C#-Code

using System;
using static System.Console;
using System.Numerics; //muss zu Verweisen hinzugefügt werden
using System.Diagnostics;

namespace pi_183{

    class Program    {

        static string NL = Environment.NewLine, LZ = " ";

        static void Main()        {

            Stopwatch sw = new Stopwatch();
            sw.Start();

            int genauigkeit = 1001;

            WriteLine("Garantierte Genauigkeit: " + (genauigkeit - 1).ToString("n0") + " Nachkommastellen" + NL);

            int n = 10;                      
            BigInteger pi1 = 0, pi2 = 0;

            bool bo_ok = false;
            while (!bo_ok)            {  
                              
                pi1 = pi2;

                rational L2 = new rational();
                L2 = Lambert(1, n + 1);

                BigInteger p2 = BigInteger.Pow(10, L2.zaehler.ToString().Length);
                pi2 = p2 * 4 * L2.nenner / L2.zaehler;

                int identisch = pi2.identisch_bis(pi1);
                bo_ok = genauigkeit <= identisch;
                
                n++;              
            }            

            WriteLine(pi2.ToString().MitCharAnzahl(genauigkeit));
            WriteLine(NL + Math.PI * Math.Pow(10, 14));

            sw.Stop();
            TimeSpan ts = sw.Elapsed;
            string elapsedTime = String.Format("{0:00}:{1:00}:{2:00}.{3:00}",
                ts.Hours, ts.Minutes, ts.Seconds,
                ts.Milliseconds / 10);
            WriteLine(NL + "Rechenzeit " + elapsedTime);

            ReadKey();
        }

        #region biginteger region

        public struct rational 
        {
            public BigInteger zaehler;
            public BigInteger nenner;
        }

        public static rational div(rational z1, rational z2)        {
            rational erg;
            erg.zaehler = z1.zaehler * z2.nenner;
            erg.nenner = z1.nenner * z2.zaehler;
            return erg;
        }

        public static rational add(rational z1, rational z2)        {
            rational erg;
            erg.nenner = z1.nenner * z2.nenner;
            erg.zaehler = z1.zaehler * z2.nenner + z2.zaehler * z1.nenner;
            return erg;
        }

        public static rational Lambert(int k, int n)        {   // nach Lamberts Kettenbruch
                                                                // lambert(k) = 2 * k - 1 + k * k / lambert(k + 1)
            rational erg;
            if (k == n)
            {
                erg.zaehler = 1;
                erg.nenner = 1;
                return erg;
            }
            rational a, b;
            a.zaehler = 2 * k - 1; a.nenner = 1;
            b.zaehler = k * k; b.nenner = 1;

            erg = add(a, div(b, Lambert(k + 1, n)));

            return erg;
        }
    
        #endregion
    }

    public static class MyExtensions    {

        public static string MitCharAnzahl(this string s, int chars)        {
            string result = string.Empty;
            for (var i = 0; i < s.Length; i++)
                if (i < chars)
                    result += s[i].ToString();
                else
                    break;
            return result;
        }

        public static int identisch_bis(this BigInteger z1, BigInteger z2)
        {
            int result = 0;
            char[] c1 = z1.ToString().ToCharArray();
            char[] c2 = z2.ToString().ToCharArray();
            for (var i = 0; i < Math.Min(c1.Length, c2.Length); i++)
                if (c1[i] == c2[i])
                    result++;
                else
                    break;
            return result;
        }
    }
}

C#-Code

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace PI
{
    class Program
    {
        static void Main(string[] args)
        {
            double pi = 4/1;
            bool b = false;
            double i = 1;
            while(i < Double.MaxValue)
            {
                i += 2;
                pi -= (4 / i);
                i += 2;
                pi += (4 / i);
                Console.WriteLine("" + pi);
            }
            Console.WriteLine("" + pi);
            Console.ReadLine();
        }
    }
}

C#-Code

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace pi
{
    class Program
    {
        static void Main(string[] args)
        {
            Random zufall = new Random();
            double innerhlab = 0;
            int tropfen = Convert.ToInt32(Console.ReadLine());
            for(int i = 0; i <= tropfen;i++)
            {
                double x = zufall.NextDouble();
                double y = zufall.NextDouble();
                if(x*x+y*y < 1.0)
                {
                    innerhlab++;
                }
            }
            double  pi = 4*innerhlab / tropfen;
            Console.WriteLine(pi);
            Console.Read();
        }
    }
}

C#-Code

using System;

namespace CS_Aufgabe_183_Pi
{
    class Program
    {
        static void Main(string[] args)
        {
            double n = 1e8;
            int counter = 0;
            Random rnd = new Random();

            for (int i = 0; i < n; i++)
            {
                double x = rnd.NextDouble();
                double y = rnd.NextDouble();

                if (Math.Sqrt(Math.Pow(x, 2e0) + Math.Pow(y, 2e0)) < 1e0) {
                    counter++;
                }
            }

            Console.WriteLine($"pi = {4e0 * counter / n}");
            Console.ReadKey();
        }
    }
}