Use LINQ to fill Dictionary with async method

Question

I am trying to fill in the Dictionary<long, Data> . Method GetData which returns data is async so I use following code to get the dictionary:

var myDictionary = entities
  .GroupBy(e => e.EntityId)
  .Select(e => e.First())
  .ToDictionary(e => e.EntityId, async entity => await GetData(entity));

Unfortunately myDictionary is of type Dictionary<long, Task<Data>>

How to fill in the using async lambda in ToDictionary?

The simplest, but arguably incorrect workaround is to use `GetData(entity).GetAwaiter().Result`, as this simply forces all the asynchronous operations to execute synchronously. But there is no support in either LINQ or PLINQ (that I know of) for properly constructing collections asynchronously. — Jeroen Mostert, Apr 13 '18 at 14:10
@JeroenMostert: yeah, but I lose all the benefits of async/await — michal.jakubeczy, Apr 13 '18 at 14:14
You need `.ToDictionaryAsync`, but such a method does not exist (except in EF, but then it's never a fully generic version). [Duplicate](https://stackoverflow.com/q/37796139/4137916), albeit with no really good answers. — Jeroen Mostert, Apr 13 '18 at 14:22

Magnus · Answer 1 · 2018-04-13T14:52:43.910

4

Try this:

var myDictionary = 
                (await Task.WhenAll(entities
                .GroupBy(e => e.EntityId)
                .Select(e => e.First())
                .Select(async e => new {e.EntityId, Data = await GetData(e)})))
                  .ToDictionary(e => e.EntityId, e => e.Data);

edited Apr 13 '18 at 14:52

answered Apr 13 '18 at 14:11

Magnus

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GetData returns Task so it returns Dictionary> – michal.jakubeczy Apr 13 '18 at 14:13
And there I thought, this is section is for answers and not for questions. ;) – Rand Random Apr 13 '18 at 14:13
Wasn't Dictionary> what was desired? Or did I missunderstand – Magnus Apr 13 '18 at 14:14
@Servy The tasks are awaited: `await Task.WhenAll(...` – Magnus Apr 13 '18 at 14:29
@Servy "extremely terrible", come on... – Magnus Apr 13 '18 at 14:35
@Magnus: thanks, will try this one and let you know the outcome. – michal.jakubeczy Apr 13 '18 at 14:37
@jabko87 You really shouldn't reward people for both posting incorrect answers, and just copying solutions from the duplicate into their own answer. – Servy Apr 13 '18 at 14:38
@Servy I did not copy from your duplicate, just wrote it. But if it is incorrect please let me know why. – Magnus Apr 13 '18 at 14:40
@Magnus "Just remove the await?" which does nothing at all. – Servy Apr 13 '18 at 14:43
@Servy This is a community where we try to help each other the best we can. Being obnoxious does not help anyone. – Magnus Apr 13 '18 at 14:45
@Magnus And how does posting an answer with an incorrect solution and duplicating a solution posted earlier helping anyone? Also note this is a site to create a useful repository of knowledge, not a help desk. – Servy Apr 13 '18 at 14:47
@Servy I know you are an extremely good developer, I have read many of your answers. But you really should try to be a little bit more humble. – Magnus Apr 13 '18 at 14:51

Use LINQ to fill Dictionary with async method

1 Answers1