How to delete a line from a text file in C

Question

I have a problem  how can I delete a line from a text file in C

User · Answer

I agree with John Saunders  this isn t really C  specific  However  to answer your question  you basically need to rewrite the file  There are two ways you can do this    Read the whole file into memory  e g  with File ReadAllLines  Remove the offending line  in this case it s probably easiest to convert the string array into a List lt string gt  then remove the line  Write all the rest of the lines back  e g  with File WriteAllLines  - potentially convert the List lt string gt  into a string array again using ToArray   That means you have to know that you ve got enough memory though  An alternative    Open both the input file and a new output file  as a TextReader TextWriter  e g  with File OpenText and File CreateText  Read a line  TextReader ReadLine  - if you don t want to delete it  write it to the output file  TextWriter WriteLine  When you ve read all the lines  close both the reader and the writer  if you use using statements for both  this will happen automatically  If you want to replace the input with the output  delete the input file and then move the output file into place

User · Answer

For very large files I d do something like this  string tempFile   Path GetTempFileName     using var sr   new StreamReader  file txt    using var sw   new StreamWriter tempFile         string line       while  line   sr ReadLine       null                 if line     removeme                sw WriteLine line            File Delete  file txt    File Move tempFile   file txt      Update I originally wrote this back in 2009 and I thought it might be interesting with an update  Today you could accomplish the above using LINQ and deferred execution  var tempFile   Path GetTempFileName    var linesToKeep   File ReadLines fileName  Where l   gt  l     removeme     File WriteAllLines tempFile  linesToKeep    File Delete fileName   File Move tempFile  fileName     The code above is almost exactly the same as the first example  reading line by line and while keeping a minimal amount of data in memory   A disclaimer might be in order though  Since we re talking about text files here you d very rarely have to use the disk as an intermediate storage medium  If you re not dealing with very large log files there should be no problem reading the contents into memory instead and avoid having to deal with the temporary file   File WriteAllLines fileName       File ReadLines fileName  Where l   gt  l     removeme   ToList       Note that The  ToList is crucial here to force immediate execution  Also note that all the examples assume the text files are UTF-8 encoded

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I extended what Markus Olsson suggested  and came up with this class that adds multiple search strings and a couple of event   public static class TextLineRemover       public static void RemoveTextLines IList lt string gt  linesToRemove  string filename  string tempFilename                   Initial values         int lineNumber   0          int linesRemoved   0          DateTime startTime   DateTime Now              Read file         using  var sr   new StreamReader filename                            Write new file             using  var sw   new StreamWriter tempFilename                                    Read lines                 string line                  while   line   sr ReadLine       null                                        lineNumber                           Look for text to remove                     if   ContainsString line  linesToRemove                                                    Keep lines that does not match                         sw WriteLine line                                             else                                                  Ignore lines that DO match                         linesRemoved                            InvokeOnRemovedLine new RemovedLineArgs   RemovedLine   line  RemovedLineNumber   lineNumber                                                                               Delete original file         File Delete filename                   and put the temp file in its place          File Move tempFilename  filename               Final calculations         DateTime endTime   DateTime Now          InvokeOnFinished new FinishedArgs  LinesRemoved   linesRemoved  TotalLines   lineNumber  TotalTime   endTime Subtract startTime                private static bool ContainsString string line  IEnumerable lt string gt  linesToRemove                foreach  var lineToRemove in linesToRemove                        if line Contains lineToRemove                   return true                    return false             public static event RemovedLine OnRemovedLine      public static event Finished OnFinished       public static void InvokeOnFinished FinishedArgs args                Finished handler   OnFinished          if  handler    null  handler null  args              public static void InvokeOnRemovedLine RemovedLineArgs args                RemovedLine handler   OnRemovedLine          if  handler    null  handler null  args            public delegate void Finished object sender  FinishedArgs args    public class FinishedArgs       public int TotalLines   get  set        public int LinesRemoved   get  set        public TimeSpan TotalTime   get  set       public delegate void RemovedLine object sender  RemovedLineArgs args    public class RemovedLineArgs       public string RemovedLine   get  set        public int RemovedLineNumber   get  set        Usage    TextLineRemover OnRemovedLine     o  removedLineArgs    gt  Console WriteLine string Format  Removed    0    at line  1    removedLineArgs RemovedLine  removedLineArgs RemovedLineNumber    TextLineRemover OnFinished     o  finishedArgs    gt  Console WriteLine string Format   0  of  1  lines removed  Time used   2    finishedArgs LinesRemoved  finishedArgs TotalLines  finishedArgs TotalTime ToString      TextLineRemover RemoveTextLines new List lt string gt     aaa    bbb     fileName  fileName     tmp

