[release/10.0] PersistedAssemblyBuilder: Fix encoding of custom modifiers. (#123925)

[jkotas]

Backport of #119935, #121128, #123925 to release/10.0

Customer Impact

• Customer reported
• Found internally

Unable to emit references to methods and fields with signatures that include modopts and modreqs using PersistedAssemblyBuilder (e.g. method signatures that use C# readonly ref or in).

Regression

• Yes
• No

This is a bug in PersistedAssemblyBuilder that shipped in .NET 9. Adoption blocker with no workaround.

Testing

Dedicated tests.

Risk

Medium. The change is a combination of 3 different PRs that fixed different aspects of the bug. The size of the delta makes is a medium risk.

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[Copilot]

Pull request overview

This PR fixes a critical bug in PersistedAssemblyBuilder where custom modifiers (like InAttribute on in parameters) were not being correctly encoded in method signatures when using types obtained via GetModifiedParameterType(). The issue manifested as a TypeLoadException when loading assemblies with interface implementations that have custom modifiers on parameters.

Changes:

• Refactored signature encoding to always extract custom modifiers from types when modifier arrays are null
• Added a test to verify interface overrides with custom modifiers work correctly
• Improved code clarity with better variable naming and simplified array encoding

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/libraries/System.Reflection.Emit/src/System/Reflection/Emit/SignatureHelper.cs	Core fix: Modified WriteSignatureForType to extract custom modifiers from types when modifier arrays are null; unified signature encoding logic across fields, methods, properties, and parameters; simplified array encoding
src/libraries/System.Reflection.Emit/tests/PersistedAssemblyBuilder/AssemblySaveTypeBuilderTests.cs	Added regression test SaveInterfaceOverrideWithCustomModifier that validates the fix by creating an interface override with in parameter modifiers and verifying the assembly loads without errors
src/libraries/System.Reflection.Emit/tests/PersistedAssemblyBuilder/AssemblySaveILGeneratorTests.cs	Renamed variable nestedParam to sectorParam for better code clarity
src/libraries/System.Reflection.Emit/tests/System.Reflection.Emit.Tests.csproj	Ensured project reference to System.Reflection.Emit source project

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.

[Copilot]

Pull request overview

Copilot reviewed 14 out of 14 changed files in this pull request and generated 5 comments.

[Copilot]

FunctionPointer ctor uses non-nullable parameters with null defaults (e.g., Type[] conventions = null, Type customReturnType = null). With repo-wide TreatWarningsAsErrors=true, this will fail compilation (CS8625). Make these parameters nullable (Type[]?, Type?, etc.) or remove the null defaults.

[Copilot]

ModifiedType ctor also uses non-nullable array parameters with null defaults (Type[] requiredMods = null, Type[] optionalMods = null), which will produce nullable warnings treated as errors. Update the parameter types to nullable (Type[]?) or avoid null defaults.

Suggested change

	public ModifiedType(Type delegatingType, Type[] requiredMods = null, Type[] optionalMods = null)
	public ModifiedType(Type delegatingType, Type[]? requiredMods = null, Type[]? optionalMods = null)

[Copilot]

field2 is dereferenced before the null-check: field2Type = field2.GetModifiedFieldType(); runs before Assert.NotNull(field2). If the field lookup fails, this will throw NullReferenceException instead of a test assertion. Move the Assert.NotNull(field2) before calling GetModifiedFieldType().