Is it possible to clone an IEnumerable<T> instance, saving a copy of the iteration state?

Question

I'd like to create a copy of an IEnumerator<T> so that I can restart the enumeration process from a particular location in the collection. Clearly, there is no benefit to doing so for collections that implement IList, since we can remember the index of interest.

Is there a clever way to accomplish this task using a combination of yield statements and Linq functions? I could not find a suitable Clone() method to copy the enumerator, and would like to avoid using Enumerable.Skip() to reposition a new enumerator to the desired resumption point.

Also, I'd like to keep the solutions as generic as possible, and not have to depend on state from any concrete collections.

Answer 1

The best you could do is write something that keeps a buffer (perhaps a Queue<T>) of the data consumed from one and not the other (which would get messy/expensive if you advanced one iterator by 1M positions, but left the other alone). I really think you would be better off rethinking the design, though, and just using GetEnumerator() (i.e. another foreach) to start again - or buffer the data (if short) in a list/array/whatever.

Nothing elegant built in.

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Update: perhaps an interesting alternative design here is "PushLINQ"; rather than clone the iterator, it allows multiple "things" to consume the same data-feed at the same time.

In this example (lifted from Jon's page) we calculate multiple aggregates in parallel:

// Create the data source to watch
DataProducer<Voter> voters = new DataProducer<Voter>();

// Add the aggregators
IFuture<int> total = voters.Count();
IFuture<int> adults = voters.Count(voter => voter.Age >= 18);
IFuture<int> children = voters.Where(voter => voter.Age < 18).Count();
IFuture<int> youngest = voters.Min(voter => voter.Age);
IFuture<int> oldest = voters.Select(voter => voter.Age).Max();

// Push all the data through
voters.ProduceAndEnd(Voter.AllVoters());

// Write out the results
Console.WriteLine("Total voters: {0}", total.Value);
Console.WriteLine("Adult voters: {0}", adults.Value);
Console.WriteLine("Child voters: {0}", children.Value);
Console.WriteLine("Youngest vote age: {0}", youngest.Value);
Console.WriteLine("Oldest voter age: {0}", oldest.Value);