Notes: Delegates as custom attribute argument

note.md

Notes: Add support for delegates as attribute arguments

Original proposal

About dotnet/runtime

• Don't open the repo in vscode with the c/c++ extension enabled, it will hoard enormous amounts of resources.

• Be sure to build clr+libs in Debug the first time; then you can build just the clr (as long as you don't touch the libs, duh).

• To try the changes out, use artifacts/bin/coreclr/linux.x64.Debug/corerun, though you'll probably first have to copy the contents of linux.x64.Debug/libs into linux.x64.Debug/. You should probably also set CORE_LIBRARIES to artifacts/bin/runtime/net-9.0-linux-Debug-x64.

• Workflow guide

• Debugging the runtime on linux

• docs/ Index

• Testing the runtime

Tools

• CFF Explorer
• mdv, the metadata visualizer
• ilasm and ildasm from dotnet/runtime

Articles

• ECMA-335 Partition II: Metadata
• Anatomy of a .NET Assembly – Custom attribute encoding
• Anatomy of a .NET Assembly – CLR metadata 1 (and the series as a whole)
• The .NET File Format

Interesting files/places

dotnet/runtime

current tree

• inc/caparser.h -- helper to turn the raw data from blobs into the target type (int, string, type, etc...)
  • methods like GetData also use inc/strgpooli.h(29): CPackedLen to decode some compressed data
• inc/corhdr.h(932): CorSerializationType -- list of types that can be serialized for attributes
• vm/customattribute.cpp(222): ParseCaValue -- used in loader to parse attr arguments
• md/ -- heart of the metadata loading/reading
  • md/compiler/custattr.h
  • md/compiler/custattr_import.cpp
  • md/inc/cahlprinternal.h(56): CaValue -- structure holding the actual value

Problems

• what should we actually encode (answer: probably just MD tokens)
• how do we construct the actual delegate from what we encoded
  • if MD token, probably use ResolveMethodHandle to get RuntimeMethodHandleInternal -> RuntimeMethodInfo.ctor to get MethodInfo -> .CreateDelegate
• delegate could end-up in object[], how do we handle boxing in that case?
• encoding generic methods and/or methods on generic types
• what restrictions have to be in place for this to work out? (e.g. cannot be instance lambda/method group, obviously)
• what restrictions should be in place to make our lives easier?
• we'll also have to change a bunch of other things/tools: compiler back-end + mid-end (for ex. GetAttributeData could now return IMethodInfo), mdv, ilasm+ildasm, System.Reflection.Metadata/Emit
• do we even want to give a delegate??? Like a MethodInfo would allow getting instance delegates for example, it'd then be up to the user to actually instantiate them
• how do we reference methods from another assembly?

Misc

• types are encoded as strings -- probably not a great sign that they chose the "stringly-typed" road instead of using MD tokens ;-;
• attribute ctors are called lazily when introspecting (e.g. typeof(C).GetCustomAttributes() will run the ctors of all attributes on class C)
• getting RVA from MethodBase