Reflection Emit doesn't allow nested types in interface, even though C# 8 does

[KalleOlaviNiemitalo]

C# 8 allows "member declarations that declare constants, operators, static constructors, and nested types" in an interface, according to Default interface methods - C# feature specifications. However, the reflection emit APIs still do not allow setting an interface as the parent of a type. SharpLab

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

static class Program
{
    static void Main()
    {
    	AssemblyBuilder assemblyCreator = AssemblyBuilder.DefineDynamicAssembly(new AssemblyName("TestAssembly"), AssemblyBuilderAccess.Run);

    	ModuleBuilder moduleBuilder = assemblyCreator.DefineDynamicModule("TestModule");

        TypeBuilder interfaceBuilder = moduleBuilder.DefineType("TestInterface", TypeAttributes.Interface | TypeAttributes.Abstract | TypeAttributes.Public);

    	TypeBuilder classBuilder = moduleBuilder.DefineType("TestClass", TypeAttributes.Class | TypeAttributes.Public);
    	classBuilder.SetParent(interfaceBuilder);
    }
}

System.ArgumentException: Cannot set parent to an interface.
   at System.Reflection.Emit.RuntimeTypeBuilder.SetParentCore(Type parent)
   at Program.Main()

Code that disallows interfaces is here:

runtime/src/coreclr/System.Private.CoreLib/src/System/Reflection/Emit/RuntimeTypeBuilder.cs

Lines 1691 to 1692 in 9d5a6a9

	if (parent.IsInterface)
	throw new ArgumentException(SR.Argument_CannotSetParentToInterface);

runtime/src/mono/System.Private.CoreLib/src/System/Reflection/Emit/RuntimeTypeBuilder.Mono.cs

Lines 1487 to 1488 in 9d5a6a9

	if (parent.IsInterface)
	throw new ArgumentException(SR.Argument_CannotSetParentToInterface);

runtime/src/libraries/System.Reflection.Emit/src/System/Reflection/Emit/TypeBuilderImpl.cs

Lines 565 to 568 in 9d5a6a9

	if (parent.IsInterface)
	{
	throw new ArgumentException(SR.Argument_CannotSetParentToInterface);
	}

runtime/src/libraries/System.Private.CoreLib/src/Resources/Strings.resx

Lines 960 to 962 in 9d5a6a9

	<data name="Argument_CannotSetParentToInterface" xml:space="preserve">
	<value>Cannot set parent to an interface.</value>
	</data>

runtime/src/libraries/System.Reflection.Emit/src/Resources/Strings.resx

Lines 123 to 125 in 9d5a6a9

	<data name="Argument_CannotSetParentToInterface" xml:space="preserve">
	<value>Cannot set parent to an interface.</value>
	</data>

Noticed while looking at #112072. I'm posting this as a discussion rather than an issue because this is more a missing feature than a bug, and because I have not needed the feature. Nested types were not mentioned in #50129 and #9478.

[huoyaoyuan]

You may open an issue to summarize missing cases for nested type support.

[KalleOlaviNiemitalo]

I misunderstood the meaning of SetParent -- it's for setting the base type from which members are inherited, not the enclosing type of a nested type.