Create new DbContext dynamically when using ASP.Net Core dependency injection

Question

I've configured my DbContext with services.AddDbContext() in the Startup class and constructor injection in my controllers works very well.

By default it's a scoped service, but I have one place in the app where I want to update a single property of an entity in a separate scope of work. So I need to create a new DbContext in the controller, but I'm not sure how. I want it to be created by the DI so I don't have to manually call the constructor and provide all the options needed. Is there a way to do that? Maybe there's a way to get the db context options from the DI? Then I could construct the DbContext easily.

You can get the DbContextOptions from DI and then create a new DbContext with it. — Brad
If it is in your controller, then why not DI? Why not creating a service that abstracts that away from your controller and you let take Di take care of the rest? — Roelant M
I want to use DI, but the DbContext is configured as a scoped service, so the DI always returns me the same instance in the context of a single request. But I want a new one and I'm not sure how to ask DI for a new instance. Getting DbContextOptions from DI sounds promising. I'm also thinking about create a new DI scope and then ask it for a new DbContext instance. — Michał Fronczyk

Matt Matt · Accepted Answer · 2019-01-10T17:05:14

The normal method of injecting a DbContext into your Controller works fine, as long as you are doing a reasonable amount of work during an HTTP request. However, you might want to create a DbContext if you are running a background/parallel operation, or if you are querying and modifying a lot of records (causing SaveChangesAsync() to get bogged down because DbContext.ChangeTracker is tracking a lot of objects). If so, you can create a scoped DbContext for each operation. Here is an example ASP.NET Core Controller method:

    /// <summary>
    /// An endpoint that processes a batch of records.
    /// </summary>
    /// <param name="provider">The service provider to create scoped DbContexts.
    /// This is injected by DI per the FromServices attribute.</param>
    /// <param name="records">The batch of records.</param>
    public async Task<IActionResult> PostRecords(
        [FromServices] IServiceProvider provider,
        Record[] records)
    {
        // The service scope factory is used to create a scope per iteration
        var serviceScopeFactory = 
            provider.GetRequiredService<IServiceScopeFactory>();

        foreach (var record in records)
        {
            // At the end of the using block, scope.Dispose() will be called,
            // releasing the DbContext so it can be disposed/reset.
            using (var scope = serviceScopeFactory.CreateScope())
            {
                var context = scope.ServiceProvider.GetService<MainDbContext>();

                // Query and modify database records as needed

                await context.SaveChangesAsync();
            }
        }

        return Ok();
    }

Also, I would recommend switching from AddDbContext() to AddDbContextPool() in Startup.cs to avoid creating/destroying DbContext objects for each request. The DbContextPool will reset the DbContext objects to a clean state after they go out of scope. (In case you were interested, DbContextPool calls DbContext.ResetState() and DbContext.Resurrect(), but I wouldn't recommend calling those directly from your code, as they will probably change in future releases.) https://github.com/aspnet/EntityFrameworkCore/blob/v2.2.1/src/EFCore/Internal/DbContextPool.cs#L157

Create new DbContext dynamically when using ASP.Net Core dependency injection

2 Answers