Develop Reference

Reading and changing a file

I'm reading a file using C# and class TextReader
TextReader reader = new StreamReader(stream);
string line;
while ((line = reader.ReadLine()) != null)
{
if (someCondition)
{
// I want to change "line" and save it into the file I'm reading from
}
}
In a code there is a question: how do I save a changed line to a file I'm reading from and continue reading?

Fast and dirty solution would be:
TextReader reader = new StreamReader(stream);
string line;
StringBuilder sb = new StringBuilder();
while ((line = reader.ReadLine()) != null)
{
if (someCondition)
{
//Change variable line as you wish.
}
sb.Append(line);
}
using (StreamWriter sw = new StreamWriter("filePath"))
{
sw.Write(sb.ToString());
}
or
TextReader reader = new StreamReader(stream);
string line;
String newLines[];
int index = 0;
while ((line = reader.ReadLine()) != null)
{
if (someCondition)
{
//Change variable line as you wish.
}
newLines[index] = line;
index++;
}
using (StreamWriter sw = new StreamWriter("filePath"))
{
foreach (string l in newLines)
{
sw.WriteLine(l);
}
}
If memory is too important you can try this too:
TextReader reader = new StreamReader(stream);
string line;
while ((line = reader.ReadLine()) != null)
{
if (someCondition)
{
//Change variable line as you wish.
}
using (StreamWriter sw = new StreamWriter("filePath"))
{
sw.WriteLine(line);
}
}

The easiest thing is to write a new file, then when finished, replace the old file with the new file. This way you only do writes in one file.
If you try to read/write in the same file, you will run into problems when the content you want to insert is not the exact same size as the content it is replacing.
There is nothing magic about text files. They are just a stream of bytes representing characters in a text encoding. There are no line concept in the file, just separators in the form of newline characters.

If the file is not too large, you should simply rewrite the whole file:
var lines = File.ReadAllLines(path)
.Where(l => someCondition);
File.WriteAllLines(path, lines);

A very simple solution
void Main()
{
var lines = File.ReadAllLines("D:\\temp\\file.txt");
for(int x = 0; x < lines.Length; x++)
{
// Of course this is an example of the condtion
// you should implement your checks
if(lines[x].Contains("CONDITION"))
{
lines[x] = lines[x].Replace("CONDITION", "CONDITION2");
}
}
File.WriteAllLines("D:\\temp\\file.txt", lines);
}
The drawback is the memory usage caused by the in memory lines, but, if we stay around 50MB, it should be handled effortlessly by modern PC.

Writing a specific line from one text file to other text file using c#

I Am using sharp develop. I am making a Win App using C# . I want my program check a text file named test in drive c: and find the line which contains "=" and then write this line to other newly created text file in drive c: .

Try this one-liner:
File.WriteAllLines(destinationFileName,
File.ReadAllLines(sourceFileName)
.Where(x => x.Contains("=")));

Here's another simple way using File.ReadLines, Linq's Where and File.AppendAllLines
var path1 = #"C:\test.txt";
var path2 = #"C:\test_out.txt";
var equalLines = File.ReadLines(path1)
.Where(l => l.Contains("="));
File.AppendAllLines(path2, equalLines.Take(1));

using(StreamWriter sw = new StreamWriter(#"C:\destinationFile.txt"))
{
StreamReader sr = new StreamReader(#"C:\sourceFile.txt");
string line = String.Empty;
while ((line = sr.ReadLine()) != null)
{
if (line.Contains("=")) { sw.WriteLine(line)); }
}
sr.Close();
}

Have you tried something?
Here are two ways to read a file:
Use static methods available in File class. ReadAllLines to be specific. This is good enough if you are dealing with small files. Next, once you have the array, just find the item with "=" using LINQ or by any other iteration method. Once you got the line, again use File class to create and write data to the file.
If you are dealing with large files, use Stream. Rest remains fairly same.

if (File.Exists(txtBaseAddress.Text))
{
StreamReader sr = new StreamReader(txtBaseAddress.Text);
string line;
string fileText = "";
while ((line = sr.ReadLine()) != null)
{
if (line.Contains("="))
{
fileText += line;
}
}
sr.Close();
if (fileText != "")
{
try
{
StreamWriter sw = new StreamWriter(txtDestAddress.Text);
sw.Write(fileText);
sw.Close();
}
catch (Exception ex)
{
MessageBox.Show(ex.Message);
}
}
}

a bit edited Furqan's answer
using (StreamReader sr = new StreamReader(#"C:\Users\Username\Documents\a.txt"))
using (StreamWriter sw = new StreamWriter(#"C:\Users\Username\Documents\b.txt"))
{
int counter = 0;
string line = String.Empty;
while ((line = sr.ReadLine()) != null)
{
if (line.Contains("="))
{
sw.WriteLine(line);
if (++counter == 4)
{
sw.WriteLine();
counter = 0;
}
}
}
}

How to change some text in a file by StreamWriter

I'm trying to change some text in a .CSV file.
StreamReader sReader = new StreamReader(path1);
while (sReader.Peek() != -1)
{
rowValue = sReader.ReadLine();
if (rowValue == "25")
{
sWriter = new StreamWriter(path1);
rowValue = "27";
sWriter.WriteLine(rowValue);
}
}
Nothing happened. What is the correct way, please?

