Source coming from multiple threads?

[ejlevin1]

Is the sourcing of the stream thread safe? I.e. would the below situation cause any issues I'm unaware of. I have a situation where I need to make a large amount of async requests that generate the async stream, and want to do the sourcing requests in parallel (unordered), and then push into the async stream. Will that create any negative side effects I'm not seeing?

[Fact]
        public void TestAsyncStreams()
        {
            IList<string> urls = new List<string>() {
                "http://www.google.com",
                "http://www.microsoft.com",
                "http://www.github.com"
            };


            var result = new ConcurrentBag<string>();

            var asyncEnumerableCollection = ProduceWebPages(urls, CancellationToken.None);
            asyncEnumerableCollection.ForEachAsync(async page => {
                result.Add(page);
            }).Wait();

            Assert.Equal(urls.Count, result.Count);
        }

        static IAsyncEnumerable<string> ProduceWebPages(IEnumerable<string> urls, CancellationToken cancellationToken)
        {
            return new AsyncEnumerable<string>(async yield => {

                await urls.ParallelForEachAsync(
                    async uri =>
                    {
                        using (var client = new HttpClient())
                        {
                            var str = await client.GetStringAsync(uri);
                            await yield.ReturnAsync(str);
                        }
                    },
                    maxDegreeOfParalellism: 5,
                    cancellationToken: cancellationToken);

                Console.WriteLine("Done");
            });
        }

[kind-serge]

@ejlevin1, the yield.Return is not thread-safe, so the sample code certainly leads to an unpredicted behavior.

Having multiple producers and one (or multiple) consumers is a slightly more complex problem, which is discussed in issue #32.

I posted an example on StackOverflow how to implement it:
https://stackoverflow.com/questions/54402544/c-sharp-asyncenumerable-running-awaiting-multiple-tasks-never-finishes/56189780#56189780

It seems that the solution to this problem can be a good candidate for a helper/extension method.