Activator.cs

using System;
using System.Globalization;
using System.Reflection;

namespace TestActivator
{
    class Program
    {
        static void Main(string[] args)
        {
            Activator.CreateInstance(
                Type.GetType("TestActivator.Test1", true),
                BindingFlags.CreateInstance | BindingFlags.Public | BindingFlags.Instance | BindingFlags.OptionalParamBinding,
                null,
                new object[] { },
                CultureInfo.CurrentCulture);

            Activator.CreateInstance(
                Type.GetType("TestActivator.Test2", true),
                BindingFlags.CreateInstance | BindingFlags.Public | BindingFlags.Instance | BindingFlags.OptionalParamBinding,
                null,
                new object[] { },
                CultureInfo.CurrentCulture);

            Activator.CreateInstance(
                Type.GetType("TestActivator.Test2", true),
                BindingFlags.CreateInstance | BindingFlags.Public | BindingFlags.Instance | BindingFlags.OptionalParamBinding,
                null,
                new object[] { "Dimitris" },
                CultureInfo.CurrentCulture);
        }
    }


    public class Test1
    {
        public Test1()
        {
            Console.WriteLine("Hello World!!!");
        }
    }

    public class Test2
    {
        public Test2(string name = null)
        {
            Console.WriteLine($"Hello {name ?? "NoName"}!!!");
        }
    }
}