ILGenerator does not properly handle function pointers

[jgh07]

Description

When using the new PersistedAssemblyBuilder introduced in .NET 9 and trying to load a field whose type is a function pointer, invalid IL gets generated, causing the application to crash with a System.MissingFieldException when executed.

(On another note, is there currently a way of generating such fields using ILGenerator? How do you get a Type object for a function pointer at runtime?)

Reproduction Steps

The following code reproduces the problem in .NET 9. It doesn't produce a working executable by itself since I did not set the entry point, so to execute code from the generated assembly, you would need to reference it by some other assembly.

using System;
using System.Reflection;
using System.Reflection.Emit;

PersistedAssemblyBuilder ab = new(new("Test"), typeof(object).Assembly);
ModuleBuilder mb = ab.DefineDynamicModule("Test.dll");
TypeBuilder tb = mb.DefineType("Program");

MethodBuilder methb = tb.DefineMethod("Main", MethodAttributes.Public | MethodAttributes.Static);
methb.SetReturnType(typeof(void));

ILGenerator il = methb.GetILGenerator();

il.Emit(OpCodes.Call, typeof(Container).GetMethod("Init"));

il.Emit(OpCodes.Ldc_I4_2);
il.Emit(OpCodes.Ldc_I4_3);
il.Emit(OpCodes.Ldsfld, typeof(Container).GetField("Method"));
il.EmitCalli(OpCodes.Calli, System.Runtime.InteropServices.CallingConvention.Winapi, typeof(int), [typeof(int), typeof(int)]);
il.Emit(OpCodes.Call, typeof(Console).GetMethod("WriteLine", BindingFlags.Public | BindingFlags.Static, null, [typeof(int)], []));

il.Emit(OpCodes.Ret);

tb.CreateType();
ab.Save("Test.dll");

public unsafe static class Container
{
    public static delegate*<int, int, int> Method;

    public static int Add(int a, int b) => a + b;
    public static void Init() => Method = &Add;
}

Expected behavior

The following IL is generated as the method body for Test::Main() when running under .NET Framework 4.8.1:

call void [Issue]Container::Init()
ldc.i4.2
ldc.i4.3
ldsfld method int32 *(int32,int32) [Issue]Container::Method
calli unmanaged stdcall int32(int32,int32)
call void [mscorlib]System.Console::WriteLine(int32)
ret

The generated application runs correctly and prints 5.

Actual behavior

The following IL is generated when running under .NET 9:

call void [Issue]Container::Init()
ldc.i4.2
ldc.i4.3
ldsfld class [System.Private.CoreLib]$TR$2 [Issue]Container::Method
calli int32(int32,int32)
call void [System.Console]System.Console::WriteLine(int32)
ret

The ldsfld instruction, instead of correctly using a function pointer as the field type, generates some garbage type (as is displayed by ildasm).

When executed, the following exception is thrown:

Unhandled exception. System.MissingFieldException: Field not found: 'Container.Method'.
   at Program.Main()

Regression?

API was newly introduced with .NET 9. Works fine in .NET Framework 4.8.1.

Known Workarounds

Don't know of any.

Configuration

• .NET 9.0.100
• Windows 11 23H2
• Debug, x64

Other information

No response

[jgh07]

Based on some further tesing, when declaring new fields of function pointer types, .NET 9 creates the same non-existent type as above and .NET Framework creates fields of type native int. While that is not ideal either, it is at least better than in .NET 9, where the program will crash no matter what.

I noticed that there is (active?) work going on regarding function pointer types in Reflection.Emit, but I do feel like this can be considered a separate issue since it is dealing with functionality that used to work fine.