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LINQ Projections: ProjectTo<T> ​

AutoMappic v0.6.0 introduces the Performance King: ProjectTo<T>. This feature allows object mapping to occur directly inside the database query (SQL), drastically reducing database I/O and memory pressure by only selecting the columns required for the DTO.

1. How It Works ​

Traditional mapping requires fetching the entire entity from the database into memory, then mapping it to a DTO.

csharp
// ❌ UNOPTIMIZED: Selects all columns from 'Users' (including large Blobs/Passwords)
var users = dbContext.Users.ToList(); 
var dtos = mapper.Map<List<UserDto>>(users);

ProjectTo emits a LINQ Expression Tree that translates the mapping directly into a SELECT statement before the database executes it.

csharp
using AutoMappic;

// ✅ OPTIMIZED: Translates to 'SELECT Id, Name FROM Users'
var dtos = dbContext.Users.ProjectTo<UserDto>().ToList();

2. Key Benefits ​

  • Reduced Data Transfer: Only specific columns are sent over the network from the database.
  • Lower Memory Usage: No intermediate "fat" entity objects are materialized in memory.
  • Native Performance: AutoMappic generates the expression tree at compile-time, eliminating the runtime overhead that other mappers incur when building dynamic expressions for LINQ.

3. Supported Features ​

AutoMappic's projection engine supports:

  • Direct Assignments: Simple property matches.
  • Flattening: Deep property access (e.g., User.Address.City).
  • Recursive Nested Objects: Mapping a child entity to a child DTO (e.g., User -> UserDto containing AddressDto).
  • Collections and Lists: Deeply projected nested collections (e.g., List<TodoItem> -> List<TodoItemDto>).
  • Convention-Based Naming: Automatic mapping for similarly named properties.

4. Unsupported Scenarios ​

By design, Projections are limited to what the database (SQL) can translate. The following will trigger AM0008 (Unsupported ProjectTo Feature):

  • Procedural Logic: BeforeMap, AfterMap, or complex custom resolvers.
  • In-Memory Hooks: Predicates that rely on local state or non-translatable C# methods.

5. Performance Benchmarks ​

In comparison to standard IMapper.Map, ProjectTo demonstrates:

  • 5x - 10x faster query execution on tables with 20+ columns.
  • 90% reduction in allocation pressure during DB materialization.

Next: Standalone Mapping ->