mapperly

# Mapperly Conventions and null-handling semantics for Mapperly source-generated mappers. ## Mapper Conventions - `[Mapper(RequiredMappingStrategy = RequiredMappingStrategy.None)]` on every mapper. Generated DTOs have many properties we don't map; this silences unmapped-property warnings. - One mapper class per service (Sonarr/Radarr). Generated DTO types share names but are distinct types from separate packages. - Namespace aliases disambiguate: `using SonarrApi = Recyclarr.Api.Sonarr`. - Mappers are `internal static partial class`. ## Null Handling (nullable DTO -> non-nullable domain) Generated Refit DTOs mark all properties as nullable (`T?`). Domain models use non-nullable types for fields that should always have a value. Mapperly handles the mismatch differently depending on the target shape. ### Constructor parameters (positional records) Mapperly substitutes type-appropriate defaults when the source is null: - `string?` -> `string`: `""` (empty string) - `T?` where `T` has a parameterless constructor: `new T()` - `T?` value type: `default(T)` ### Init properties without initializers Mapperly generates `throw ArgumentNullException` when the source is null. This is hardcoded for `required init` properties regardless of mapper settings. ### Init/setter properties with initializers (e.g. `= ""`, `= []`) Mapperly skips the assignment when the source is null. The initializer value is preserved. This is the safest pattern for "optional with sensible default" semantics. ### Setter properties without initializers Mapperly skips the assignment. The property stays at `default(T)`, which for reference types is `null`. This silently violates the non-nullable contract. Avoid this shape for non-nullable reference types. ### Nullable target properties Assigned directly, including null. No special handling. ## RMG089 Diagnostic "Mapping nullable source property X to target property Y which is not nullable." Default severity: Info. This diagnostic is informational; the generated code handles the mismatch using the rules above. No suppression or severity promotion is needed. ## Inspecting Generated Code To see what Mapperly actually generates, temporarily add to `Directory.Build.props`: ```xml <EmitCompilerGeneratedFiles>true</EmitCompilerGeneratedFiles> ``` Output appears in `obj/Debug/<tfm>/generated/Riok.Mapperly/Riok.Mapperly.MapperGenerator/`. Remove the property when done.