Entity framework async issues context or query?

Question

I have async issue with my below query. I have singleton context and i am trying to execute below query:

var query = await (from parent in Context.ParentTable
                   join child in Context.ChildTable
                   on parent.ID equals child.ID
                   into allResult
                   from ResultValue in allResult.DefaultIfEmpty()
                   where ResultValue.TenantId == tenantId
                   select new Result
                   {
                      Code = parent.Code,
                      Type = parent.Type,
                      ID = ResultValue == null ? 0 : ResultValue.Id
                   }).ToListAsync();

My singleton context looks like this:

public class BaseRepository
{
    private readonly IConfigurationContextFactory configurationContextFactory;

    private IConfigurationContext context;

    protected IConfigurationContext Context
    {
        get
        {
            return context ?? (context = configurationContextFactory.Context);
        }
    }

    public BaseRepository(IConfigurationContextFactory configurationContextFactory)
    {
        this.configurationContextFactory = configurationContextFactory;
    }
}

The configuration context factory returns Context like this:

private ConfigurationContext Get()
{
    logger.WriteEntrySync(LogLevel.Information,
                          null != context ? "Config Context: Using existing context." : "Config Context: Wiil create new context.");
    return context ?? (context = new ConfigurationContext(connectionString));
}

In this i get intermittent issue with following 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.

Answer 1

I have singleton context

This is your problem. DbContext isn't thread-safe, and is designed to execute one query at a time. Since you're sharing your DbContext, you're probably attempting to invoke another query concurrently, which isn't "legal" in DbContext terms.

You can even see it in the remarks of ToListAsync:

Multiple active operations on the same context instance are not supported. Use 'await' to ensure that any asynchronous operations have completed before calling another method on this context.

What you should do is not re-use your context with a global singleton, by create a new context each time you want to query your database.

Edit:

Instead of getting a single Context via your factory method, simply allocate a new one for every query:

using (var context = new ConfigurationContext(connectionString))
{
    var query = await (from feature in context.Features
                join featureFlag in context.FeatureFlags
                on feature.FeatureId equals featureFlag.FeatureId
                into allFeatures
                from featureFlagValue in allFeatures.DefaultIfEmpty()
                where featureFlagValue.TenantId == tenantId
                select new BusinessEntities.FeatureFlag
                {
                   Code = feature.Code,
                   Type = feature.Type,
                   FeatureFlagId = featureFlagValue == null ? 0 : featureFlagValue.FeatureFlagId
                }).ToListAsync();
}

Answer 2

A DbContext should live for one business transaction (unit of work), no more no less. A business transaction is usually a request, a page or a form. DbContext is not thread safe, it keeps an internal entity cache and tracks changes so you cannot share it accros multiple request.

You really need to read Entity Framework documentation : Working with DbContext.

I wouldn't create an instance per repository as a business transaction can manipulate several repositories.

You can inject the context in your repository :

public class BaseRepository
{
    private IConfigurationContext context;
    public BaseRepository(IConfigurationContext context)
    {
        this.context = context;
    }
    //...
}

Change your factory so it creates an instance each time :

public interface IConfigurationContextFactory
{
    IConfigurationContext CreateContext();
}

// ...
public ConfigurationContext CreateContext()
{
    return new ConfigurationContext(connectionString);
}

Then configure your dependency resolver to inject a IConfigurationContext per unit of work. Let's say you are working on an ASP.NET application using unity.

container.RegisterType<IConfigurationContext>(
    //Instance per http request (unit of work)
    new PerRequestLifetimeManager(),
    //Create using factory
    new InjectionFactory(c => c.Resolve<IConfigurationContextFactory>.CreateContext()));

Don't forget to call SaveChangesAsync at the end of your business transaction when needed : operation succeeded and modifications need to be persisted.