How would you integration-test an ASP.NET Core API with `WebApplicationFactory`?

Integration-test ASP.NET Core with WebApplicationFactory

Uses the real ASP.NET Core pipeline while replacing only environment-specific boundaries deliberately.

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TL;DR

I derive a custom WebApplicationFactory<Program>, create an HttpClient, and call the API over HTTP. The test boots the real service registration and request pipeline with test configuration. I replace external services and the database only through explicit test registrations, then assert status code, headers, and response contract rather than calling the endpoint method directly.

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Application hosting, configuration overrides, middleware, authentication, dependencies, disposal, and HTTP assertions.

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A handler can be correct while routing, middleware order, model binding, authentication, or serialization is wrong. Hosting the application catches those composition failures.
I derive a custom WebApplicationFactory<Program>, create an HttpClient, and call the API over HTTP. The test boots the real service registration and request pipeline with test configuration. I replace external services and the database only through explicit test registrations, then assert status code, headers, and response contract rather than calling the endpoint method directly.
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The factory can remove a production registration and add a controlled fake or test container. For authentication I use a test scheme that creates known claims, while separate tests still verify real token configuration where needed. I keep one factory setup readable and give each test isolated data. This level is especially useful for validation ProblemDetails, authorization policies, middleware ordering, and JSON options.

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Details

Calling controller methods and calling it integration testing.
Replacing every registration with a mock.
Sharing one mutable database state.
Ignoring response headers and error contracts.

How would you integration-test an ASP.NET Core API with `WebApplicationFactory`?

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