c# - Using FileStream.Seek

Question

I am trying to work with FileStream.Seek to quickly jump to a line and read it.

However, I am not getting the right results. I have tried to look at this for a while and can't understand what I am doing wrong.

Environment:
OS: Windows 7
Framework: .NET 4.0
IDE: Visual C# Express 2010

Sample Data in file location: C:TempTemp.txt

0001|100!2500
0002|100!2500
0003|100!2500
0004|100!2500
0005|100!2500
0006|100!2500
0007|100!2500
0008|100!2500
0009|100!2500
0010|100!2500

The code:

class PaddedFileSearch
{
    private int LineLength { get; set; }
    private string FileName { get; set; }

    public PaddedFileSearch()
    {
        FileName = @"C:TempTemp.txt";     // This is a padded file.  All lines are of the same length.

        FindLineLength();
        Debug.Print("File Line length: {0}", LineLength);

        // TODO: This purely for testing.  Move this code out.
        SeekMethod(new int[] { 5, 3, 4 });
        /*  Expected Results:
         *  Line No     Position        Line
         *  -------     --------        -----------------
         *  3           30              0003|100!2500
         *  4           15              0004|100!2500
         *  5           15              0005|100!2500 -- This was updated after the initial request.
         */

        /* THIS DOES NOT GIVE THE EXPECTED RESULTS */
        SeekMethod(new int[] { 5, 3 });
        /*  Expected Results:
         *  Line No     Position        Line
         *  -------     --------        -----------------
         *  3           30              0003|100!2500
         *  5           30              0005|100!2500
         */
    }

    private void FindLineLength()
    {
        string line;

        // Add check for FileExists

        using (StreamReader reader = new StreamReader(FileName))
        {
            if ((line = reader.ReadLine()) != null)
            {
                LineLength = line.Length + 2;
                // The 2 is for NewLine(
)
            }
        }

    }

    public void SeekMethod(int[] lineNos)
    {
        long position = 0;
        string line = null;

        Array.Sort(lineNos);

        Debug.Print("");
        Debug.Print("Line NoPositionLine");
        Debug.Print("--------------------------------");

        using (FileStream fs = new FileStream(FileName, FileMode.Open, FileAccess.Read, FileShare.None))
        {
            using (StreamReader reader = new StreamReader(fs))
            {
                foreach (int lineNo in lineNos)
                {
                    position = (lineNo - 1) * LineLength - position;
                    fs.Seek(position, SeekOrigin.Current);

                    if ((line = reader.ReadLine()) != null)
                    {
                        Debug.Print("{0}{1}{2}", lineNo, position, line);
                    }
                }
            }
        }
    }
}

The output I get:

File Line length: 15

Line No     Position        Line
-------     --------        -----------------
3           30              0003|100!2500
4           15              0004|100!2500
5           45              0005|100!2500

Line No     Position        Line
-------     --------        -----------------
3           30              0003|100!2500
5           30              0004|100!2500

My problem is with the following output:

Line No     Position        Line
-------     --------        -----------------
5           30              0004|100!2500

The output for Line should be: 0005|100!2500

I don't understand why this is happening.

Am I doing something wrong? Is there a workaround? Also are there faster ways to do this using something like seek?
(I am looking for code based options and NOT Oracle or SQL Server. For the sake of argument lets also say that the file size 1 GB.)

Any help is greatly appreciated.

Thanks.

UPDATE:
I found 4 great answers here. Thanks a lot.

Sample Timings:
Based on a few runs the following are the methods from best to good. Even the good is very close to best.
In a file that contains 10K lines, 2.28 MB. I searched for same 5000 random lines using all the options.

1. Seek4: Time elapsed: 00:00:00.0398530 ms -- Ritch Melton
2. Seek3: Time elapsed: 00:00:00.0446072 ms -- Valentin Kuzub
3. Seek1: Time elapsed: 00:00:00.0538210 ms -- Jake
4. Seek2: Time elapsed: 00:00:00.0889589 ms -- bitxwise

Shown below is the code. After saving the code you can simply call it by typing TestPaddedFileSeek.CallPaddedFileSeek();. You will also have to specify the namespace and the "using references".

