Why does Task.WhenAll return void?

Question

This is the code from the image above:

if (claims != null && claims.Any())
{
    // firstly, why doesn't this work?
    // var _claimResults = from claim in claims select UserManager.AddClaimAsync(user.Id, claim);

    // but this does...
    List<Task> _claimResults = new List<Task>();
    foreach (var claim in claims)
    {
        _claimResults.Add(UserManager.AddClaimAsync(user.Id, claim));   
    }

    // secondly, why does Task.WhenAll return void when it clearly says it returns Task?
    Task claimsResult = await Task.WhenAll(_claimResults);
}

1. Why doesn't the LINQ expression work, yet the foreach does. The LINQ expression gives me a "underlying provider failed to open" exception on execution.
2. Why does Task.WhenAll() return void when it says it's return type is Task?

Edit: claims is a List<Claim> which I think is List<System.Security.Claim>.

Answer 1

WhenAll returns a Task, but then you're awaiting that task. Awaiting a plain Task (rather than a Task<T>) gives no result. So you either want:

Task claimsResult = Task.WhenAll(_claimResults);

or

await Task.WhenAll(_claimResults);

My suspicion is that the LINQ problem is because your foreach approach materializes the query immediately - the LINQ equivalent would be:

var _claimsResults = claims.Select(claim => UserManager.AddClaimAsync(user.Id, claim))
                           .ToList();

... where the ToList() method materializes the results immediately. (I've used the method call syntax rather than query expression syntax because query expressions are pretty pointless for trivial queries like this.)