Private constructor does not prevent derivation at IL level.

Program.cs

// MIT license.

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

public class Base { private Base() { } }

class Program
{
  static void EnsureBaseCtorNotAccessible(string asmname, string modname)
  {
    try
    {
      var ab = AssemblyBuilder.DefineDynamicAssembly(new AssemblyName(asmname), AssemblyBuilderAccess.Run);
      var mb = ab.DefineDynamicModule(modname);
      var tb = mb.DefineType("Derived", TypeAttributes.Public, typeof(Base));
      var cb = tb.DefineDefaultConstructor(MethodAttributes.Public);
      var t = tb.CreateType();
      Activator.CreateInstance(t);
    }
    catch (Exception ex)
    {
      Console.WriteLine(ex);
    }
    Console.WriteLine();
  }

  static void EnsureBaseCtorNotNeeded(string asmname, string modname)
  {
    var ab = AssemblyBuilder.DefineDynamicAssembly(new AssemblyName(asmname), AssemblyBuilderAccess.Run);
    var mb = ab.DefineDynamicModule(modname);
    var tb = mb.DefineType("Derived", TypeAttributes.Public, typeof(Base));
    var cb = tb.DefineConstructor(MethodAttributes.Public, CallingConventions.Standard, new Type[0]);
    var il = cb.GetILGenerator();
    il.Emit(OpCodes.Ret);
    var t = tb.CreateType();
    var obj = Activator.CreateInstance(t);
    Console.WriteLine("Constructed " + obj);
    Console.WriteLine("Base type is " + obj.GetType().BaseType);
    Console.WriteLine("(obj is Base) == " + (obj is Base));
  }

  static void Main()
  {
    EnsureBaseCtorNotAccessible("test1", "test1mod");
    EnsureBaseCtorNotNeeded("test2", "test2mod");
  }
}

/* Output:

System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation.
 ---> System.MethodAccessException: Attempt by method 'Derived..ctor()' to access method 'Base..ctor()' failed.
   at Derived..ctor()
   at System.RuntimeType.CreateInstanceDefaultCtor(Boolean publicOnly, Boolean wrapExceptions)
   --- End of inner exception stack trace ---
   at System.RuntimeType.CreateInstanceDefaultCtor(Boolean publicOnly, Boolean wrapExceptions)
   at Program.EnsureBaseCtorNotAccessible(String asmname, String modname) in [path redacted]

Constructed Derived
Base type is Base
(obj is Base) == True

*/