Tag: dependency-injection

AddDbContextPool hands out one scoped DbContext per DI scope, so it cannot run two queries at once. AddDbContextFactory and AddPooledDbContextFactory give you a context per call, which is what parallel queries need. Measured on EF Core 11 RC 1: the pooled factory creates a context in 342 ns and 40 B versus 17 us and 44 KB.

Before Aspire 13.5.4, every AppHost health check on an AddKafka resource created a new Confluent producer that nobody disposed, so polling threads piled up and the AppHost burned CPU. The fix, and the HealthCheckRegistration.Factory trap behind it, applies to your own health checks too.

A singleton cannot inject a DbContext, but it can inject IDbContextFactory<T>, because AddDbContextFactory registers the factory as a singleton by default. Create and dispose one context per call, never cache the instance.

Implement IValidateOptions<T>, register it in DI, and chain ValidateOnStart so a bad appsettings.json kills the process instead of the first request that touches it. Covers the .NET 11 Validate<TValidator>() overload, async validation through IAsyncValidateOptions<T>, and the three places ValidateOnStart silently does nothing.

Default to IOptions<T>. Use IOptionsMonitor<T> when a singleton has to see config reloads, and IOptionsSnapshot<T> only when a scoped consumer wants a value that is stable for one request. The deciding axis is the lifetime of the consumer, not the shape of the settings.

EF Core throws this when AddDbContext never ran, ran after Build, or your context's constructor takes the wrong DbContextOptions. Register before Build and use DbContextOptions<YourContext>.

Register more than one implementation of the same interface under a key with AddKeyedSingleton/Scoped/Transient, then resolve them with [FromKeyedServices], GetRequiredKeyedService, or KeyedService.AnyKey. The keyed and non-keyed registries are separate, which is the gotcha that bites most people.

A BackgroundService is a singleton, so it cannot inject a scoped service like a DbContext directly. Take IServiceScopeFactory, open one scope per unit of work with CreateAsyncScope, resolve inside it, and dispose it when the work is done.

A step-by-step checklist to remove MediatR 12-14 and replace IRequest handlers, ISender, pipeline behaviors, and INotification with plain service classes and constructor injection.

For new code, plain service classes are the better default. MediatR's July 2025 license change matters only if you sit above the $5M Community threshold or refuse the RPL-1.5 copyleft. Keep MediatR when pipeline behaviors are load-bearing.