How do DI lifetimes work in ASP.NET Core, and what problems can the wrong lifetime cause in an API?

How do dependency injection lifetimes affect ASP.NET Core APIs?

Explains transient, scoped, and singleton lifetimes in ASP.NET Core APIs and production bugs caused by wrong lifetime choices.

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TL;DR

ASP.NET Core commonly uses transient, scoped, and singleton lifetimes. Transient services are created each time they are requested, scoped services live for one request scope, and singletons live for the application lifetime. In APIs, scoped is common for request-oriented services and EF Core DbContext. A wrong lifetime can create bugs like shared state between users or using a disposed dependency.

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Request scope, service lifetime boundaries, state leaks, thread safety, background services, and production behavior.

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Why this matters
DI lifetimes are not just container settings. In web APIs, wrong lifetimes can leak state across users, capture disposed services, or make background services fail in subtle ways.
ASP.NET Core commonly uses transient, scoped, and singleton lifetimes. Transient services are created each time they are requested, scoped services live for one request scope, and singletons live for the application lifetime. In APIs, scoped is common for request-oriented services and EF Core DbContext. A wrong lifetime can create bugs like shared state between users or using a disposed dependency.
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I use scoped services for request work, transient for lightweight stateless helpers, and singleton only for truly thread-safe long-lived services. For example, DbContext is usually scoped because it represents a unit of work for a request. The trade-off is that singletons can be efficient for expensive thread-safe objects, but they are dangerous if they capture scoped services or mutable request state. I always check the dependency chain before changing lifetimes.

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Registering everything as singleton for performance.
Injecting DbContext into a singleton.
Storing user/request state in singleton services.
Assuming transient is always safer.
Ignoring background service scope creation.

How do DI lifetimes work in ASP.NET Core, and what problems can the wrong lifetime cause in an API?

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