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How would you unit test a service that depends on multiple injected interfaces without spinning up the full DI container?

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Answer

Unit tests typically construct the service under test directly, passing mock or fake implementations of its dependencies (created via a mocking library like Moq or NSubstitute, or hand-written test doubles) directly to the constructor -- bypassing the DI container entirely, since unit tests should test the class's logic in isolation, not the wiring itself (that's what integration tests using WebApplicationFactory are for).

[Fact]
public async Task ProcessOrder_SendsConfirmationEmail() {
    var mockEmailSender = new Mock<IEmailSender>();
    var sut = new OrderService(mockEmailSender.Object);  // direct construction, no DI container

    await sut.ProcessOrderAsync(new Order());

    mockEmailSender.Verify(e => e.SendAsync(It.IsAny<string>(), It.IsAny<string>()), Times.Once);
}

Real-world example A test suite for OrderService directly constructs it with a mocked IEmailSender and IPaymentGateway, verifying business logic behavior in milliseconds without any DI container overhead or database/network dependencies.

Common follow-ups: When would an integration test actually need to resolve services through the real DI container?;How do you test a service with many dependencies without excessive mock setup boilerplate?

Testing in .NET (xUnit Integration & Unit Testing);Dependency Injection