Mock HttpMessageHandler, Not Interfaces, for Better HttpClient Testing in .NET

A common but flawed practice among .NET developers involves wrapping HttpClient behind a custom interface to enable unit testing, which bypasses real serialization, headers, and error handling. Since HttpClient accepts an HttpMessageHandler in its constructor, developers can inject a mock handler directly to intercept and validate outgoing HTTP requests without losing built-in features. This approach allows tests to verify request methods, URIs, status codes, and response deserialization in a realistic way. Helper classes can further simplify test setup by creating pre-configured mock clients for both success and error scenarios. The technique avoids unnecessary abstractions while keeping tests accurate and maintainable.