Akka.Streams.Tests.Dsl.FlowSplitWhenSpec.SplitWhen_must_fail_substream_if_materialized_twice C# (CSharp) Method

        public void SplitWhen_must_fail_substream_if_materialized_twice()
        {
            this.AssertAllStagesStopped(() =>
            {
                var task = Source.Single(1).SplitWhen(_ => true).Lift()
                    .SelectAsync(1, source =>
                    {
                        source.RunWith(Sink.Ignore<int>(), Materializer);
                        // Sink.ignore+mapAsync pipes error back
                        return Task.Run(() =>
                        {
                            source.RunWith(Sink.Ignore<int>(), Materializer).Wait(TimeSpan.FromSeconds(3));
                            return 1;
                        });
                    })
                    .RunWith(Sink.Ignore<int>(), Materializer);
                task.Invoking(t => t.Wait(TimeSpan.FromSeconds(3)))
                    .ShouldThrow<IllegalStateException>();
            }, Materializer);
        }