How to execute a dynamic list of async functions in a sequential way?

Question

I have refactored a Web API to rely on async/await in ASP.NET Core 3.1 and I have the following scenario: a statistics method is sequentially computing a list of indicators which are defined in a list.

readonly Dictionary<StatisticItemEnum, Func<Task<SimpleStatisticItemApiModel>>> simpleItemActionMap =
    new Dictionary<StatisticItemEnum, Func<Task<SimpleStatisticItemApiModel>>>();

private void InitSimpleStatisticFunctionsMap()
{
    simpleItemActionMap.Add(StatisticItemEnum.AllQuestionCount, GetAllQuestionCountApiModel);
    simpleItemActionMap.Add(StatisticItemEnum.AllAnswerCount, GetAllAnswerCountApiModel);
    simpleItemActionMap.Add(StatisticItemEnum.AverageAnswer, GetAverageAnswer);
    // other mappings here
}

private async Task<SimpleStatisticItemApiModel> GetAllQuestionCountApiModel()
{
    // await for database operation
}

private async Task<SimpleStatisticItemApiModel> GetAllAnswerCountApiModel()
{
    // await for database operation
}

private async Task<SimpleStatisticItemApiModel> GetAverageAnswer()
{
    // await for database operation
}

The code sequentially goes through each item and computes it and after the refactoring it is looking like this:

itemIds.ForEach(itemId =>
{
    var itemEnumValue = (StatisticItemEnum) itemId;
    if (simpleItemActionMap.ContainsKey(itemEnumValue))
    {
        var result = simpleItemActionMap[itemEnumValue]().Result;
        payload.SimpleStatisticItemModels.Add(result);
    }
});

I know that Task.Result might lead to deadlocks, but I could not find any other way to make this work.

Question: How to execute a dynamic list of async functions in a sequential way?

Stephen Cleary · Accepted Answer · 2020-01-21T03:05:35.483

1

You should change the ForEach call to a regular foreach, and then you can use await:

foreach (var itemId in itemIds)
{
  var itemEnumValue = (StatisticItemEnum) itemId;
  if (simpleItemActionMap.ContainsKey(itemEnumValue))
  {
    var result = await simpleItemActionMap[itemEnumValue]();
    payload.SimpleStatisticItemModels.Add(result);
  }
}

Do not make the ForEach lambda async; that will result in an async void method, and you should avoid async void.

edited Jan 21 '20 at 03:05

answered Jan 20 '20 at 16:50

Stephen Cleary

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675
810

yes, my final solution is indeed using `foreach` instead of `ForEach` to simplify things. Now I also understand that it is the only way. Thanks. – Alexei - check Codidact Jan 20 '20 at 17:23

score 0 · Answer 2 · answered Jan 20 '20 at 09:48

0

I think you can do this:

itemIds.ForEach(async itemId =>
{
    var itemEnumValue = (StatisticItemEnum) itemId;
    if (simpleItemActionMap.ContainsKey(itemEnumValue))
    {
        var result = await simpleItemActionMap[itemEnumValue]();
        payload.SimpleStatisticItemModels.Add(result);
    }
});

answered Jan 20 '20 at 09:48

Michal Diviš

2,008
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Yes, I have tried using await, but I simply could figure out where the `async` was missing. Of course, on the lambda expression / "closure" itself. Thanks. – Alexei - check Codidact Jan 20 '20 at 10:22
Cool! I'm glad it helped. – Michal Diviš Jan 20 '20 at 11:26

How to execute a dynamic list of async functions in a sequential way?

2 Answers2