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

class FStreamLock
{
    static void Main()
    {
        UnicodeEncoding uniEncoding = new UnicodeEncoding();
        string lastRecordText =
            "The last processed record number was: ";
        int textLength = uniEncoding.GetByteCount(lastRecordText);
        int recordNumber = 13;
        int byteCount =
            uniEncoding.GetByteCount(recordNumber.ToString());
        string tempString;

        using(FileStream fileStream = new FileStream(
            "Test#@@#.dat", FileMode.OpenOrCreate,
            FileAccess.ReadWrite, FileShare.ReadWrite))
        {
            // Write the original file data.
            if(fileStream.Length == 0)
            {
                tempString =
                    lastRecordText + recordNumber.ToString();
                fileStream.Write(uniEncoding.GetBytes(tempString),
                    0, uniEncoding.GetByteCount(tempString));
            }

            // Allow the user to choose the operation.
            char consoleInput = 'R';
            byte[] readText = new byte[fileStream.Length];
            while(consoleInput != 'X')
            {
                Console.Write(
                    "\nEnter 'R' to read, 'W' to write, 'L' to " +
                    "lock, 'U' to unlock, anything else to exit: ");

                if((tempString = Console.ReadLine()).Length == 0)
                {
                    break;
                }
                consoleInput = char.ToUpper(tempString[0]);
                switch(consoleInput)
                {
                    // Read data from the file and
                    // write it to the console.
                    case 'R':
                        try
                        {
                            fileStream.Seek(0, SeekOrigin.Begin);
                            fileStream.Read(
                                readText, 0, (int)fileStream.Length);
                            tempString = new String(
                                uniEncoding.GetChars(
                                readText, 0, readText.Length));
                            Console.WriteLine(tempString);
                            recordNumber = int.Parse(
                                tempString.Substring(
                                tempString.IndexOf(':') + 2));
                        }

                        // Catch the IOException generated if the
                        // specified part of the file is locked.
                        catch(IOException e)
                        {
                            Console.WriteLine("{0}: The read " +
                                "operation could not be performed " +
                                "because the specified part of the " +
                                "file is locked.",
                                e.GetType().Name);
                        }
                        break;

                    // Update the file.
                    case 'W':
                        try
                        {
                            fileStream.Seek(textLength,
                                SeekOrigin.Begin);
                            fileStream.Read(
                                readText, textLength - 1, byteCount);
                            tempString = new String(
                                uniEncoding.GetChars(
                                readText, textLength - 1, byteCount));
                            recordNumber = int.Parse(tempString) + 1;
                            fileStream.Seek(
                                textLength, SeekOrigin.Begin);
                            fileStream.Write(uniEncoding.GetBytes(
                                recordNumber.ToString()),
                                0, byteCount);
                            fileStream.Flush();
                            Console.WriteLine(
                                "Record has been updated.");
                        }

                        // Catch the IOException generated if the
                        // specified part of the file is locked.
                        catch(IOException e)
                        {
                            Console.WriteLine(
                                "{0}: The write operation could not " +
                                "be performed because the specified " +
                                "part of the file is locked.",
                                e.GetType().Name);
                        }
                        break;

                    // Lock the specified part of the file.
                    case 'L':
                        try
                        {
                            fileStream.Lock(textLength - 1, byteCount);
                            Console.WriteLine("The specified part " +
                                "of file has been locked.");
                        }
                        catch(IOException e)
                        {
                            Console.WriteLine(
                                "{0}: The specified part of file is" +
                                " already locked.", e.GetType().Name);
                        }
                        break;

                    // Unlock the specified part of the file.
                    case 'U':
                        try
                        {
                            fileStream.Unlock(
                                textLength - 1, byteCount);
                            Console.WriteLine("The specified part " +
                                "of file has been unlocked.");
                        }
                        catch(IOException e)
                        {
                            Console.WriteLine(
                                "{0}: The specified part of file is " +
                                "not locked by the current process.",
                                e.GetType().Name);
                        }
                        break;

                    // Exit the program.
                    default:
                        consoleInput = 'X';
                        break;
                }
            }
        }
    }
}