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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?

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?

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?

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?

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?

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?

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?

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?

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?

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?

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