C# - Random write to file - writing the second line before the first line

Question

I am trying to write to a file using a FileStream and want to write the second line and then write the first line. I use Seek() to go back to the beginning after writing the second line and then write the first line. It replaces the second line ( or part of it depending on the length of the first line.) How do I not make it replace the second line?

        var fs = new FileStream("my.txt", FileMode.Create);
        byte[] stringToWrite = Encoding.UTF8.GetBytes("string that should be in the end");
        byte[] stringToWrite2 = Encoding.UTF8.GetBytes("first string\n");
        fs.Write(stringToWrite, 0, stringToWrite.Length);
        fs.Seek(0, SeekOrigin.Begin);
        fs.Write(stringToWrite2, 0, stringToWrite2.Length);

Following is written to the file:

first string
hould be in the end

I want it to be

first string
string that should be in the end

Thanks

When you seek to the start of the stream you are setting the position you will write to. If there is data already there it will be overwritten. — Russell Troywest, Apr 17 '11 at 10:12

score 4 · Accepted Answer · answered Apr 17 '11 at 10:08

4

You first need to seek into the file equal to the length of the first string.

fs.Seek(stringToWrite2.Length, SeekOrigin.Begin);
fs.Write(stringToWrite, 0, stringToWrite.Length);
fs.Seek(0, SeekOrigin.Begin);

Also, don't forget to dispose of your stream (using).

answered Apr 17 '11 at 10:08

Talljoe

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But the reason why I am writing so is because I do not know first string ( and hence the length ) when I am writing the second string – manojlds Apr 17 '11 at 10:15
In that case you'll have to re-read what you already wrote, write it again further into the file, then write the string that goes in the front. – Talljoe Apr 17 '11 at 10:29
That makes sense. Accepting as answer. – manojlds Apr 17 '11 at 10:30

score 3 · Answer 2 · answered Apr 17 '11 at 10:14

3

You can't insert into a file and push the existing contents back. You can only overwrite or extend.

You therefore can't write a piece of the file until you know the contents of all that precedes it.

answered Apr 17 '11 at 10:14

David Heffernan

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Or at least the length of the preceding content. – Svish Apr 20 '12 at 12:41

score 2 · Answer 3 · edited May 23 '17 at 11:44

Depending on what you are trying to achieve, you may need to write to two different files, one being a temporary file.

Create a temporary file with "second" content
Create a new file with the "first" content
Open the first file, read its content and append to the second file

If this a recurring requirement in a bigger solution, maybe what you want is some kind of database? Maybe a file-based database like SqlLite or BerkeleyDb.

A similar problem is discussed here.

C# - Random write to file - writing the second line before the first line

3 Answers3