Dependency injection lifetimes in ASP.NET Core — SOLID Principles with Examples

6. Dependency injection lifetimes in ASP.NET Core

Intermediate★ Must knowLesson 6 of 8

Lifetime	One instance per	Typical use
`AddTransient`	Every resolution	Lightweight, stateless services
`AddScoped`	HTTP request (scope)	`DbContext`, unit of work, per-request state
`AddSingleton`	Application	Configuration, caches, `HttpClient` handlers, thread-safe services

csharp

builder.Services.AddSingleton<IClock, SystemClock>();
builder.Services.AddScoped<IOrderRepository, SqlOrderRepository>();
builder.Services.AddTransient<IEmailBuilder, EmailBuilder>();
builder.Services.AddKeyedSingleton<IPaymentGateway, StripeGateway>("stripe");   // .NET 8 keyed services

Captive dependency bug: a singleton that takes a scoped service (e.g. DbContext) keeps that one instance forever, so it is shared across requests and threads. The default container throws in Development (ValidateScopes). Fix: inject IServiceScopeFactory and create a scope, or change the lifetimes.