Edit a specific Line of a Text File in C

Question

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

User · Answer

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

User · Answer

You can t rewrite a line without rewriting the entire file  unless the lines happen to be the same length   If your files are small then reading the entire target file into memory and then writing it out again might make sense  You can do that like this   using System  using System IO   class Program       static void Main string   args                int line to edit   2     Warning  1-based indexing          string sourceFile    source txt           string destinationFile    target txt               Read the appropriate line from the file          string lineToWrite   null          using  StreamReader reader   new StreamReader sourceFile                         for  int i   1  i  lt   line to edit    i                  lineToWrite   reader ReadLine                       if  lineToWrite    null              throw new InvalidDataException  Line does not exist in     sourceFile               Read the old file          string   lines   File ReadAllLines destinationFile               Write the new file over the old file          using  StreamWriter writer   new StreamWriter destinationFile                         for  int currentLine   1  currentLine  lt   lines Length    currentLine                                if  currentLine    line to edit                                        writer WriteLine lineToWrite                                     else                                       writer WriteLine lines currentLine - 1                                                        If your files are large it would be better to create a new file so that you can read streaming from one file while you write to the other  This means that you don t need to have the whole file in memory at once  You can do that like this   using System  using System IO   class Program       static void Main string   args                int line to edit   2          string sourceFile    source txt           string destinationFile    target txt           string tempFile    target2 txt               Read the appropriate line from the file          string lineToWrite   null          using  StreamReader reader   new StreamReader sourceFile                         for  int i   1  i  lt   line to edit    i                  lineToWrite   reader ReadLine                       if  lineToWrite    null              throw new InvalidDataException  Line does not exist in     sourceFile               Read from the target file and write to a new file          int line number   1          string line   null          using  StreamReader reader   new StreamReader destinationFile           using  StreamWriter writer   new StreamWriter tempFile                         while   line   reader ReadLine       null                                if  line number    line to edit                                        writer WriteLine lineToWrite                                     else                                       writer WriteLine line                                     line number                                        TODO  Delete the old file and replace it with the new file here            You can afterwards move the file once you are sure that the write operation has succeeded  no excecption was thrown and the writer is closed    Note that in both cases it is a bit confusing that you are using 1-based indexing for your line numbers  It might make more sense in your code to use 0-based indexing  You can have 1-based index in your user interface to your program if you wish  but convert it to a 0-indexed before sending it further   Also  a disadvantage of directly overwriting the old file with the new file is that if it fails halfway through then you might permanently lose whatever data wasn t written  By writing to a third file first you only delete the original data after you are sure that you have another  corrected  copy of it  so you can recover the data if the computer crashes halfway through   A final remark  I noticed that your files had an xml extension  You might want to consider if it makes more sense for you to use an XML parser to modify the contents of the files instead of replacing specific lines

User · Answer

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

User · Answer

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

User · Answer

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

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

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