expr-debug

name	expr-debug
description	Inspect and debug EggMapper's compiled expression trees for a type pair
disable-model-invocation	true
user-invocable	true
allowed-tools	Bash, Read, Grep, Glob

Expression Tree Debugger

Inspect what expression tree EggMapper compiles for a specific source→destination type pair. Helps diagnose why a property is unmapped, null, or taking the slow path.

Arguments

• $ARGUMENTS — the type pair or symptom to debug (e.g., "OrderDto to Order", "why is Address null", "which path does UserDto use")

Steps

1. Find the relevant types in the codebase:

   • Search src/EggMapper.UnitTests/ and src/EggMapper.Benchmarks/ for the types mentioned
   • Identify the source and destination types

2. Trace the compilation path in src/EggMapper/Execution/ExpressionBuilder.cs:

   • Check if the type pair would qualify for ctx-free path (TryBuildCtxFreeDelegate) — no hooks, no conditions, no MaxDepth, has parameterless ctor
   • Check if it would use typed delegate path (TryBuildTypedDelegate) — same constraints but boxed
   • Or if it falls to flexible path (BuildFlexibleDelegate) — per-property action arrays

3. For each destination property, trace which assignment method handles it:

   • Direct same-type assignment?
   • Numeric conversion?
   • Nullable unwrap/wrap?
   • Inlined nested object (TryBuildCtxFreeNestedAssign)?
   • Inlined collection (TryBuildCtxFreeCollectionAssign)?
   • Flattened property (TryBuildTypedFlattenedAssign)?
   • Or falls through unmapped?

4. Report findings:

   Expression Debug: {SourceType} → {DestType}

   Delegate path: ctx-free / typed / flexible (and WHY)

   Property mapping:

   Dest Property	Source	Method	Notes

   Potential issues:

   • Properties that fall through unmapped
   • Properties forcing the map to flexible path
   • Type mismatches that could be auto-converted

5. If a property is unexpectedly null/unmapped, explain exactly why and suggest the fix (ForMember, type conversion, missing CreateMap for nested type, etc.)