using System;
using System.Reflection;

namespace Test
{
    public class A { }
    public class B : A { }

    public class Class
    {
        public A a = new A();
    }

    class Program
    {
        static void Main(string[] args)
        {
            var y = typeof(B);
            var x = typeof(Class).GetField("a");
            var type = x.GetType();

            Console.WriteLine(x);

            FieldInfo fieldInfo = null;
            for (; type != null && fieldInfo == null; type = type.BaseType)
                fieldInfo = type.GetField("type", BindingFlags.Instance | BindingFlags.NonPublic);
            fieldInfo.SetValue(x, y);

            Console.WriteLine(typeof(Class).GetField("a"));

            Console.Read();
        }
    }
}