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

namespace ConsoleApp1
{
    class Program
    {
        internal class Data
        {
            private int P1 { get; set; }
            private string X { get; set; }
            private byte Something { get; set; }
            // ...
            private double P10 { get; set; }
        }

        static void Main()
        {
            IDictionary<int, Data> dic = new Dictionary<int, Data>();
            for (var i = 1; i <= 300_00; i++)
            {
                dic.Add(i, new Data());
            }

            var tp = dic.GetType().GetGenericArguments()[1].GetProperties((BindingFlags.NonPublic | BindingFlags.Instance));
            List<PropertyInfo> lProperties = new List<PropertyInfo>();
            Array.ForEach(tp, (p) =>
            {
                if (p.Name != "Something")
                {
                    lProperties.Add(p);
                }
            });

            foreach (var kvp in dic)
            {
                object[] @out = new object[lProperties.Count];

                foreach (var property in lProperties)
                {
                    @out[p] = property.GetValue(kvp.Value);
                }
            }
            Console.WriteLine("Done");
        }
    }
}