`

/// <summary>
/// This class multiple options of reading a by line number in a padded file (all lines are the same length).
/// The idea is to quick jump to the file.
/// Details about the discussions is available at: http://stackoverflow.com/questions/5201414/having-a-problem-while-using-filestream-seek-in-c-solved
/// </summary>
class PaddedFileSeek
{
    public FileInfo File {get; private set;}
    public int LineLength { get; private set; }

    #region Private methods
    private static int FindLineLength(FileInfo fileInfo)
    {
        using (StreamReader reader = new StreamReader(fileInfo.FullName))
        {
            string line;
            if ((line = reader.ReadLine()) != null)
            {
                int length = line.Length + 2;   // The 2 is for NewLine(
)
                return length;
            }
        }
        return 0;
    }

    private static void PrintHeader()
    {
       /*
        Debug.Print("");
        Debug.Print("Line NoLine");
        Debug.Print("---------------------------------");
       */ 
    }

    private static void PrintLine(int lineNo, string line)
    {
        //Debug.Print("{0}{1}", lineNo, line);
    }

    private static void PrintElapsedTime(TimeSpan elapsed)
    {
        Debug.WriteLine("Time elapsed: {0} ms", elapsed);
    }
    #endregion

    public PaddedFileSeek(FileInfo fileInfo)
    {
        // Possibly might have to check for FileExists
        int length = FindLineLength(fileInfo);
        //if (length == 0) throw new PaddedProgramException();
        LineLength = length;
        File = fileInfo;
    }

    public void CallAll(int[] lineNoArray, List<int> lineNoList)
    {
        Stopwatch sw = new Stopwatch();

        #region Seek1
        // Create new stopwatch
        sw.Start();

        Debug.Write("Seek1: ");
        // Print Header
        PrintHeader();

        Seek1(lineNoArray);

        // Stop timing
        sw.Stop();

        // Print Elapsed Time
        PrintElapsedTime(sw.Elapsed);

        sw.Reset();
        #endregion

        #region Seek2
        // Create new stopwatch
        sw.Start();

        Debug.Write("Seek2: ");
        // Print Header
        PrintHeader();

        Seek2(lineNoArray);

        // Stop timing
        sw.Stop();

        // Print Elapsed Time
        PrintElapsedTime(sw.Elapsed);

        sw.Reset();
        #endregion

        #region Seek3
        // Create new stopwatch
        sw.Start();

        Debug.Write("Seek3: ");
        // Print Header
        PrintHeader();

        Seek3(lineNoArray);

        // Stop timing
        sw.Stop();

        // Print Elapsed Time
        PrintElapsedTime(sw.Elapsed);

        sw.Reset();
        #endregion

        #region Seek4
        // Create new stopwatch
        sw.Start();

        Debug.Write("Seek4: ");

        // Print Header
        PrintHeader();

        Seek4(lineNoList);

        // Stop timing
        sw.Stop();

        // Print Elapsed Time
        PrintElapsedTime(sw.Elapsed);

        sw.Reset();
        #endregion

    }

    /// <summary>
    /// Option by Jake
    /// </summary>
    /// <param name="lineNoArray"></param>
    public void Seek1(int[] lineNoArray)
    {
        long position = 0;
        string line = null;

        Array.Sort(lineNoArray);

        using (FileStream fs = new FileStream(File.FullName, FileMode.Open, FileAccess.Read, FileShare.None))
        {
            using (StreamReader reader = new StreamReader(fs))
            {
                foreach (int lineNo in lineNoArray)
                {
                    position = (lineNo - 1) * LineLength;
                    fs.Seek(position, SeekOrigin.Begin);

                    if ((line = reader.ReadLine()) != null)
                    {
                        PrintLine(lineNo, line);
                    }

                    reader.DiscardBufferedData();
                }
            }
        }

    }

    /// <summary>
    /// option by bitxwise
    /// </summary>
    public void Seek2(int[] lineNoArray)
    {
        string line = null;
        long step = 0;

