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AutoMapper · Quick recall Q&A

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Quick recall Q&A

Q: When is AutoMapper a good fit vs hand-written mapping? A: Use AutoMapper when mappings are mostly 1:1 and you want centralized profiles plus query projections. If mappings involve conditional branching, heavy domain logic, or large object graphs where clarity matters, hand-written mapping is safer.
Q: How do profiles improve maintainability? A: Profiles group mapping configuration per aggregate or feature, keeping conventions together and discoverable. They can be scanned automatically via AddAutoMapper, ensuring new mappings only require adding a profile class.
Q: What’s the benefit of .ProjectTo<T>() with EF Core? A: It translates mapping expressions into SQL so the database returns DTO-shaped data directly, eliminating extra materialization and reducing memory usage in the API layer.
Q: How do you customize property names that don’t match? A: Use .ForMember(dest => dest.Property, opt => opt.MapFrom(src => src.OtherName)) or .ForPath for nested members. AutoMapper falls back to conventions for matching names but explicit configuration handles mismatches cleanly.
Q: How do you avoid runtime mapping errors? A: Call mapper.ConfigurationProvider.AssertConfigurationIsValid() during startup/tests, and enable CreateMissingTypeMaps = false so AutoMapper throws configuration errors early.
Q: What are best practices for DI registration? A: Register profiles via services.AddAutoMapper(typeof(SomeProfile)) or assembly scanning. Inject IMapper where needed; avoid static mapper instances so you respect scoped dependencies in custom value resolvers.
Q: How do you handle mapping collections efficiently? A: Map queryables using .ProjectTo<T>() to avoid per-item mapping in memory. For in-memory collections, IMapper.Map<IEnumerable<Dest>>(source) reuses configuration and executes efficiently without additional setup.
Q: Can AutoMapper handle complex value conversions like formatting currency? A: Yes, via ForMember(...).ConvertUsing(...), custom value resolvers, or type converters. Keep heavy logic in domain services; use converters for presentation formatting or simple transforms (e.g., decimal to string).
Q: How do you map inheritance hierarchies? A: Configure Include/IncludeBase to map derived types and ensure discriminators map to correct DTOs. AutoMapper can project derived types so long as the EF model supports it.
Q: How do you test mappings? A: Instantiate the mapper configuration in tests, call AssertConfigurationIsValid(), and perform sample Map/ProjectTo calls on fixture data to ensure custom resolvers behave as intended.

See also