How to await a method in a Linq query

Question

Trying to use the await keyword in a LINQ query and I get this:

The 'await' operator may only be used in a query expression within the first collection expression of the initial 'from' clause or within the collection expression of a 'join' clause

Sample Code:

var data = (from id in ids
            let d = await LoadDataAsync(id)
            select d);

Is it not possible to await something in a LINQ query, or does it need to be structured a different way?

Answer 1

LINQ has very limited support for async/await. For LINQ-to-objects, the only really useful operation I know of is to do a Select with an async delegate (which results in a sequence of tasks).

List<T> data = new List<T>();
foreach (var id in ids)
  data.Add(await LoadDataAsync(id));

If you can do LoadDataAsync in parallel safely, your example could be rewritten as:

T[] data = await Task.WhenAll(ids.Select(id => LoadDataAsync(id)));