Entity Framework - async select with where condition

Question

I'm using ASP.NET Core with Entity Framework.

First I select an employee, and then all employees that satisfy a condition (for the purpose of displaying what works):

var a = db.Employee.FirstOrDefault();
var b = db.Employee.Where(x => x.FirstName == "Jack");

Now I try the same, but asynchronously:

var c = await db.Employee.FirstOrDefaultAsync();
var d = await db.Employee.Where(x => x.FirstName == "Jack");

However, for the "WHERE" there's no async version, and the second line of code doesn't compile - I get an error

... does not contain a definition for GetAwaiter ...

How do I perform a SELECT with a WHERE condition in this case?

OK, from the answers I see that ToListAsync() will resolve the "var d = ..." line. However, there's a continuation to this issue, I wasn't aware before that it matters. In this case I'm just trying to select a set of records that will be deleted, I'm not interested in accessing the data for the purpose of manipulating it further in the code. So I amended all 4 code versions with purpose to delete one or more records, synchronously or asynchronously. Why does only the last one need a ToListAsync(), won't that actually retrieve the records from the database?

var a = db.Employee.FirstOrDefault();
db.Employee.Remove(a);
// db.Employee.RemoveRange(a); <- this also works?
db.SaveChanges();

var b = db.Employee.Where(x => x.FirstName == "Jack");
db.Employee.RemoveRange(b);
db.SaveChanges();

var c = await db.Employee.FirstOrDefaultAsync();
db.Employee.Remove(c);
await db.SaveChangesAsync();

var d = await db.Employee.Where(x => x.FirstName == "Jack").ToListAsync();
db.Employee.RemoveRange(d);
await db.SaveChangesAsync();

RemoveRange will first pull all items from database, then mark as deleted. So there is no harm to call ToListAsync first - otherwise it will be done (synchronously) by RemoveRange itself. — Evk
– Evk, Commented Nov 20, 2016 at 8:58
But there's no point in pulling all the records from database just to delete them... — Marko
– Marko, Commented Nov 20, 2016 at 9:51
I agree but that is what you do, and that is how EF works with deletes. There are workarounds though, you can google for them (example: github.com/loresoft/EntityFramework.Extended/blob/master/…) — Evk
– Evk, Commented Nov 20, 2016 at 9:56

Sampath · Accepted Answer · 2016-11-19 21:43:36Z

94

You can do it like this.

If you need to retrieve one object then :

var d = await db.Employee.FirstOrDefaultAsync(x => x.FirstName == "Jack");

If you need to retrieve list then :

 var d = await db.Employee.Where(x => x.FirstName == "Jack").ToListAsync();

edited Nov 19, 2016 at 21:43

answered Nov 19, 2016 at 21:32

Sampath

66.2k70 gold badges329 silver badges462 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

variable Over a year ago

How to convert the list into an object to return as a response?

symbiont Over a year ago

@variable not sure what you are using and this is probably too late, but you could try returning the list as a response

Collectives™ on Stack Overflow

Entity Framework - async select with where condition

1 Answer 1

2 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related