Mono Expression Emit

Target directory: Runtime/Mono/Emit/ (from Phase 3) Il2Cpp counterpart: STUBS-IL2CPP.md (build-time stub tables) Constraints: ../MONO.md D3, D6, D7

1. Design principles

At type bind time (after MetaBinding.EnsureBinding, when a Lua closure is first needed), Mono Editor generates a unique Lua C# callback for each public field, property accessor, method, and constructor:

System.Linq.Expressions → Compile() → LuaCSFunction / delegate
    → lua_pushcfunction → write into three tables (method / fieldGetter / fieldSetter)

Unlike Il2Cpp ReducedType signature reuse, Mono does not maintain a global stub hash table; each (Type, MemberInfo, isStatic, isByVal) combo gets one specialized bridge.

2. Failure policy (D3)

Case	Behavior
Signature in Expression-expressible set	Normal Compile
Unsupported signature (e.g. open generic method, certain byref combos)	Bind-time InvalidOperationException, message includes type and member full names
Forbidden	Silent fallback to Method.Invoke + object[]

Known supported sets are covered by Tests/Lua/**; new signature kinds must extend Emit first, then open binding.

3. Implementation order (Phase 3)

../MONO.md suggests simple → complex by dependency:

Step	Member	Write table	Notes
1	Field static/instance	getter → fieldGetterTable; writable → fieldSetterTable	Expression or emit IL for FieldInfo.Get/SetValue
2	Property no-arg	Same as Field; call property getter/setter MethodInfo	Indexer property → methodTable (dispatch)
3	Method single overload	methodTable direct closure	Capture MethodInfo + typed pop/push
4	Method multi-overload	methodTable dispatch closure	Calls managed MethodOverloadResolver (align ../marshal/OVERLOAD-RESOLVER.md)
5	Constructor	SMT.__call	Reuse ctor overload grouping
6	Aliases	Extra keys in methodTable	[LuaAlias], zlua.register_method

Run corresponding Lua cases after each step before continuing.

4. Closure shape (D7)

4.1 Forbidden upvalue pattern

Il2Cpp direct method closures use:

• ClosureKind + TypeBinding* + MethodMarshalCtx*

Mono Emit must not copy MethodMarshalCtx / methodId into Lua upvalues. Specialized bridges should:

• Directly Compile to (IntPtr L) => int delegates;
• Hold statically or capture in closures already resolved MethodInfo/FieldInfo and prebuilt pop/push delegates;
• Overload dispatch captures a read-only candidate array, not runtime string lookup.

4.2 Writing the three tables

After Emit:

// Pseudocode
LuaDll.lua_rawgeti(L, RegistryIndex, binding.StaticTables.MethodTableRef);
LuaDll.lua_pushcfunction(L, compiledDelegate);
LuaDll.lua_setfield(L, -2, methodName);
LuaDll.lua_pop(L, 1);

Aligned with ../metatable/INDEXER-MONO.md: method tables store the closure itself, not a wrapper.

5. this and ByVal / ByObj

Instance method Emit must branch:

Target kind	this source
class ByObj	ObjectRegistry.PopThis(L, 1)
struct ByVal	InstanceTarget equivalent: payload pointer / copy
struct ByObj	boxed ByObj userdata

Align with Il2Cpp InstanceTarget::GetResolveMethodThisFunc; helpers live in Mt/InstanceTarget.cs (placeholder exists).

6. Marshal and Emit cooperation

Pop/push logic reuses typed APIs in Marshaling/*Marshaling*.cs; do not re-encode spec rules inside Emit:

• At bind time, build a delegate chain per parameter/return (same idea as Il2Cpp MarshalMetaInfo writers);
• Hot path must not allocate object[] per call (unless boxing is required by the spec itself).

[LuaMarshalAs]: Emit reads attributes to choose pop/push strategy (align ../../spec/marshal/02-MARSHAL-AS.md.

7. Boundary with GetFunction / Delegate

Path	Implementation
Lua→C# members	Emit/ (this doc)
C#→Lua GetFunction	DelegateImpl/ + LuaCallInvoker (not Emit)
Lua function → C# delegate	DelegateImpl/ Expression (Phase 4)

Do not store C#→Lua delegate bridge logic under the Emit directory.

8. Suggested layout

Emit/
├── FieldEmitter.cs
├── PropertyEmitter.cs
├── MethodEmitter.cs
├── ConstructorEmitter.cs
├── MemberTableEmitter.cs   // Fill three tables + BindCall
├── BridgeMarshaling.cs
├── EmitMethods.cs          // MethodInfo cache for Expression
├── ClosurePin.cs
└── EmitException.cs

Namespace: ZLua.Emit. Finer overload (better-member) and OverloadDispatchEmitter may split later.

9. Acceptance

• Unsupported signatures at bind → EmitException; no silent Invoke
• Three-table members match Il2Cpp member set (same type public API)
• Hot path has no Method.Invoke, no object[] arg arrays (Field uses FieldInfo.Get/SetValue)
• Overload selects same overload as Il2Cpp (currently arity MVP)
• MetaBinding / closures have no methodId upvalue
• Tests/Lua/manifest.lua Phase 3 cases all green

10. Related docs

• Three-table attach: ../metatable/INDEXER-MONO.md
• Overload: ../marshal/OVERLOAD-RESOLVER.md
• Il2Cpp stub counterpart: STUBS-IL2CPP.md