AddSingleton vs AddScoped vs AddTransient — when to use which?

AddTransient — a new instance every time it's resolved:

services.AddTransient<IGuidGenerator, GuidGenerator>();

Use for stateless services that are cheap to create.

AddScoped — one instance per HTTP request:

services.AddScoped<IUnitOfWork, EfUnitOfWork>();

Use for anything that holds per-request state (DbContext, current-user context).

AddSingleton — one instance for the entire app lifetime:

services.AddSingleton<IMemoryCache, MemoryCache>();
services.AddSingleton<IHttpClientFactory, ...>();

Use for thread-safe utilities, caches, and configuration.

The lifetime trap:

services.AddSingleton<EmailService>();
public class EmailService(AppDbContext db) { /* ... */ }   // ❌

A singleton holding a scoped DbContext leaks: the context outlives the request, the connection never returns to the pool, and you'll see ObjectDisposedException randomly.

Rule: the dependency's lifetime must be ≥ the dependent's lifetime. Singleton can only depend on Singleton.