c# - 反射发射生成简单的继承

Question

我正在尝试通过 reflection.emit 动态生成以下类层次结构:

public class Main
{
    public class A { A(){} }
    public class B : A { B() : base() {} }
}

这是我试图在 .net core 框架上运行的程序(在 .net 上使用了类似的程序，但异常是相同的):

using System;
using System.Reflection;
using System.Reflection.Emit;    
public class Program
{
    public static void Main()
    {
        AssemblyBuilder assemBuilder = AssemblyBuilder.DefineDynamicAssembly(new AssemblyName() { Name = "DynamicAssembly" }, AssemblyBuilderAccess.Run);
        TypeBuilder main_builder = assemBuilder.DefineDynamicModule("DynamicModule").DefineType("Main", TypeAttributes.Public);

        TypeBuilder a_builder = main_builder.DefineNestedType("A", TypeAttributes.NestedPublic);
        TypeBuilder b_builder = main_builder.DefineNestedType("B", TypeAttributes.NestedPublic, a_builder);

        var a_constr = a_builder.DefineDefaultConstructor(MethodAttributes.Public);
        var b_cnstr = b_builder.DefineConstructor(MethodAttributes.Public, CallingConventions.Standard, Type.EmptyTypes);

        {
            //  Creation b constructor    
            var il_gen = b_cnstr.GetILGenerator();
            il_gen.Emit(OpCodes.Ldarg_0);
            il_gen.Emit(OpCodes.Call, a_constr);
            il_gen.Emit(OpCodes.Ret);
        }
        a_builder.CreateType();
        b_builder.CreateType();
        main_builder.CreateType();
    }
}

但是在执行过程中，程序在 b_builder.CreateType() 处失败并出现异常消息(这是我无法调试的 .net core 函数):

是 reflection.emit 错误吗？我不知道我的代码有什么问题

Answer 1

文档中TypeBuilder.CreateType备注部分的第一行告诉我们:

If this type is a nested type, the CreateType method must be called on the enclosing type before it is called on the nested type

https://learn.microsoft.com/en-us/dotnet/api/system.reflection.emit.typebuilder.createtype

所以改变调用顺序，它应该可以工作:

main_builder.CreateType();
a_builder.CreateType();
b_builder.CreateType();

参见 https://dotnetfiddle.net/bxYCQb