When would you organize a .NET API by vertical feature slices instead of Controllers, Services, and Repositories folders?

Vertical slices versus horizontal application layers

Chooses code organization from change locality, shared policy, and cross-cutting needs rather than adopting one folder structure universally.

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I prefer a slice when a request, its validation, mapping, handler, and tests evolve together. A developer can follow CreateOrder without searching several broad folders, and unrelated features are less likely to share a generic service accidentally. I still keep shared domain rules, infrastructure adapters, and genuine cross-cutting behavior in explicit common boundaries. Vertical slicing is organization, not permission to duplicate invariants.

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Change locality, feature ownership, shared domain rules, cross-cutting behavior, coupling, and discoverability.

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Vertical slices place the code for one use case close together, while horizontal layers group code by technical role. The better choice depends on what changes together and which rules truly need sharing.
I prefer a slice when a request, its validation, mapping, handler, and tests evolve together. A developer can follow CreateOrder without searching several broad folders, and unrelated features are less likely to share a generic service accidentally. I still keep shared domain rules, infrastructure adapters, and genuine cross-cutting behavior in explicit common boundaries. Vertical slicing is organization, not permission to duplicate invariants.
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A mature slice can contain its endpoint, command or query, validator, handler, DTOs, and focused tests. Shared pipeline behavior can handle logging, authorization metadata, or validation when that makes execution clearer. I would avoid placing all business logic in handlers if several slices must enforce the same invariant; that belongs in a domain policy or cohesive service. Horizontal layers remain reasonable for small applications or technical libraries whose components genuinely change together.

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Copying shared invariants into every slice.
Treating handlers as a new generic service layer.
Letting slices access each other's persistence directly.
Reorganizing folders without reducing coupling.

When would you organize a .NET API by vertical feature slices instead of Controllers, Services, and Repositories folders?

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