User · Answer

Why can t use this  First  create an array   string   lines   File ReadAllLines openFileDialog1 FileName     Then look up the line you need to delete and replace it with       lines x  Replace lines x          Done

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Remove a block of code from multiple files  To expand on  Markus Olsson s answer  I needed to remove a block of code from multiple files  I had problems with Swedish characters in a core project  so I needed to install System Text CodePagesEncodingProvider nuget package and use System Text Encoding GetEncoding 1252  instead of System Text Encoding UTF8       public static void Main string   args                try                       var dir     C  Test                 Get all html and htm files             var files   DirSearch dir               foreach  var file in files                                RmCode file                                   catch  Exception e                        Console WriteLine e Message               throw                        private static void RmCode string file                string tempFile   Path GetTempFileName             using  var sr   new StreamReader file  Encoding UTF8           using  var sw   new StreamWriter new FileStream tempFile  FileMode Open  FileAccess ReadWrite   Encoding UTF8                         string line               var startOfBadCode     lt div gt                var endOfBadCode     lt  div gt                var deleteLine   false               while   line   sr ReadLine       null                                if  line Contains startOfBadCode                                         deleteLine   true                                    if   deleteLine                                        sw WriteLine line                                      if  line Contains endOfBadCode                                         deleteLine   false                                                     File Delete file           File Move tempFile  file              private static List lt String gt  DirSearch string sDir                List lt String gt  files   new List lt String gt             try                       foreach  string f in Directory GetFiles sDir                                 files Add f                             foreach  string d in Directory GetDirectories sDir                                 files AddRange DirSearch d                                    catch  System Exception excpt                        Console WriteLine excpt Message                      return files Where s   gt  s EndsWith   htm      s EndsWith   html    ToList

User · Answer

Read the file  remove the line in memory and put the contents back to the file  overwriting   If the file is large you might want to read it line for line  and creating a temp file  later replacing the original one

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To remove an item from a text file  first move all the text to a list and remove whichever item you want  Then write the text stored in the list into a text file   List lt string gt  quotelist File ReadAllLines filename  ToList    string firstItem  quotelist 0   quotelist RemoveAt 0   File WriteAllLines filename  quotelist ToArray     return firstItem

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string fileIN     quot C  myTextFile txt quot   string fileOUT     quot C  myTextFile Out txt quot   if  File Exists fileIN         string   data   File ReadAllLines fileIN       foreach  string line in data          if   line Equals  quot my line to remove quot                File AppendAllText fileOUT  line       File Delete fileIN       File Move fileOUT  fileIN

User · Answer

I d very simply    Open the file for read write Read seek through it until the start of the line you want to delete Set the write pointer to the current read pointer Read through to the end of the line we re deleting and skip the newline delimiters  counting the number of characters as we go  we ll call it nline  Read byte-by-byte and write each byte to the file When finished truncate the file to  orig length - nline

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I wrote a method to delete lines from files   This program uses using System IO   See my code   void File DeleteLine int Line  string Path        StringBuilder sb   new StringBuilder        using  StreamReader sr   new StreamReader Path                 int Countup   0          while   sr EndOfStream                        Countup                if  Countup    Line                                using  StringWriter sw   new StringWriter sb                                         sw WriteLine sr ReadLine                                                 else                               sr ReadLine                                      using  StreamWriter sw   new StreamWriter Path                 sw Write sb ToString

[c#] How to delete a line from a text file in C#?

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