Reflection with list property

How can I change the value of the class property in the list?

My variable classPropValueList always comes null.

I'm trying to make a reflection of every class that inherits from Coisable , to change its string properties.

You can find the following properties in a class that inherits from Coisable :

I then managed, through the code below, to solve scenarios 1 and 2, but I had problems with the 3rd scenario. In the debug section of the code, I saw that in the example case, it is possible to enter the third condition, but when trying to get the list, it always comes null.

using System;
using System.Collections.Generic;
using System.Linq;

public class Program
{
    public static void Main()
    {
        var coisa1 = new MyClass3
        {
            MyProperty1 = "Valor Original 1",
            MyProperty2 = new MyClass1
            {
                MyProperty = "Valor Original 2"
            },
            MyProperty3 = new List<MyClass2> 
            {
                new MyClass2 
                {
                    MyProperty = "Valor Original 3"
                }
            }
        };

        Console.WriteLine("--- Valores Originais ---");
        Console.WriteLine(coisa1.MyProperty1);
        Console.WriteLine(coisa1.MyProperty2.MyProperty);
        Console.WriteLine(coisa1.MyProperty3.First().MyProperty);

        Console.WriteLine();

        coisa1.Coise();


        Console.WriteLine("--- Novos Valores ---");
        Console.WriteLine(coisa1.MyProperty1);
        Console.WriteLine(coisa1.MyProperty2.MyProperty);
        Console.WriteLine(coisa1.MyProperty3.First().MyProperty);

    }
}

// #region Minhas Classes

    public class Coisable
    {
    }


    public class MyClass1 : Coisable
    {
       public string MyProperty { get; set; }
    }

    public class MyClass2 : Coisable
    {
       public string MyProperty { get; set; }
    }

    public class MyClass3 : Coisable
    {
        public string MyProperty1 { get; set; }
        public MyClass1 MyProperty2 { get; set; }
        public List<MyClass2> MyProperty3 { get; set; }
    }

    // #endregion

    // #region Reflection

    public static class CoisableExt
    {
        public static void Coise(this Coisable coisableClass) {
            foreach (var propertyInfo in coisableClass.GetType().GetProperties())
            {
                if(typeof(string).IsAssignableFrom(propertyInfo.PropertyType))
                {
                    var propValue = propertyInfo.GetValue(coisableClass, null).ToString();
                    propValue = "Novo Valor";
                    propertyInfo.SetValue(coisableClass, propValue, null);
                }
                else if(typeof(Coisable).IsAssignableFrom(propertyInfo.PropertyType))
                {
                    var classPropValue = propertyInfo.GetValue(coisableClass, null) as Coisable;
                    classPropValue.Coise();
                }
                else if (typeof(IEnumerable<Coisable>).IsAssignableFrom(propertyInfo.PropertyType))
                {
                    var classPropValueList = propertyInfo.GetValue(coisableClass, null) as List<Coisable>;
                    if(classPropValueList != null && classPropValueList.Any())
                    {
                        classPropValueList.ForEach(classPropValueItem =>
                        {
                           classPropValueItem.Coise();
                        });
                    }
                }
            }
        }
    }

    // #endregion