How to read and write Text file at the same time.

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Hello,
How can i read and write a text file at the same time.

I have the following code but the problem is that it does writes at the end of the file contents. but i want it to write at after "two". i.e. if the read line contains "two" then write "test" after "two".

FileStream fsrw = new FileStream(path, FileMode.OpenOrCreate, FileAccess.ReadWrite);
StreamReader sr = new StreamReader(fsrw);
StreamWriter sw = new StreamWriter(fsrw);
test = "Dummy";

 while (test != null)
{
 if (test.Contains("two"))
    {
    sw.Write("test");
    }
    test = sr.ReadLine();
}
sw.Close();
}

Posted 2-Sep-10 0:18am

asjadazeez

Updated 13-Aug-23 18:55pm

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Kubajzz · Answer 1 · 2010-09-02T00:40:00

Reading a file that is being edited at the same time sounds like a really bad idea.

I'm afraid you could get unexpected results and impossible-to-find bugs...

Imagine the following scenario:

StreamWriter writes data to the end of the stream by default so the word "test" would most probably appear at the end of the file. Maybe it is possible to force StreamWriter to start writing at an arbitrary location, but that would not solve the whole problem... You would have to keep track of the current position, which is pretty difficult considering the fact that the file is changing in time. And don't forget that both StreamReader and StreamWriter use buffers to load/write data efficiently, which is adds even more complexity...

I suggest reading the entire file first, perform the changes and then write the file (or, if speed is important, perform the changes on the fly while reading or writing). If the file is too large, use a temporary file, but do not read and write at the same time...

User 13075200 · Answer 2 · 2023-08-13T18:55:00

As mentioned by Kubajzz, reading a file that is being edited at the same time may have the possibility of yielding unexpected results.

If you wish to edit files as they are being edited, your best bet is to use FileSystemWatcher class:

C#

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

class Program
{
    static void Main(string[] args)
    {
        string filePath = "full_path_to_your_file.txt";

        // NOTE: There is `FileSystemWatcher(DirectoryPath, Filter)` constructor as well
        FileSystemWatcher watcher = new FileSystemWatcher();
        watcher.Path = Path.GetDirectoryName(filePath);
        watcher.Filter = Path.GetFileName(filePath);

        // Callback when file is last written (aka, on file saved).
        watcher.NotifyFilter = NotifyFilters.LastWrite;
        watcher.Changed += OnFileChanged;

        watcher.EnableRaisingEvents = true;

        Console.WriteLine("Press Enter to exit.");
        Console.ReadLine();
    }

    private static void OnFileChanged(object sender, FileSystemEventArgs e)
    {
        string filePath = e.FullPath;

        try
        {
            // Read the entire content, manipulate, then dump back.
            string fileContent = File.ReadAllText(filePath);
            string updatedContent = StringManipulationStuff(fileContent);

            File.WriteAllText(filePath, updatedContent);
        }
        catch (Exception ex)
        {
            // Error handle...
        }
    }

    private static string StringManipulationStuff(string input)
    {
        string pattern = @"\btwo\b";
        string replacement = "twotest";

        return Regex.Replace(input, pattern, replacement);
    }
}

How to read and write Text file at the same time.

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