class Program
{
    static void Main(string[] args)
    {
        MyClass m = new MyClass();

        PropertyInfo? prop = m.GetType().GetProperty("PrivateList", BindingFlags.Instance | BindingFlags.NonPublic);

        Assembly assembly = Assembly.GetExecutingAssembly();
        Type subType = assembly.GetTypes().First(t => t.FullName.EndsWith("PrivateSubClass"));

        object? obj = prop.GetValue(m);
        foreach (object item in (IEnumerable)obj)
        {
            object? x = subType.GetField("x", BindingFlags.Instance | BindingFlags.Public).GetValue(item);
            Console.WriteLine(x);
        }
    }
}

class MyClass
{
    private List<PrivateSubClass> PrivateList { get; } = new List<PrivateSubClass>()
    {
        new PrivateSubClass() { x = 0 },
        new PrivateSubClass() { x = 1 },
    };

    private class PrivateSubClass
    {
        public int x;
    }
}