Create new DbContext dynamically when using ASP.Net Core dependency injection - asp.net-core

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.

Answer 1

The normal method of injecting a DbContext into your Controller works fine, as long as you are doing a small amount of work during an HTTP request. However, you might want to create a DbContext for a long-running a operation that queries/modifies a lot of records (causing SaveChangesAsync() to get bogged down because DbContext.ChangeTracker is tracking a lot of objects). In that case, you can create a scoped DbContext for each operation ("unit of work"). 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
Finally, be aware that there are a few pitfalls of creating multiple DbContexts:
Using a large number of DbContexts in parallel may cause the database server to run out of active connections, since many EF database providers open a database connection per DbContext. (Requesting and releasing pooled DbContext objects in a loop should be fine.)
There may be more efficient ways to do the same thing. On my project, I tested and found that running a single "upsert" on a single DbContext was significantly faster than running a SELECT and INSERT/UPDATE on a separate DbContext for each record. There are a number of implementations of upsert for EF Core. For example, here are two that I have used:
FlexLabs.Upsert: https://github.com/artiomchi/FlexLabs.Upsert
EF Extensions BulkMerge: https://entityframework-extensions.net/bulk-merge

Answer 2

One option is to inject IDbContextFactory into your comtroller to create contexts within using blocks.
https://msdn.microsoft.com/en-us/library/hh506876(v=vs.113).aspx