Multi-async in Entity Framework 6?

Question

This is my code:

var banner = context.Banners.ToListAsync()
var newsGroup = context.NewsGroups.ToListAsync()
await Task.WhenAll(banner, newsGroup);

But when i called the function from controller. It showed error

A second operation started on this context before a previous asynchronous operation completed. Use 'await' to ensure that any asynchronous operations have completed before calling another method on this context. Any instance members are not guaranteed to be thread safe.

Please help me solve this issue.

I have 2 task. If i run each task. it's success. but if i run like my code above. It's error — An Hv
– An Hv, Commented Dec 17, 2013 at 8:16

Stephen Cleary · Accepted Answer · 2013-12-17 13:10:20Z

122

The exception explains clearly that there is only one asynchronous operation per context allowed at a time.

So, you either have to await them one at a time as the error message suggests:

var banner = await context.Banners.ToListAsync();
var newsGroup = await context.NewsGroups.ToListAsync();

Or you can use multiple contexts:

var banner = context1.Banners.ToListAsync();
var newsGroup = context2.NewsGroups.ToListAsync();
await Task.WhenAll(banner, newsGroup);

answered Dec 17, 2013 at 13:10

Stephen Cleary

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22 Comments

Pedro.The.Kid Over a year ago

just a note, if you have a Lazy variable that uses the context in the query even with the await it will throw the same error, just get the property before the query, it was a pain to find out this.

Stephen Cleary Over a year ago

@Pedro.The.Kid: As a general rule, don't use lazy loading with asynchronous DB access. Lazy loading is always synchronous, so it's far better to use Include or separate queries for the additional data.

Zapnologica Over a year ago

Is there any specific reason why you need a context per async query? I feel like this becomes quite a bit of a limiting factor.

Stephen Cleary Over a year ago

@Zapnologica: It's just the way ES6 was designed. Each context can only handle one query at a time. So if you finish one query before the next one begins, you'd only need one context. It's only a problem if you want to do multiple queries at the same time.

Fabio Milheiro Over a year ago

@StephenCleary, I am having a hard time finding that query as I there isn't anything immediately before the exception. Is there a way for us to find what is currently being executed? Thanks

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Vitaliy Ulantikov · Accepted Answer · 2017-09-08 05:55:07Z

4

If you are using IoC container for your Data Provider injection, consider to use "transient" or "PerWebRequest" type for your lifecycle.

For example: https://github.com/castleproject/Windsor/blob/master/docs/lifestyles.md

edited Sep 8, 2017 at 5:55

Vitaliy Ulantikov

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answered Nov 11, 2016 at 20:38

Teoman shipahi

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Comments

Ogglas · Accepted Answer · 2017-07-18 09:30:24Z

3

If you use Unity for dependency injection with for example repository pattern you will get the following error using two or more contexts with create/update/delete:

The relationship between the two objects cannot be defined because they are attached to different ObjectContext objects.

This can be solved using PerRequestLifetimeManager. More info here:

C# EF6 make multiple async calls to one context using Unity - Asp.Net Web Api

container.RegisterType<DbContext>(new PerRequestLifetimeManager());
container.RegisterType<ISupplierRepository, SupplierRepository>();
container.RegisterType<IContactRepository, ContactRepository>();

answered Jul 18, 2017 at 9:30

Ogglas

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