LINQ select new Async/Await

Question

I currently have a asynchronous query as follows which is fine and allows me to use the FirstOrDefaultAsync / ToListAsync methods.

 public async Task<X> FindXAsync(int x)
 {
      var q = from c in context.X
              where c.Id == x
              select c;

      return await q.FirstOrDefaultAsync();
 }

However I am attempting to extend that query to select into a new class

public async Task<XClass> FindXAsync(int x)
{
     var q = from c in context.X
            where c.Id == x
            select new XClass (
            c.Id,c.Header .........
            );

     return await q.FirstOrDefaultAsync();
}

For the above you can no longer use the FirstOrDefaultAsync() only FirstOrDefault(), I was wondering what would be the most efficient way to get this functionality into an asynchronous method. Thanks, Chris

@MihailStancescu because he want to project the result with select — tede24, Mar 10 '16 at 11:29
Ok you are right. I thought that XClass is the same as `context.X`. — Mihail Stancescu, Mar 10 '16 at 11:30
But your syntax inside select seems wrong you wrote "new class XClass{c.Id.." But should be "new class {Id=c.Id.." — tede24, Mar 10 '16 at 11:33
@tede24 xClass has a constructor that takes in those as parameters — Chris, Mar 10 '16 at 11:42
@Chris You should not be using the class constructor for EF query projection — Ivan Stoev, Mar 10 '16 at 11:48
You can't use class Initialization there as part of linq to entities. Do you really need it? Why? — tede24, Mar 10 '16 at 11:48
Anonymous async is not supported Take a look this answer http://stackoverflow.com/a/252426/1876572 — Eldho, Mar 10 '16 at 11:49

score 3 · Accepted Answer · answered Mar 10 '16 at 11:51

3

In this case, the easiest answer is probably just to do the (asynchronous) first selection, and then create the type you need:

public async Task<XClass> FindXAsync(int x)
{
  var q = from c in context.X
          where c.Id == x
          select c;
  var c = await q.FirstOrDefaultAsync();
  return new XClass (
           c.Id,c.Header .........
  );
}

answered Mar 10 '16 at 11:51

Stephen Cleary

437,863
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Thanks Stephen, realized the problem after reading the comments but thanks for the answer. p.s. Concurrency In C# Cookbook was an excellent reference book for starting to work with asynchronous programming. – Chris Mar 10 '16 at 12:04
On a side note how would this work with the result of a stored procedure as the result is an IEnumerable ? – Chris Mar 10 '16 at 12:13
@Chris: I don't have much experience with sprocs, but I believe it should work fine; it would be exposed as an IQueryable over the network and I'd expect EF's `FirstOrDefaultAsync` to work. – Stephen Cleary Mar 10 '16 at 12:41

score 1 · Answer 2 · answered Mar 10 '16 at 11:34

1

Try it with generic typed parameter:

return await q.FirstOrDefaultAsync<XClass>();

answered Mar 10 '16 at 11:34

Mihail Stancescu

4,088
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q does not contain a definition for FirstOrDefaultAsync – Chris Mar 10 '16 at 11:45
Add reference to `System.Data.Entity` from EntittyFramework. – Mihail Stancescu Mar 10 '16 at 11:49
Additionally you need to use the parameterless constructor for `XClass`. ie `new XClass { Id = c.Id, ...}` rather than `new XClass(c.Id,...)` – Bob Vale Jan 11 '18 at 12:31

LINQ select new Async/Await

2 Answers2