        Array.Sort(lineNoArray);

        using (FileStream fs = new FileStream(File.FullName, FileMode.Open, FileAccess.Read, FileShare.None))
        {
            // using (StreamReader reader = new StreamReader(fs))
            // If you put "using" here you will get WRONG results.
            // I would like to understand why this is.
            {
                foreach (int lineNo in lineNoArray)
                {
                    StreamReader reader = new StreamReader(fs);
                    step = (lineNo - 1) * LineLength - fs.Position;
                    fs.Position += step;

                    if ((line = reader.ReadLine()) != null)
                    {
                        PrintLine(lineNo, line);
                    }
                }
            }
        }
    }

    /// <summary>
    /// Option by Valentin Kuzub
    /// </summary>
    /// <param name="lineNoArray"></param>
    #region Seek3
    public void Seek3(int[] lineNoArray)
    {
        long position = 0; // totalPosition = 0;
        string line = null;
        int oldLineNo = 0;

        Array.Sort(lineNoArray);

        using (FileStream fs = new FileStream(File.FullName, FileMode.Open, FileAccess.Read, FileShare.None))
        {
            using (StreamReader reader = new StreamReader(fs))
            {
                foreach (int lineNo in lineNoArray)
                {
                    position = (lineNo - oldLineNo - 1) * LineLength;
                    fs.Seek(position, SeekOrigin.Current);
                    line = ReadLine(fs, LineLength);
                    PrintLine(lineNo, line);
                    oldLineNo = lineNo;

                }
            }
        }

    }

    #region Required Private methods
    /// <summary>
    /// Currently only used by Seek3
    /// </summary>
    /// <param name="stream"></param>
    /// <param name="length"></param>
    /// <returns></returns>
    private static string ReadLine(FileStream stream, int length)
    {
        byte[] bytes = new byte[length];
        stream.Read(bytes, 0, length);
        return new string(Encoding.UTF8.GetChars(bytes));
    }
    #endregion
    #endregion

    /// <summary>
    /// Option by Ritch Melton
    /// </summary>
    /// <param name="lineNoArray"></param>
    #region Seek4
    public void Seek4(List&l

Answer 1

I'm confused by your expected positions, Line 5 at position 30 and 45, with Line 4 at 15, and 3 at 30?

Here's the core of the read logic:

    var offset = (lineNumber - 1) * LineLength;

    fs.Seek(offset, SeekOrigin.Begin);

    var data = new byte[LineLength];
    fs.Read(data, 0, LineLength);

    var text = DecodeData(data);
    Debug.Print("{0,-12}{1,-16}{2}", lineNumber, offset, text);

The full sample is here:

class PaddedFileSearch
{
    public int LineLength { get; private set; }
    public FileInfo File { get; private set; }

    public PaddedFileSearch(FileInfo fileInfo)
    {
        var length = FindLineLength(fileInfo);
        //if (length == 0) throw new PaddedProgramException();
        LineLength = length;
        File = fileInfo;
    }

    private static int FindLineLength(FileInfo fileInfo)
    {
        using (var reader = new StreamReader(fileInfo.FullName))
        {
            string line;
            if ((line = reader.ReadLine()) != null)
            {
                var length = line.Length + 2;
                return length;
            }
        }

        return 0;
    }

    public void SeekMethod(List<int> lineNumbers)
    {

        Debug.Print("");
        Debug.Print("Line NoPositionLine");
        Debug.Print("--------------------------------");

        lineNumbers.Sort();

        using (var fs = new FileStream(File.FullName, FileMode.Open))
        {
            lineNumbers.ForEach(ln => OutputData(fs, ln));
        }
    }

    private void OutputData(FileStream fs, int lineNumber)
    {
        var offset = (lineNumber - 1) * LineLength;

        fs.Seek(offset, SeekOrigin.Begin);

        var data = new byte[LineLength];
        fs.Read(data, 0, LineLength);

        var text = DecodeData(data);
        Debug.Print("{0,-12}{1,-16}{2}", lineNumber, offset, text);
    }

    private static string DecodeData(byte[] data)
    {
        var encoding = new UTF8Encoding();
        return encoding.GetString(data);
    }
}

class Program
{
    static void Main(string[] args)
    {
        var seeker = new PaddedFileSearch(new FileInfo(@"D:DesktopTest.txt"));

        Debug.Print("File Line length: {0}", seeker.LineLength);

        seeker.SeekMethod(new List<int> { 5, 3, 4 });
        seeker.SeekMethod(new List<int> { 5, 3 });
    }
}