[c#] Edit a specific Line of a Text File in C#

I have two text files, Source.txt and Target.txt. The source will never be modified and contain N lines of text. So, I want to delete a specific line of text in Target.txt, and replace by an specific line of text from Source.txt, I know what number of line I need, actually is the line number 2, both files.

I haven something like this:

string line = string.Empty;
int line_number = 1;
int line_to_edit = 2;

using (StreamReader reader = new StreamReader(@"C:\source.xml"))
{
    using (StreamWriter writer = new StreamWriter(@"C:\target.xml"))
    {
        while ((line = reader.ReadLine()) != null)
        {
            if (line_number == line_to_edit)
            {
                writer.WriteLine(line);
            } 

            line_number++;
        }
    }
}

But when I open the Writer, the target file get erased, it writes the lines, but, when opened, the target file only contains the copied lines, the rest get lost.

What can I do?

You need to Open the output file for write access rather than using a new StreamReader, which always overwrites the output file.

StreamWriter stm = null;
fi = new FileInfo(@"C:\target.xml");
if (fi.Exists)
   stm = fi.OpenWrite();

Of course, you will still have to seek to the correct line in the output file, which will be hard since you can't read from it, so unless you already KNOW the byte offset to seek to, you probably really want read/write access.

FileStream stm = fi.Open(FileMode.OpenOrCreate, FileAccess.ReadWrite, FileShare.None);

with this stream, you can read until you get to the point where you want to make changes, then write. Keep in mind that you are writing bytes, not lines, so to overwrite a line you will need to write the same number of characters as the line you want to change.

I guess the below should work (instead of the writer part from your example). I'm unfortunately with no build environment so It's from memory but I hope it helps

using (var fs = File.Open(filePath, FileMode.Open, FileAccess.ReadWrite)))
        {
            var destinationReader = StreamReader(fs);
            var writer = StreamWriter(fs);
            while ((line = reader.ReadLine()) != null)
            {
              if (line_number == line_to_edit)
                {
                    writer.WriteLine(lineToWrite);
                }
                else
                {
                    destinationReader .ReadLine();
                }
                line_number++;
            }
        }

the easiest way is :

static void lineChanger(string newText, string fileName, int line_to_edit)
{
     string[] arrLine = File.ReadAllLines(fileName);
     arrLine[line_to_edit - 1] = newText;
     File.WriteAllLines(fileName, arrLine);
}

usage :

lineChanger("new content for this line" , "sample.text" , 34);

When you create a StreamWriter it always create a file from scratch, you will have to create a third file and copy from target and replace what you need, and then replace the old one. But as I can see what you need is XML manipulation, you might want to use XmlDocument and modify your file using Xpath.