C# - PrimeNumberGenerator

using System;
using System.Text;
using System.IO;
using System.Diagnostics;

/// <summary>
/// A static class for generating prime numbers
/// Calculated prime numbers are stored in a binary file
/// </summary>
/// <remarks>
/// Usage: 
/// 
/// static void Main(string[] args)
/// {
///    PrimeNumberGenerator.PrimeDatabaseFileName = @"C:\myprimes.bin";
///    PrimeNumberGenerator.StartBackgroundPrimeNumberGenerator();
///    ...
///    run your program as usual
///    ...
///    PrimeNumberGenerator.StopBackgroundPrimeNumberGenerator();
/// }
/// </remarks>
public static class PrimeNumberGenerator
{
/// <summary>
/// Returns the greatest prime number found in the file
/// </summary>
public static long GreatestKnownPrimeNumber
{
    get
    {
        if (File.Exists(PrimeDatabaseFileName))
        {
            long buf;
            // open it for reading
            using (BinaryReader reader = new BinaryReader(new FileStream(PrimeDatabaseFileName, FileMode.Open, FileAccess.Read, FileShare.ReadWrite)))
            {
                // seek to the last long
                reader.BaseStream.Seek(-sizeof(long), SeekOrigin.End);
                // read the last long, it's the last found prime number
                buf = reader.ReadInt64();
                // close
                reader.Close();
            }
            return buf;
        }
        else
        {
            return 1;
        }
    }                
}
// how many numbers are tested in each run of Eratosthenes()
private static long _NumbersToTest = 100000;
/// <summary>
/// Minimum value for NumbersToTest
/// </summary>
public const long MinNumbersToTest = 1000;
/// <summary>
/// Maximum value for NumbersToTest
/// </summary>
public const long MaxNumbersToTest = 1000000000;
/// <summary>
/// Get/set the count of numbers to test in a run of Eratosthenes()
/// <para>The value is automatically corrected to be in the bounds given by MinNimbersToTest and MaxNumbersToTest</para>
/// </summary>
public static long NumbersToTest
{
    get
    {
        return _NumbersToTest;
    }
    set
    {
        // TODO: enter code setting bounds to this value
        if (value < MinNumbersToTest)
        {
            _NumbersToTest = MinNumbersToTest;
        }
        else
        {
            if (value > MaxNumbersToTest)
            {
                _NumbersToTest = MaxNumbersToTest;
            }
            else
            {
                _NumbersToTest = value;
            }
        }
    }
}
// true if Eratosthenes() is running as background thread
private static bool RunThreaded = false;
// the background thread itself
private static System.Threading.Thread thread;
private static string _FileName;
/// <summary>
/// The name of the file in which the prime numbers are stored
/// </summary>
public static string PrimeDatabaseFileName
{
    get { return _FileName; }
    set
    {
        if (!RunThreaded)
        {
            _FileName = value;
        }
        else
        {
            throw new InvalidOperationException("Stop generator using StopBackgoundPNG() first");
        }
    }
}
/// <summary>
/// Initializes the prime number generator with a default filename
/// <para>FileName = Environment.GetFolderPath(Environment.SpecialFolder.CommonApplicationData) + "\\PrimeNumberDatabase.bin";</para>
/// </summary>
static PrimeNumberGenerator()
{
    _FileName = Environment.GetFolderPath(Environment.SpecialFolder.CommonApplicationData) + "\\PrimeNumberDatabase.bin";
}
/// <summary>
/// Tests NumberToTest numbers for prime and saves found primes to PrimeDatabaseFileName
/// </summary>
public static void Eratosthenes()
{
    // do at least once
    do
    {
        // file access variables
        BinaryWriter writer = null;
        BinaryReader reader = null;
        // we do not need any exception here
        try
        {
            // every number to test is an element in this array
            bool[] Zahlen = new bool[NumbersToTest];
            // initializes the array to false meaning the number is prime
            Zahlen.Initialize();
            // initialize last found prime number with 1
            // although 1 is not a prime number it is necessary here
            // anyway, algorithm starts by successor
            long LastFoundPrimeNumber = 1;
            // is prime number file existing?
            if (File.Exists(PrimeDatabaseFileName))
            {
                // if yes:
                // open it for reading
                reader = new BinaryReader(new FileStream(PrimeDatabaseFileName, FileMode.Open, FileAccess.Read, FileShare.ReadWrite));
                // seek to the last long
                reader.BaseStream.Seek(-sizeof(long), SeekOrigin.End);
                // read the last long, it's the last found prime number
                LastFoundPrimeNumber = reader.ReadInt64();
                // seek back to beginning
                reader.BaseStream.Seek(0, SeekOrigin.Begin);
            }
#if DEBUG
            Debug.WriteLine("last found prime number is " + LastFoundPrimeNumber.ToString());
            Debug.WriteLine("looking for primes lower than " + ((long)(LastFoundPrimeNumber + NumbersToTest)).ToString());
#endif
            // first number to be test
            // this is the number represented by Zahlen[0]
            long ZIB = LastFoundPrimeNumber + 1;
            // calc last number which multiples must be tested
            long MaxSieb = Convert.ToInt64(Math.Ceiling(Math.Sqrt(LastFoundPrimeNumber + NumbersToTest)));
            // the number of which multiples are beeing eliminated
            long AktSieb = 0;
            // if we have to look up prime numbers in Zahlen
            // start at this index
            long StartIndex = 0;
            // repeat until all multiples of all primes lower than MaxSieb are eliminated
            do
            {
                // current index
                long SuchIndex = StartIndex;
                // test, where the next prime number can be retrieved
                // it has to be taken from file if
                // -ZIB is greater than 2 (meaning that are already prime numbers in the file)
                // -and current prime (AktSieb) is lower than the greatest prime in file
                // -and there is actually a file opened
                if ((ZIB > 2) && (AktSieb < LastFoundPrimeNumber) && (reader != null))
                {
                    // load from file
                    do
                    {
                        // read a number
                        SuchIndex = reader.ReadInt64();
                    }
                    // until the number is greater than the current prime number
                    // or end of file
                    while ((SuchIndex <= AktSieb) && (reader.BaseStream.Position < reader.BaseStream.Length));
                    // current prime number is the loaded
                    AktSieb = SuchIndex;
#if DEBUG
                    Debug.WriteLine("prime number loaded from file: " + AktSieb.ToString());
#endif
                }
                else
                {
                    // find in Zahlen
                    // find first element that is not true (meaning it is not a multiple of any prime number)
                    while (Zahlen[SuchIndex] == true)
                    {
                        SuchIndex++;
                    }
                    // current prime number is the number represented bei Zahlen[SuchIndex]
                    // (remember: ZIB is the number represented by Zahlen[0])
                    AktSieb = SuchIndex + ZIB;
#if DEBUG
                    Debug.WriteLine("prime number found in current list: " + AktSieb.ToString());
#endif
                }
#if DEBUG
                Debug.WriteLine("now eliminating multiples of " + AktSieb.ToString() + " in current block");
#endif
                // find next multiple of the current prime
                long Sieb = LastFoundPrimeNumber + AktSieb - (LastFoundPrimeNumber % AktSieb);
                // repeat if the multiple is in the current block
                while (Sieb < (LastFoundPrimeNumber + NumbersToTest + 1))
                {
                    // do not "unprime" the prime itself
                    if (Sieb != AktSieb)
                    {
                        // "unprime" the multiple
                        Zahlen[Sieb - ZIB] = true;
                    }
                    // elevate to next mutliple
                    Sieb += AktSieb;
                }
#if DEBUG
                Debug.WriteLine("elimination finished");
#endif
                // elevate look up start point for next loop
                StartIndex++;
            }
            // loop until we have checked all possible primes
            while (AktSieb <= MaxSieb);

