Category: task

Could you spot the problem in transforming one code that is sequential into a Task ( threaded ) one?

First , this is the sequential one:

            var ret = new List<Indicator>();
            var jsonData = await data.JsonData();
            var jo = JArray.Parse(jsonData);
            var page = jo[0].ToObject<Pagination>();
            var array = jo[1].ToObject<Indicator[]>();
            ret.AddRange(array);
            var currentPage = 1;
            while (currentPage < page.pages)
            {
                currentPage++;
                jsonData = await data.JsonData(currentPage);
                jo = JArray.Parse(jsonData);
                array = jo[1].ToObject<Indicator[]>();
                ret.AddRange(array);

            }
            Debug.Assert(ret.Count == page.total, $"{nameof(ret.Count)} : {ret.Count} should be equal {nameof(page.total)} : {page.total}");
            return ret.ToArray();

This is the modified with task:

            var jsonData = await data.JsonData();
            var jo = JArray.Parse(jsonData);
            var page = jo[0].ToObject<Pagination>();
            var array = jo[1].ToObject<Indicator[]>();
            var ret = new List<Indicator>(array);

            var currentPage = 1;
            var downloads = new List<Task>();
            while (currentPage < page.pages)
            {
                currentPage++;
                var itemPage = currentPage;
                var task = data.JsonData(itemPage)
                    .ContinueWith(it =>
                    {
                        var data = JArray.Parse(it.Result);
                        var pageNr = data[0].ToObject<Pagination>();
                        var indicators = data[1].ToObject<Indicator[]>();
                       ret.AddRange(indicators);

                    }
                );

                downloads.Add(task);
            }
            await Task.WhenAll(downloads);
            Debug.Assert(ret.Count == page.total, $"{nameof(ret.Count)} : {ret.Count} should be equal {nameof(page.total)} : {page.total}");
            return ret.ToArray();

Ok. If you do not know , here is a helper:
Sometimes, at line

 ret.AddRange(indicators)

it gives the error:

System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. —> System.ArgumentException: Source array was not long enough. Check srcIndex and length, and the array’s lower bounds.
at System.Array.Copy(Array sourceArray, Int32 sourceIndex, Array destinationArray, Int32 destinationIndex, Int32 length, Boolean reliable)
at System.Collections.Generic.List`1.set_Capacity(Int32 value)
at System.Collections.Generic.List`1.InsertRange(Int32 index, IEnumerable`1 collection)

Ok. I let you think.
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The answer is simple : List < T > is not Thread safe. If you have small quantities of data, does not matter. I was having 16174 records – and sometimes , when multiple threads have come to .AddRange , the internal re-dimensioning of array it does not fulfill the task ( imagine one thread redim to 100, then other 3 threads coming with request of 50+ data)

Solution 1.

Use ConcurrentBag< T > instead of List < T > . This is thread safe and does the trick

Solution 2.
Add to the array in the final of data ( not use ContinueWith) . The code is slightly low performant

           var jsonData = await data.JsonData();
            var jo = JArray.Parse(jsonData);
            var page = jo[0].ToObject<Pagination>();
            var array = jo[1].ToObject<Indicator[]>();
            var ret = new List<Indicator>(array);

            var currentPage = 1;
            var downloads = new List<Task<string>>();
            while (currentPage < page.pages)
            {
                currentPage++;
                var itemPage = currentPage;
                var task = data.JsonData(itemPage);
                downloads.Add(task);
            }
            await Task.WhenAll(downloads);

            foreach (var t in downloads)
            {
                var data = JArray.Parse(t.Result);
                //var pageNr = data[0].ToObject<Pagination>();
                var indicators = data[1].ToObject<Indicator[]>();
                //Console.WriteLine($"reading {pageNr.page} with {item.Length}");
                ret.AddRange(indicators);

            }
            //Console.WriteLine($"total records {ret.Count}");
            Debug.Assert(ret.Count == page.total, $"{nameof(ret.Count)} : {ret.Count} should be equal {nameof(page.total)} : {page.total}");
            return ret.ToArray();

You will find the code at IndicatorRepository in https://github.com/ignatandrei/WorldBankAPi