Add Prime number calculation

prim_number/CSharp.cs

@@ -0,0 +1,83 @@
+using System;
+using System.Collections.Concurrent;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.Linq;
+using System.Threading.Tasks;
+
+namespace Program
+{
+    internal class Program
+    {
+        private static void Main(string[] args)
+        {
+            Console.WriteLine(
+                "max Primzahl    nr. Primzahlen     ein Thread      zwei Threads      drei Threads      vier Threads");
+            for (int i = 100000; i <= 51200000; i += 100000)
+            {
+                Dictionary<int, long> times = new Dictionary<int, long>();
+                int anz = 0;
+                for (int threads = 1; threads <= 4; threads++)
+                {
+                    Stopwatch watch = new Stopwatch();
+                    watch.Start();
+                    anz = new ConcurrentPrimCalculatior().Check(i, threads);
+                    watch.Stop();
+                    times[threads] = watch.ElapsedMilliseconds;
+                }
+                Console.WriteLine($"{i,-15} {anz,-18} {times[1],-15} {times[2],-17} {times[3],-17} {times[4],-12}");
+            }
+
+            Console.WriteLine("Done");
+            Console.ReadLine();
+        }
+    }
+
+    public class ConcurrentPrimCalculatior
+    {
+        public int Check(int maxToCheck, int maxParralell)
+        {
+            ConcurrentDictionary<int, bool> validPrims = new ConcurrentDictionary<int, bool>();
+            // Create list of valids
+            for (int i = 2; i < maxToCheck; i++)
+                validPrims[i] = true;
+
+            int lastToRemove = (int) Math.Sqrt(maxToCheck);
+
+            // Remove everythin before lastToRemove Single-Threaded
+            int currentPrim = 2;
+            while (currentPrim <= lastToRemove)
+            {
+                int toRemove = currentPrim;
+                for (int i = 2; i * currentPrim <= lastToRemove; i++)
+                {
+                    bool useless;
+                    validPrims.TryRemove(i * currentPrim, out useless);
+                }
+
+                currentPrim = validPrims.Keys.ToList()[validPrims.Keys.ToList().IndexOf(currentPrim) + 1];
+            }
+
+            List<int> primsBeforeLastToCheck = new List<int>();
+            foreach (int prim in validPrims.Keys)
+                if (prim > lastToRemove)
+                    break;
+                else
+                    primsBeforeLastToCheck.Add(prim);
+
+            // Now remove mutiples of 0 - lastToRemove
+            ParallelOptions options = new ParallelOptions {MaxDegreeOfParallelism = maxParralell};
+            Parallel.ForEach(primsBeforeLastToCheck, options, toCheck =>
+            {
+                for (int mutiiple = toCheck * 2; mutiiple <= maxToCheck; mutiiple += toCheck)
+                    lock (validPrims)
+                    {
+                        bool useless;
+                        validPrims.TryRemove(mutiiple, out useless);
+                    }
+            });
+
+            return validPrims.Count;
+        }
+    }
+}