program1(Exception.InternalToString).cs

using System;
using System.Prig;
using System.Reflection;
using Urasandesu.Prig.Framework;

namespace program1
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("★入れ替え前: {0}", InternalToString());
            using (new IndirectionsContext())
            {
                PException.InternalToString.Body = () => "にゃんぱすー";
                Console.WriteLine("★入れ替え中: {0}", InternalToString());
            }
            Console.WriteLine("★戻し:       {0}", InternalToString());
        }

        static string InternalToString()
        {
            var e = new NotImplementedException();
            var internalToStringInfo = typeof(NotImplementedException).GetMethod("InternalToString", BindingFlags.Instance | BindingFlags.NonPublic);
            return (string)internalToStringInfo.Invoke(e, new object[0]);
        }
    }
}