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A response can be correct for the current request and still corrupt what a later reader sees. If an ASP.NET Core response filter translates values directly on a DTO that is also held in a shared cache, the next request may receive the first caller’s translated text instead of the authoritative source value. Keep cached data unchanged; build request-specific output that belongs to the response.
How a correct-looking response creates a later bug
Imagine a catalogue cache containing a product description in its source language. A request asks for another language, and a response filter replaces the DTO’s description with a translation. The first caller sees the expected output, so a test that checks only that response passes.
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The problem is object ownership. If the response and cache refer to the same in-process DTO instance, assigning the translated string changes the cached object too. A later source-language request—or a background consumer reading the same object—can now see or persist presentation text as if it were source data. The response was right; the shared state was not.
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Separate authoritative data, request context, and output
Keep three roles distinct:
- Cached or domain data: the authoritative input, such as the source-language catalogue text.
- Request context: the selector for presentation rules, such as the requested language.
- HTTP response payload: request-specific output that can be transformed without changing shared or caller-owned objects.
This boundary is useful beyond localization: any per-request presentation change can create the same ownership bug when it is written back into an object shared with a cache or another consumer. The practical rule from Ivan Rossouw’s article is: “The practical rule is short: if a value is shared, treat it as immutable.”
#1 Best Overall
Choose a response projection that fits the application
The right implementation depends on the application’s serializer contract and constraints. The invariant is more important than a particular API: request-specific work must not mutate cache-owned or caller-owned objects.
Map into a dedicated response model
Create a response-specific model from the authoritative DTO, applying presentation changes to the new model. This makes the ownership boundary visible in the types and keeps the cached representation intact. It does require maintaining a mapping and response model.
Rank #2
Clone before changing values
Copy the object or object graph, then apply transformations to the copy. Ensure the clone covers nested objects and collections; a shallow copy can leave nested mutable values shared with the cache.
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Substitute presentation values as the response is serialized or otherwise materialized, rather than assigning them to the source DTO. This can avoid maintaining a separate model, but it must work with the application’s actual result types and serializer configuration.
Rank #3
Keep the unchanged path cheap and define failure behavior
Projection has costs: JSON materialization, translation lookup work, and temporary allocations. If a request already uses the source representation, retain a pass-through path where appropriate. Measure transformed requests with representative payload sizes; the available comparison is qualitative and supplies no benchmark or performance percentage.
Decide which payloads are eligible before applying a response transformation. Error responses and security-sensitive payloads may need exclusion or different handling. Also decide what happens when projection fails. For a presentation feature, returning an untransformed response may be an acceptable fallback—or a feature failure. For redaction, an untransformed fallback can expose data that should not be visible, so the system may need to fail closed instead.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Test the next reader, not just the first response
A snapshot of the transformed response cannot establish that shared state stayed unchanged. Test the sequence that exposes the ownership bug:
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- Request a transformed response and verify that the response contains the expected presentation value.
- Read the cached object again and verify that its source value has not changed.
- Request the source representation and verify that it still contains the original value.
Where practical, exercise this sequence through the real result filter and serializer configuration, not just a mapping helper. Add focused cases for nested collections, response wrappers, explicit JSON results, errors, exempt payloads, and projection failure. Those cases help reveal where a pipeline’s behavior differs from the ordinary DTO path.
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What the source establishes
Ivan Rossouw’s “Project the Response, Not the Cache” was listed on DEV Community as posted October 1, 2026. The guidance describes an ownership invariant and qualitative implementation trade-offs; it does not report a measured performance comparison. The article’s indexed substantive text is the basis for this summary.
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