How i was learning C# - random code, simple array operations

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.IO;
using System.Text;

namespace CalculateMatrix
{
    class Program
    {
        const int ConsoleW = 120;
        const int ConsoleH = 40;

        private static readonly Object __TASKLOCK__ = new Object();

        private static Random rng = new Random();
        private static Dictionary<string, Sequence> watchList = new Dictionary<string, Sequence>();
        private static Stopwatch watch = new Stopwatch();
        private static StreamWriter stream;
        private static int userXPos, userYPos, userRNG, userRNGElements;

        static void Main(string[] args)
        {
            Console.SetWindowSize(ConsoleW, ConsoleH);

            WelcomeMsg();

            int[,] XYmatrix = CreateMatrix(userXPos, userYPos, 1, userRNG);
            Console.WriteLine($"Random number generated matrix elements sum = {CalculateMatrix(XYmatrix)}\n");

            string RandomElements = GetRandomElements(XYmatrix, userRNGElements).ToString();
            string FullMatrix = ShowMatrix(XYmatrix).ToString();

            Console.WriteLine("File saving logs:\n");

            LockWrite(__TASKLOCK__, "random", RandomElements);
            LockWrite(__TASKLOCK__, "fullMatrix", FullMatrix);

            Console.ReadLine();
        }

        private static void LockWrite(object obj, string instance, string variable)
        {
            lock (obj)
            {
                WriteToFile(instance, variable);
                Console.WriteLine($"Instance \"{instance}\" writed to file: {watchList[instance].filename}\nOperation time: {watchList[instance].watch}");
            }
        }

        private static int GetNumericValue(string message)
        {
            Console.Write(message);
            int value;
            while (!Int32.TryParse(Console.ReadLine(), out value))
            {
                Console.Write("Wrong character type. You need to provide numeric value: ");
            }
            return value;
        }

        private static void WelcomeMsg()
        {
            Console.ForegroundColor = ConsoleColor.White;
            Console.WriteLine(@"Application will create 2D[Xpos,Ypos] array, fill it with random numbers from 1 to [RNG Amount].");
            userXPos = GetNumericValue("Xpos (array max X position number): ");
            userYPos = GetNumericValue("Ypos (array max Y position number): ");
            userRNG = GetNumericValue("Max range for random number generator: ");
            Console.WriteLine("Application will save to files user provided amount of random matrix elements.");
            userRNGElements = GetNumericValue("Amount of elements: ");
        }

        private static string GenerateFile(string instance)
        {
            string fDateElement = DateTime.Now.Day + "-" +
                                  DateTime.Now.Month + "-" +
                                  DateTime.Now.Year + "-" +
                                  DateTime.Now.Hour + "-" +
                                  DateTime.Now.Minute + "-" +
                                  DateTime.Now.Second;
            Guid UUID = Guid.NewGuid();
            string UniqueUUID = Convert.ToBase64String(UUID.ToByteArray());
            UniqueUUID = UniqueUUID.Replace("+", "");
            UniqueUUID = UniqueUUID.Replace("=", "");
            string filename = $"{instance}-{fDateElement}-{UniqueUUID}.txt";
            return filename;
        }

        private static void WriteToFile(string instance, string text)
        {
            watch.Start();
            string fname = GenerateFile(instance);
            stream = new StreamWriter(fname);
            stream.Write(text);
            stream.Close();
            watch.Stop();
            watchList.Add(instance, new Sequence { filename = fname, watch = watch.Elapsed });
        }

        private static StringBuilder ShowMatrix(int[,] matrix)
        {
            StringBuilder item = new StringBuilder();

            for (int i = 0; i < matrix.GetLength(0); i++)
            {
                for (int j = 0; j < matrix.GetLength(1); j++)
                {
                    item.Append($"Item position [{i},{j}] = {matrix[i, j]}\r\n");
                }
            }

            return item;
        }

        private static ulong CalculateMatrix(int[,] matrix)
        {
            ulong sum = 0;

            for (int i = 0; i < matrix.GetLength(0); i++)
            {
                for (int j = 0; j < matrix.GetLength(1); j++)
                {
                    sum += Convert.ToUInt64(matrix[i, j]);
                }
            }

            return sum;
        }

        private static int[,] CreateMatrix(int XPos, int YPos, int RNGmin, int RNGmax)
        {
            int[,] matrix = new int[XPos, YPos];

            for (int i = 0; i < matrix.GetLength(0); i++)
            {
                for (int j = 0; j < matrix.GetLength(1); j++)
                {
                    matrix[i, j] = rng.Next(RNGmin, RNGmax);
                }
            }

            return matrix;
        }

        private static StringBuilder GetRandomElements(int[,] matrix, int amount)
        {
            StringBuilder text = new StringBuilder();
            int counter = 1;

            do
            {
                int cX = rng.Next(1, matrix.GetLength(0));
                int cY = rng.Next(1, matrix.GetLength(1));
                text.Append($"| Counter: {counter} | Matrix Position [{cX},{cY}] = {matrix[cX, cY]}\r\n");
                counter++;
            } while (counter != amount+1);

            return text;
        }
    }

    struct Sequence
    {
        public string filename { get; set; }
        public TimeSpan watch { get; set; }
    }
}