c# - StreamReader and seeking

Question

Can you use StreamReader to read a normal textfile and then in the middle of reading close the StreamReader after saving the current position and then open StreamReader again and start reading from that poistion ?

If not what else can I use to accomplish the same case without locking the file ?

I tried this but it doesn't work:

var fs = File.Open(@ "C:\testfile.txt", FileMode.Open, FileAccess.Read);
var sr = new StreamReader(fs);

Debug.WriteLine(sr.ReadLine()); //Prints:firstline

var pos = fs.Position;

while (!sr.EndOfStream) 
{
    Debug.WriteLine(sr.ReadLine());
}

fs.Seek(pos, SeekOrigin.Begin);

Debug.WriteLine(sr.ReadLine());
//Prints Nothing, i expect it to print SecondLine.

---

Here is the other code I also tried :

var position = -1;
StreamReaderSE sr = new StreamReaderSE(@ "c:\testfile.txt");

Debug.WriteLine(sr.ReadLine());
position = sr.BytesRead;

Debug.WriteLine(sr.ReadLine());
Debug.WriteLine(sr.ReadLine());
Debug.WriteLine(sr.ReadLine());
Debug.WriteLine(sr.ReadLine());

Debug.WriteLine("Wait");

sr.BaseStream.Seek(position, SeekOrigin.Begin);
Debug.WriteLine(sr.ReadLine());

Answer 1

I realize this is really belated, but I just stumbled onto this incredible flaw in StreamReader myself; the fact that you can't reliably seek when using StreamReader. Personally, my specific need is to have the ability to read characters, but then "back up" if a certain condition is met; it's a side effect of one of the file formats I'm parsing.

Using ReadLine() isn't an option because it's only useful in really trivial parsing jobs. I have to support configurable record/line delimiter sequences and support escape delimiter sequences. Also, I don't want to implement my own buffer so I can support "backing up" and escape sequences; that should be the StreamReader's job.

This method calculates the actual position in the underlying stream of bytes on-demand. It works for UTF8, UTF-16LE, UTF-16BE, UTF-32LE, UTF-32BE, and any single-byte encoding (e.g. code pages 1252, 437, 28591, etc.), regardless the presence of a preamble/BOM. This version will not work for UTF-7, Shift-JIS, or other variable-byte encodings.

When I need to seek to an arbitrary position in the underlying stream, I directly set BaseStream.Position and then call DiscardBufferedData() to get StreamReader back in sync for the next Read()/Peek() call.

And a friendly reminder: don't arbitrarily set BaseStream.Position. If you bisect a character, you'll invalidate the next Read() and, for UTF-16/-32, you'll also invalidate the result of this method.

public static long GetActualPosition(StreamReader reader)
{
    System.Reflection.BindingFlags flags = System.Reflection.BindingFlags.DeclaredOnly | System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance | System.Reflection.BindingFlags.GetField;

    // The current buffer of decoded characters
    char[] charBuffer = (char[])reader.GetType().InvokeMember("charBuffer", flags, null, reader, null);

    // The index of the next char to be read from charBuffer
    int charPos = (int)reader.GetType().InvokeMember("charPos", flags, null, reader, null);

    // The number of decoded chars presently used in charBuffer
    int charLen = (int)reader.GetType().InvokeMember("charLen", flags, null, reader, null);

    // The current buffer of read bytes (byteBuffer.Length = 1024; this is critical).
    byte[] byteBuffer = (byte[])reader.GetType().InvokeMember("byteBuffer", flags, null, reader, null);

    // The number of bytes read while advancing reader.BaseStream.Position to (re)fill charBuffer
    int byteLen = (int)reader.GetType().InvokeMember("byteLen", flags, null, reader, null);

    // The number of bytes the remaining chars use in the original encoding.
    int numBytesLeft = reader.CurrentEncoding.GetByteCount(charBuffer, charPos, charLen - charPos);

