FileStream and StreamWriter - How to truncate the remainder of the file after writing?

Question

var fs = new FileStream(filePath, FileMode.OpenOrCreate, FileAccess.ReadWrite);
using(var writer = new StreamWriter(fs))
    writer.Write(....);

If the file previously contained text and the newly-written text is shorter than what was already in the file, how do I make sure that the obsolete trailing content in the file is truncated?

Note that opening the file in truncate mode isn't an option in this case. The file is already open when I receive the FileStream object. The above code is just to illustrate the stream's properties.

EDIT

Expanding on the answer below, the solution is:

var fs = new FileStream(filePath, FileMode.OpenOrCreate, FileAccess.ReadWrite);
using(var writer = new StreamWriter(fs))
{
    writer.Write(....);
    writer.Flush();
    fs.SetLength(fs.Position);
}

The answer/solution code looks wrong to me: Reading fs.Position **before** having called writer.Flush or writer.Dipose() won't give you the real byte count. You have luck because the Stream automatically expands when written to, but your file could get fragmentented unnecessary due to 2 length adaptions which get applied for 1 write. — springy76, Jul 30 '12 at 16:43

score 17 · Accepted Answer · edited May 23 '17 at 12:17

17

Use SetLength to set the new length of the file - the file should get truncated.

See this answer to a related question.

edited May 23 '17 at 12:17

Community

1
1

answered Dec 11 '11 at 13:29

Oded

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Just found the answer at the same time you posted it. Cheers :) – Nathan Ridley Dec 11 '11 at 13:31

score 4 · Answer 2 · answered Dec 11 '11 at 13:29

4

you could try writer.BaseStream.SetLength(writer.BaseStream.Position) although I'm not sure how well that would work.

For a FileStream I think that should truncate the file to the current position.

answered Dec 11 '11 at 13:29

Russell Troywest

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score 1 · Answer 3 · answered Mar 16 '17 at 15:48

This code will truncate part of the log file, if the file grows above 1 MB.

using (FileStream fs = File.Open("C:\\LogFile.txt", FileMode.OpenOrCreate))
        {
            int OneMB = 1000000;

            fs.Seek(0, SeekOrigin.Begin);
            if (fs.Length > OneMB)
            { 
                int fileByte = 1;
                fs.Position  = fs.Seek(fs.Length / 2, SeekOrigin.Begin);
                List<byte> bytes = new List<byte>();

                while (fileByte > 0)
                {
                   fileByte = fs.ReadByte();
                   bytes.Add((byte)fileByte);
                }

                fs.SetLength(0);
                fs.Position = 0;
                fs.Write(bytes.ToArray(), 0, bytes.Count());

                fs.Seek(0, SeekOrigin.End);
                var stringBytes =  UTF8Encoding.ASCII.GetBytes($"Test1" + Environment.NewLine);
                fs.Write(stringBytes, 0, stringBytes.Length);  
            }
            else
            {
                fs.Seek(0, SeekOrigin.End);
                var stringBytes = UTF8Encoding.ASCII.GetBytes($"Test2" + Environment.NewLine);
                fs.Write(stringBytes, 0, stringBytes.Length); 
            }

            fs.Flush();
        }
    }

FileStream and StreamWriter - How to truncate the remainder of the file after writing?

3 Answers3

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