Creating a ValueTuple with a TypeBuilder element

[jgh07]

Let's say I have a TypeBuilder called Element that represents an empty class like this:

class Element {}

How can I use this type as the element type of a tuple? Let's just say, to make it simple, I want to build something semantically equivalent to this using Reflection.Emit:

ValueTuple<Element> item = new(null);

class Element {}

The below code is supposed to achieve this, but the code fails on line 18 with a System.NotSupportedException, since calling GetConstructor() or GetConstructors() on TypeBuilder objects is not implemented (along with many others, even if the type has been finished, which is a shame).

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

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

TypeBuilder elementType = mb.DefineType("Element");
elementType.CreateType();

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.Ldnull);
il.Emit(OpCodes.Newobj, typeof(ValueTuple<>).MakeGenericType(elementType).GetConstructor([])); // EXCEPTION
il.Emit(OpCodes.Pop);
il.Emit(OpCodes.Ret);

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

I was expecting this to be an issue only with the new .NET 9 version of PersistedAssemblyBuilder, but I am getting the same error on .NET Framework 4.8.1 with the original implementation of Reflection.Emit, which surprised me a lot.

Is there something wrong with my code or is there some workaround I need to do to achieve this? I would believe it if this was not yet supported on .NET 9 (since the implementation is still lacking quite a bit), but I think a use case as simple and common as this should be supported on .NET Framework.

It's also very much possible I am overseeing something.

[huoyaoyuan]

It's mentioned in documentation: https://learn.microsoft.com/en-us/dotnet/fundamentals/reflection/how-to-define-a-generic-type-with-reflection-emit

The GetConstructor method is not supported on a GenericTypeParameterBuilder, so it is not possible to get the constructor of List<TFirst> directly. First, it is necessary to get the constructor of the generic type definition List<T> and then to call a method that converts it to the corresponding constructor of List<TFirst>.

Although the scenario isn't exact the same with this issue (TypeBuilderInstantiation), the solution is the same:

var openCtor = typeof(ValueTuple<>).GetConstructor([typeof(ValueTuple<>).GetGenericArguments()[0]]);
var ctor = TypeBuilder.GetConstructor(typeof(ValueTuple<>).MakeGenericType(elementType), openCtor);

Constructors for generics involving type builders should be retrieved from TypeBuilder.GetConstructor.