            // if there is input file
            if (reader != null)
            {
                // close it
                reader.Close();
                // and null it for next loop
                reader = null;
            }

            // write data to file (append)
            writer = new BinaryWriter(new FileStream(PrimeDatabaseFileName, FileMode.Append, FileAccess.Write, FileShare.ReadWrite));
            // loop all elements of Zahlen
            for (long i = 0; i < Convert.ToInt64(Zahlen.LongLength); i++)
            {
                // false is meaning that the number has not been multiple of anything
                // so it is prime
                if (Zahlen[i] == false)
                {
                    // write it to file
                    // remember: ZIB is the number represented by Zahlen[0]
                    writer.Write(((long)(ZIB + i)));
                }
            }
        }
        // if we encounter any exception above
        // we have to make sure, all files are closed before the next loop
        finally
        {
            // is input file existing
            if (reader != null)
            {
                // close it
                reader.Close();
                // null it for next loop
                reader = null;
            }
            // same for output file
            if (writer != null)
            {
                writer.Flush();
                writer.Close();
                writer = null;
            }
        }
    }
    // repeat this if running as background thread
    while (RunThreaded);

}

/// <summary>
/// Signals the generator to stop after next loop
/// </summary>
public static void EndBackgroundPNG()
{
    RunThreaded = false;
}

/// <summary>
/// Immediately stops a running background prime number generator
/// </summary>
public static void StopBackgroundPNG()
{
    // if there is no thread return
    if (thread == null)
    {
        return;
    }
    // if this is false Eratosthenes() will not loop
    RunThreaded = false;
    // raise exception in Eratosthenes()
    thread.Abort();
    // wait until Eratosthenes() closes all files
    thread.Join(5000);
    // remove thread
    thread = null;
}

/// <summary>
/// starts the prime number generator to run in the background
/// </summary>
public static void StartBackgroundPNG()
{
    // if there is already a thread return
    if (thread != null)
    {
        RunThreaded = true;
        return;
    }
    // create thread for Eratosthenes()
    thread = new System.Threading.Thread(new System.Threading.ThreadStart(Eratosthenes));
    // inform Eratosthenes() about running continuosly
    RunThreaded = true;
    // it's only a background thread (will be aborted if main threads end)
    thread.IsBackground = true;
    // run with lowest priority (it shall be real background)
    thread.Priority = System.Threading.ThreadPriority.Lowest;
    // start
    thread.Start();
}
}