext())
    {
        yield return enumerator.Current;
    }
}

Answer 1

Instead of using an out parameter, you can use ValueTuple<T1, T2> (as of C# 7.0, documented here) to return two elements: the first item of the IEnumerable<T>, and the remainder as another IEnumerable<T>.

using System.Linq;

class Program {
    static void Main(string[] args) {
        (int first, IEnumerable<int> remainder) = GetFirstAndRemainder(Enumerable.Range(1, 5));
        // first = 1
        // remainder yields (2, 3, 4, 5)
    }

    // Returns the first item and the remainders as an IEnumerable
    static (T, IEnumerable<T>) GetFirstAndRemainder<T>(IEnumerable<T> sequence) {
        var enumerator = sequence.GetEnumerator();
        enumerator.MoveNext();
        return (enumerator.Current, enumerator.AsEnumerable());
    }
}

You also need to convert from an IEnumerator to an IEnumerable which I did with an extension method:

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

Note that due to your requirements, iterating once over the remainder will exhaust it even though it has the type IEnumerable<T>.

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If you need to ensure the disposal of the enumerator, then you should probably ditch these wrapping methods entirely and directly work from the IEnumerator<T> yourself. It'll save you some typing and make it more explicit that there is only one enumeration.

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It's an XY problem: from the description you give, it looks like you really need IAsyncEnumerable to be able to process items one-by-one as they come in gradually, which is probably what you're trying to do with GetFirstAndRemainder.