Entity Framework 6 Async and .Wait () method

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Entity Framework 6 Async and .Wait () method

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#1 by (3 votes)
#2 by (2 votes)

3

I have a system that is a Web Api, the control executes a class like this:

public class CrawlBO
{
    public void IniciaProcesso()
    {
        ...


        CarregaConfiguracaoCrawlBO carregaconfiguracaocrawlbo = new CarregaConfiguracaoCrawlBO(type);
        Task<ConfiguracaoCrawlModel> montarconfiguracao = carregaconfiguracaocrawlbo.MontarConfiguracaoAsync(type);

        ...

        montarconfiguracao.Wait();
        conf = montarconfiguracao.Result;

        ...
    }

}

My class CarregaConfiguracaoCrawlBO has a async method, this is the class:

public class CarregaConfiguracaoCrawlBO
{
    public async Task<ConfiguracaoCrawlModel> MontarConfiguracaoAsync(EnumOrigens type)
    {
        try
        {
            MYEntities db = new MYEntities ();
            int t = (int)type;

            CRAWL conf = await db.CRAWLs.Where(p => p.ID_ORIGEM == t).FirstOrDefaultAsync();

            ...

            return c;
        }
        catch (Exception ex)
        {

            throw ex;
        }
    }
}

The problem is that it freezes on montarconfiguracao.Wait(); . I've done all test types, and I know that within Task it freezes at the .FirstOrDefaultAsync(); line.

First I thought it might be some error, then I put try catch , but it does not get error, then I thought it might be a problem in localhost , but on the server it hangs in the same place too.

I would like to use .Wait() because I can not put all methods as async at one time.

Adding more content:

I know it for .Wait() to wait for the task, but the problem that the task should take about 4 seconds, and it freezes even, an hour later and still there in the task without doing anything

c# asp.net-web-api

asked by anonymous 12.06.2015 / 14:11

2 answers

2

When doing montarconfiguracao.Wait(); it freezes because that's what Task.Wait() does: wait for the Task to finish.

To prevent this from happening, change the code to:

public class CrawlBO
{

        public async Task IniciaProcesso()
        {

                ...

                CarregaConfiguracaoCrawlBO carregaconfiguracaocrawlbo = new CarregaConfiguracaoCrawlBO(type);
                Task<ConfiguracaoCrawlModel> montarconfiguracao = carregaconfiguracaocrawlbo.MontarConfiguracaoAsync(type);

                ...

                conf = await montarconfiguracao;
                //Após a linha anterior o método retorna

                //O código a partir daqui é executado após a Task finalizar
                // pode usar o valor de conf
                ...
        }

}

If you do not want / can use async in the method you will have to do anything like this:

public class CrawlBO
{

    public void IniciaProcesso()
    {

        ...

        CarregaConfiguracaoCrawlBO carregaconfiguracaocrawlbo = new CarregaConfiguracaoCrawlBO(type);
        Task<ConfiguracaoCrawlModel> montarconfiguracao = carregaconfiguracaocrawlbo.MontarConfiguracaoAsync(type);

        ...

        montarconfiguracao.ContinueWith(t =>
            {
                //O código aqui é executado após a Task finalizar
                conf = t.Result;
            }, TaskScheduler.FromCurrentSynchronizationContext());

         //O código aqui é executado antes da task finalizar
         ...
         ...
    }
}

12.06.2015 / 15:26

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score 3 · Accepted Answer

You are experiencing a deadlock.

Wait() and Result can lead to deadlocks, depending on the execution context.

Assuming that the MontarConfiguracaoAsync method was implemented as follows:

public Task<ConfiguracaoCrawlModel> MontarConfiguracaoAsync()
{
    var result = await something();

    return result;
}

This method will execute something on another thread / IOCP, and when something completes, the control will return to the main thread to execute the return result statement. What happens is that the main thread has been locked in MontarConfiguracaoAsync().Wait() , and therefore the return result statement can not be executed - > deadlock.

For a more detailed explanation, read this excellent Stephen Cleary blog post: Do not Block on Async Code

To answer the @ramaral question:

The first code I know does not cause deadlock, does it use async in the method and does the second cause it?

The solution using ContinueWith would not cause a deadlock. The only difference between using ContinueWith and await , as far as I know, is in exception handling. With await , if any exceptions are thrown during the execution of the task, the exception will be captured and re-released (without changing the stack trace) in the thread that started the task.

Here is a possible sequence of events:

Main thread calls MontarConfiguracaoAsync

Main thread creates an instance of MYEntities and executes a query in the database, initiating an IOC (I / O Completion Port)

Main thread returns to IniciaProcesso method and creates a "continuation" with ContinueWith to be executed on the main thread

The query terminates, and the continuation is scheduled to run on the main thread as soon as it is free

The main thread continues to execute the remaining statements of IniciaProcesso and ends, releasing the main thread

The continuation is executed in the main thread