    // For variable-byte encodings, deal with partial chars at the end of the buffer
    int numFragments = 0;
    if (byteLen > 0 && !reader.CurrentEncoding.IsSingleByte)
    {
        if (reader.CurrentEncoding.CodePage == 65001) // UTF-8
        {
            byte byteCountMask = 0;
            while ((byteBuffer[byteLen - numFragments - 1] >> 6) == 2) // if the byte is "10xx xxxx", it's a continuation-byte
                byteCountMask |= (byte)(1 << ++numFragments); // count bytes & build the "complete char" mask
            if ((byteBuffer[byteLen - numFragments - 1] >> 6) == 3) // if the byte is "11xx xxxx", it starts a multi-byte char.
                byteCountMask |= (byte)(1 << ++numFragments); // count bytes & build the "complete char" mask
            // see if we found as many bytes as the leading-byte says to expect
            if (numFragments > 1 && ((byteBuffer[byteLen - numFragments] >> 7 - numFragments) == byteCountMask))
                numFragments = 0; // no partial-char in the byte-buffer to account for
        }
        else if (reader.CurrentEncoding.CodePage == 1200) // UTF-16LE
        {
            if (byteBuffer[byteLen - 1] >= 0xd8) // high-surrogate
                numFragments = 2; // account for the partial character
        }
        else if (reader.CurrentEncoding.CodePage == 1201) // UTF-16BE
        {
            if (byteBuffer[byteLen - 2] >= 0xd8) // high-surrogate
                numFragments = 2; // account for the partial character
        }
    }
    return reader.BaseStream.Position - numBytesLeft - numFragments;
}

Of course, this uses Reflection to get at private variables, so there is risk involved. However, this method works with .Net 2.0, 3.0, 3.5, 4.0, 4.0.3, 4.5, 4.5.1, 4.5.2, 4.6, and 4.6.1. Beyond that risk, the only other critical assumption is that the underlying byte-buffer is a byte[1024]; if Microsoft changes it the wrong way, the method breaks for UTF-16/-32.

This has been tested against a UTF-8 file filled with Ažテ (10 bytes: 0x41 C5 BE E3 83 86 F0 A3 98 BA) and a UTF-16 file filled with A (6 bytes: 0x41 00 01 D8 37 DC). The point being to force-fragment characters along the byte[1024] boundaries, all the different ways they could be.

UPDATE (2013-07-03): I fixed the method, which originally used the broken code from that other answer. This version has been tested against data containing a characters requiring use of surrogate pairs. The data was put into 3 files, each with a different encoding; one UTF-8, one UTF-16LE, and one UTF-16BE.

UPDATE (2016-02): The only correct way to handle bisected characters is to directly interpret the underlying bytes. UTF-8 is properly handled, and UTF-16/-32 work (given the length of byteBuffer).

Answer 2

Yes you can, see this:

var sr = new StreamReader("test.txt");
sr.BaseStream.Seek(2, SeekOrigin.Begin); // Check sr.BaseStream.CanSeek first

Update: Be aware that you can't necessarily use sr.BaseStream.Position to anything useful because StreamReader uses buffers so it will not reflect what you actually have read. I guess you gonna have problems finding the true position. Because you can't just count characters (different encodings and therefore character lengths). I think the best way is to work with FileStream´s themselves.

Update: Use the TGREER.myStreamReader from here: http://www.daniweb.com/software-development/csharp/threads/35078 this class adds BytesRead etc. (works with ReadLine() but apparently not with other reads methods) and then you can do like this:

File.WriteAllText("test.txt", "1234\n56789");

long position = -1;

using (var sr = new myStreamReader("test.txt"))
{
    Console.WriteLine(sr.ReadLine());

    position = sr.BytesRead;
}

Console.WriteLine("Wait");

using (var sr = new myStreamReader("test.txt"))
{
    sr.BaseStream.Seek(position, SeekOrigin.Begin);
    Console.WriteLine(sr.ReadToEnd());
}

Answer 3

If you want to just search for a start position within a text stream, I added this extension to StreamReader so that I could determine where the edit of the stream should occur. Granted, this is based upon characters as the incrementing aspect of the logic, but for my purposes, it works great, for getting the position within a text/ASCII based file based upon a string pattern. Then, you can use that location as a start point for reading, to write a new file that discludes the data prior to the start point.

The returned position within the stream can be provided to Seek to start from that position within text-based stream reads. It works. I've tested it. However, there may be issues when matching to non-ASCII Unicode chars during the matching algorithm. This was based upon American English and the associated character page.

Basics: it scans through a text stream, character-by-character, looking for the sequential string pattern (that matches the string parameter) forward only through the stream. Once the pattern doesn't match the string parameter (i.e. going forward, char by char), then it will start over (from the current position) trying to get a match, char-by-char. It will eventually quit if the match can't be found in the stream. If the match is found, then it returns the current "character" position within the stream, not the StreamReader.BaseStream.Position, as that position is ahead, based on the buffering that the StreamReader does.