You can easily accomplish this task with System.IO.File.ReadAllLines and System.IO.File.WriteAllLines
string[] lines = File.ReadAllLines(path1);
for(int i = 0; i < lines.Length; i++)
{
if(lines[i] == "25") lines[i] = "27";
}
File.WriteAllLines(path1, lines);

I don't believe you can do what you're trying to do ... updating a single line. If it's working at all, you're appending the value 27 to the end of your file.
Check out the ReadAllLines and WriteAllLines methods. You need to read the entire file in, change the line(s) you want to change, then write it back.

Well, if the file to read is not a very big one, you could try to read everything in memory and write back
string[] lines = File.ReadAllLines(path1);
using(StreamWrite sw = new StreamWriter(path1))
{
foreach(string line in lines)
{
string lineOut = line;
if (line == "25")
lineOut = "27";
sw.WriteLine(lineOut);
}
sw.Flush();
}

Delete last 3 lines within while ((line = r.ReadLine()) != null) but not open a new text file to delete the lines?

This is the code I've seen so far to delete last 3 lines in a text file, but it's required to determine string[] lines = File.ReadAllLines(); which is nt necessary for me to do so.
string[] lines = File.ReadAllLines(#"C:\\Users.txt");
StringBuilder sb = new StringBuilder();
int count = lines.Length - 3; // except last 3 lines
for (int s = 0; s < count; s++)
{
sb.AppendLine(lines[s]);
}
The code works well, but I don't wanna re-read the file as I've mentioned the streamreader above :
using (StreamReader r = new StreamReader(#"C:\\Users.txt"))
Im new to C#, as far as I know, after using streamreader, and if I wanna modify the lines, I have to use this :
while ((line = r.ReadLine()) != null)
{
#sample codes inside the bracket
line = line.Replace("|", "");
line = line.Replace("MY30", "");
line = line.Replace("E", "");
}
So, is there any way to delete the last 3 lines in the file within the "while ((line = r.ReadLine()) != null)" ??
I have to delete lines, replace lines and a few more modications in one shot, so I can't keep opening/reading the same text file again and again to modify the lines. I hope the way I ask is understable for you guys >.<
Plz help me, I know the question sounds simple but I've searched so many ways to solve it but failed =(
So far, my code is :
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.IO;
namespace ConsoleApplication11
{
public class Read
{
static void Main(string[] args)
{
string tempFile = Path.GetTempFileName();
using (StreamReader r = new StreamReader(#"C:\\Users\SAP Report.txt"))
{
using (StreamWriter sw = new StreamWrite (#"C:\\Users\output2.txt"))
{
string line;
while ((line = r.ReadLine()) != null)
{
line = line.Replace("|", "");
line = line.Replace("MY30", "");
line = line.Replace("E", "");
line = System.Text.RegularExpressions.Regex.Replace(line, #"\s{2,}", " ");
sw.WriteLine(line);
}
}
}
}
}
}
Now my next task is to delete the last 3 lines in the file after these codes, and I need help on this one.
Thank you.

With File.ReadAllLines you have already read the file, so you can process each one line in the string[] (replaces and regexes), and then write them in the output. You don't have to reread them and put them in the StringBuilder.

You could keep a "rolling window" of the previous three lines:
string[] previousLines = new string[3];
int index = 0;
string line;
while ((line = reader.ReadLine()) != null)
{
if (previousLines[index] != null)
{
sw.WriteLine(previousLines[index]);
}
line = line.Replace("|", "")
.Replace("MY30", "")
.Replace("E", "");
line = Regex.Replace(line, #"\s{2,}", " ");
previousLines[index] = line;
index = (index + 1) % previousLines.Length;
}

Instead of appending the lines directly to the string builder, you could keep a list of the lines and join them later on. That way you can easily leave out the last three lines.
To reduce the amount lines you have to keep in the list, you could regularly append one line of the list and remove it from it. So you would keep a buffer of 3 lines in an array and would pop & append a line whenever the buffer contains 4 lines.

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:\target.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?

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);

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 <= 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 <= 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 <= 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.

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.

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 solution works fine. But I need to change single-line text when the same text is in multiple places. For this, need to define a trackText to start finding after that text and finally change oldText with newText.
private int FindLineNumber(string fileName, string trackText, string oldText, string newText)
{
int lineNumber = 0;
string[] textLine = System.IO.File.ReadAllLines(fileName);
for (int i = 0; i< textLine.Length;i++)
{
if (textLine[i].Contains(trackText)) //start finding matching text after.
traced = true;
if (traced)
if (textLine[i].Contains(oldText)) // Match text
{
textLine[i] = newText; // replace text with new one.
traced = false;
System.IO.File.WriteAllLines(fileName, textLine);
lineNumber = i;
break; //go out from loop
}
}
return lineNumber
}