Field offset not being set when offset is 0 on explicit layout types.

[TrueLunacy]

Description

Field offsets aren't being set in the IL for types with an offset of zero. This causes the types to fail to load at runtime.

The cause, as far as I can tell, seems to be from lines 641 in the WriteFields function in ModuleBuilderImpl.cs

if (field._offset > 0 && (typeBuilder.Attributes & TypeAttributes.ExplicitLayout) != 0)
{
    AddFieldLayout(handle, field._offset);
}

Reproduction Steps

using System.Reflection;
using System.Reflection.Emit;
using System.Runtime.Loader;

PersistedAssemblyBuilder ab = new PersistedAssemblyBuilder(new AssemblyName("MyAssembly"), typeof(object).Assembly);
ModuleBuilder mob = ab.DefineDynamicModule("MyModule");

TypeBuilder exp = mob.DefineType("ExplicitLayoutType", TypeAttributes.Public | TypeAttributes.Class | TypeAttributes.ExplicitLayout, typeof(ValueType), 8);
var f1 = exp.DefineField("first", typeof(int), FieldAttributes.Public);
f1.SetOffset(0);
var f2 = exp.DefineField("second", typeof(int), FieldAttributes.Public);
f2.SetOffset(4);
exp.CreateType();

TypeBuilder tb = mob.DefineType("MyType", TypeAttributes.Public | TypeAttributes.Class);
tb.DefineField("structfield", exp, FieldAttributes.Public | FieldAttributes.Static);

MethodBuilder meb = tb.DefineMethod("SumMethod", MethodAttributes.Public | MethodAttributes.Static,
                                                        typeof(int), new Type[] { typeof(int), typeof(int) });
ILGenerator il = meb.GetILGenerator();
il.Emit(OpCodes.Ldarg_0);
il.Emit(OpCodes.Ldarg_1);
il.Emit(OpCodes.Add);
il.Emit(OpCodes.Ret);

tb.CreateType();

using var stream = new MemoryStream();
ab.Save(stream); 
stream.Seek(0, SeekOrigin.Begin);
Assembly assembly = AssemblyLoadContext.Default.LoadFromStream(stream);
MethodInfo method = assembly.GetType("MyType").GetMethod("SumMethod");
Console.WriteLine(method.Invoke(null, new object[] { 5, 10 }));

Expected behavior

Type loads, program prints 15.

Actual behavior

Unhandled exception. System.TypeLoadException: Could not load type 'ExplicitLayoutType' from assembly 'MyAssembly, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null' because field 'first' was not given an explicit offset.
   at System.Reflection.RuntimeAssembly.GetTypeCore(QCallAssembly assembly, String typeName, ReadOnlySpan`1 nestedTypeNames, Int32 nestedTypeNamesLength, ObjectHandleOnStack retType)
   at System.Reflection.RuntimeAssembly.GetTypeCore(String typeName, ReadOnlySpan`1 nestedTypeNames, Boolean throwOnError, Boolean ignoreCase)
   at System.Reflection.TypeNameResolver.GetType(String escapedTypeName, ReadOnlySpan`1 nestedTypeNames, TypeName parsedName)
   at System.Reflection.TypeNameResolver.GetSimpleType(TypeName typeName)
   at System.Reflection.TypeNameResolver.Resolve(TypeName typeName)
   at System.Reflection.TypeNameResolver.GetType(String typeName, Boolean throwOnError, Boolean ignoreCase, Assembly topLevelAssembly)
   at Program.<Main>$(String[] args) in Program.cs:line 32

Regression?

No response

Known Workarounds

No response

Configuration

Arch Linux x64, dotnet 9.0.100-rc.1.24381.3

Other information

No response

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[buyaa-n]

The cause, as far as I can tell, seems to be from lines 641 in the WriteFields function in ModuleBuilderImpl.cs

if (field._offset > 0 && (typeBuilder.Attributes & TypeAttributes.ExplicitLayout) != 0)
{
    AddFieldLayout(handle, field._offset);
}

Thank you for reporting this @TrueLunacy! Looks you found the root cause, would you like to offer a fix (with a test repro)

[TrueLunacy]

I've made a pull request with the change and a test for it, but I'm not 100% if that's enough or I've done it right. Hopefully I have!