As indicated in the comments, this method WILL affect the position of the StreamReader, and it will be set back to the beginning (0) at the end of the method. StreamReader.BaseStream.Seek should be used to run to the position returned by this extension.

Note: the position returned by this extension will also work with BinaryReader.Seek as a start position when working with text files. I actually used this logic for that purpose to rewrite a PostScript file back to disk, after discarding the PJL header information to make the file a "proper" PostScript readable file that could be consumed by GhostScript. :)

The string to search for within the PostScript (after the PJL header) is: "%!PS-", which is followed by "Adobe" and the version.

public static class StreamReaderExtension
{
    /// <summary>
    /// Searches from the beginning of the stream for the indicated
    /// <paramref name="pattern"/>. Once found, returns the position within the stream
    /// that the pattern begins at.
    /// </summary>
    /// <param name="pattern">The <c>string</c> pattern to search for in the stream.</param>
    /// <returns>If <paramref name="pattern"/> is found in the stream, then the start position
    /// within the stream of the pattern; otherwise, -1.</returns>
    /// <remarks>Please note: this method will change the current stream position of this instance of
    /// <see cref="System.IO.StreamReader"/>. When it completes, the position of the reader will
    /// be set to 0.</remarks>
    public static long FindSeekPosition(this StreamReader reader, string pattern)
    {
        if (!string.IsNullOrEmpty(pattern) && reader.BaseStream.CanSeek)
        {
            try
            {
                reader.BaseStream.Position = 0;
                reader.DiscardBufferedData();
                StringBuilder buff = new StringBuilder();
                long start = 0;
                long charCount = 0;
                List<char> matches = new List<char>(pattern.ToCharArray());
                bool startFound = false;

                while (!reader.EndOfStream)
                {
                    char chr = (char)reader.Read();

                    if (chr == matches[0] && !startFound)
                    {
                        startFound = true;
                        start = charCount;
                    }

                    if (startFound && matches.Contains(chr))
                    {
                        buff.Append(chr);

                        if (buff.Length == pattern.Length
                            && buff.ToString() == pattern)
                        {
                            return start;
                        }

                        bool reset = false;

                        if (buff.Length > pattern.Length)
                        {
                            reset = true;
                        }
                        else
                        {
                            string subStr = pattern.Substring(0, buff.Length);

                            if (buff.ToString() != subStr)
                            {
                                reset = true;
                            }
                        }

                        if (reset)
                        {
                            buff.Length = 0;
                            startFound = false;
                            start = 0;
                        }
                    }

                    charCount++;
                }
            }
            finally
            {
                reader.BaseStream.Position = 0;
                reader.DiscardBufferedData();
            }
        }

        return -1;
    }
}

Answer 4

FileStream.Position (or equivalently, StreamReader.BaseStream.Position) will usually be ahead -- possibly way ahead -- of the TextReader position because of the underlying buffering taking place.

If you can determine how newlines are handled in your text files, you can add up the number of bytes read based on line lengths and end-of-line characters.

File.WriteAllText("test.txt", "1234" + System.Environment.NewLine + "56789");

long position = -1;
long bytesRead = 0;
int newLineBytes = System.Environment.NewLine.Length;

using (var sr = new StreamReader("test.txt"))
{
    string line = sr.ReadLine();
    bytesRead += line.Length + newLineBytes;

    Console.WriteLine(line);

    position = bytesRead;
}

Console.WriteLine("Wait");

using (var sr = new StreamReader("test.txt"))
{
    sr.BaseStream.Seek(position, SeekOrigin.Begin);
    Console.WriteLine(sr.ReadToEnd());
}

For more complex text file encodings you might need to get fancier than this, but it worked for me.

Answer 5

From MSDN:

StreamReader is designed for character input in a particular encoding, whereas the Stream class is designed for byte input and output. Use StreamReader for reading lines of information from a standard text file.

In most of the examples involving StreamReader, you will see reading line by line using the ReadLine(). The Seek method comes from Stream class which is basically used to read or handle data in bytes.

Answer 6

I found the suggestions above to not work for me -- my use case was to simply need to back up one read position (I'm reading one char at a time with a default encoding). My simple solution was inspired by above commentary ... your mileage may vary...

I saved the BaseStream.Position before reading, then determined if I needed to back up... if yes, then set position and invoke DiscardBufferedData().