Constructor Binding + [DynamoMapperConstructor]

[j-d-ha]

Priority: P2 - Important
Tier: 3 - Advanced Features
Effort: Medium/Large (8-14 hours)

Already In Codebase

• FromItem generation currently always uses an object initializer (parameterless construction): src/LayeredCraft.DynamoMapper.Generators/Templates/Mapper.scriban.
• The property analyzer currently does not distinguish init-only vs regular setters (it only checks SetMethod != null): src/LayeredCraft.DynamoMapper.Generators/PropertyMapping/PropertyAnalyzer.cs.

Goal

Improve FromItem generation to support constructing target objects via constructors when needed, with a deterministic selection policy and an explicit override attribute.

Rules

1. Prefer property assignment when possible.
2. If constructors are required/used, select a constructor as follows:
   • If exactly one constructor is marked with [DynamoMapperConstructor], use it.
   • Else use the constructor with the most parameters ("widest").
   • If multiple constructors tie for widest, emit a diagnostic.
3. Constructor parameters must be bindable to mapped members (by name) and mappable types.

New runtime API

Add a runtime attribute:

• DynamoMapperConstructorAttribute
  • Target: constructors
  • Usage: marks which constructor to use

Implementation notes

• Generator changes:
  • Extend model analysis to inspect constructors on the target model type.
  • Implement a binder that maps constructor parameters -> properties/fields (by name).
  • Generate new TargetType(arg1, arg2, ...) instead of object initializer when using constructor.
  • After construction, still assign any remaining settable non-init-only properties.
  • Init-only properties must be set during construction (not after). This likely requires checking propertySymbol.SetMethod.IsInitOnly.

Diagnostics

• Multiple constructors marked with [DynamoMapperConstructor].
• No usable constructor found for required init-only members.
• Ambiguous widest constructors.
• Constructor parameter cannot be bound.

Tests

Add verify tests in test/LayeredCraft.DynamoMapper.Generators.Tests/ covering:

• Multiple constructors, widest chosen
• [DynamoMapperConstructor] overrides widest
• Init-only properties require constructor binding
• Ambiguous widest constructors -> diagnostic

Acceptance criteria

• Constructor selection is deterministic and follows the rules above.
• Init-only properties are handled correctly (construction-time only).
• Diagnostics are emitted for ambiguous/invalid constructor scenarios.
• Snapshot tests verify generated code.