Develop Reference

Delete specific row from File

I have this little project in C# where I am manipulating with files. Now my task is that I have to delete specific rows from files.
For example my file looks like this:
1-this is the first line
2-this is the second line
3-this is the third line
4-this is the fourth line
Now how can I keep only the first two rows and delete only the last two rows?
Note- this is how I read the file from my local machine:
string[] lines = File.ReadAllLines(#"C:\Users\admin\Desktop\COMMANDS.dat");
I have tried something like this but I think it's not so "efficient"
string text = File.ReadAllText(#"C:\Users\admin\Desktop\COMMANDS.dat");
text = text.Replace(lines[2], "");
text = text.Replace(lines[3], "");
File.WriteAllText(#"C:\Users\admin\Desktop\COMMANDS.dat", text);
So this actually does the job, it replaces the lines by string with an empty character but when I take a look at the file, I don't want to have 4 lines there, even though 2 of them are real strings and the other two are just empty lines... Can I manage to do this in another way?

Try replacing the newline character with an empty string:
string text = File.ReadAllText(#"C:\Users\admin\Desktop\COMMANDS.dat");
text = text.Replace(lines[2], "").Remove(Environment.NewLine, "");
text = text.Replace(lines[3], "").Remove(Environment.NewLine , "");
File.WriteAllText(#"C:\Users\admin\Desktop\COMMANDS.dat", text);
If my answer is useful, please mark it as accepted, and upvote it.

async Task Example()
{
var inputLines = await File.ReadAllLinesAsync("path/to/file.txt");
var outputLines = inputLines.Where((l, i) => i < 2);
await File.WriteAllLinesAsync("target/file.txt", outputLines);
}
What it does
Read data but not as one string but as a collection of lines
Create a new collection containing only the lines you want in your output
Write the filtered lines
Notes:
This example is not optimized for memory usage (because we read all lines and for larger files, e.g. multiple GB, this will fail). See existing answers for memory optimized version) - but: It's totally fine to do it this way if you know you have just a few k lines. (and it's faster)
Try not to "modify" strings. This will always create a copy and needs a lot of memory.
In this "Linq style" (functional) approach, we should treat data as immutable. That means: we have one variable that represents the input file and one variable that represents the result. We use declarative Linq to describe how the output should look like. "output is input where the filter index < 2 matches" instead of "if xy remove line" in an imperative style.

Removing text above real content of CSV file

I have a CSV whose author, annoyingly enough, has decided to 'introduce' the file before the contents themselves. So in all, I have a CSV that looks like:
This file was created by XXXXYY and represents the crossover between YY and QQQ.
Additional information can be found through the website GG, blah blah blah...
Jacob, Hybrid
Dan, Pure
Lianne, Hybrid
Jack, Hatchback
So the problem here is that I want to get rid of the first few lines before the 'real content' of the CSV file begins. I'm looking for robustness here, so using Streamreader and removing all content before the 4th line for example, is not ideal (plus the length of the text can vary).
Is there a way in which one can read only what matters and write a new CSV into a directory path?
Regards,
genesis
(edit - I'm looking for C sharp code)

The solution depends on the files you have to parse. You need to look for a reliable pattern that distinguishes data from comment.
In your example, there are some possibilities that might be the same in other files:
there are 4 lines of text. But you say this isn't consistent across files
The text lives may not contain the same number of commas as the data table. But that is unlikely to be reliable for all files.
there is a blank/whitespace only line between the text and the data.
the data appears to be in the form word-comma-word. If this is true it should be easy to identify non data lines (any line which doesn't contain exactly one comma, or has multiple words etc)
You may be able to use a combination of these heuristics to more reliably detect the data.

You could scan by line (looking for the \r\n) and ignore lines that don't have a comma count that matches you csv.
You should be able to read the file into a string pretty easily unless it is really massive.
e.g.
var csv = "some test\r\nsome more text\r\na,b,c\r\nd,e,f\r\n";
var lines = csv.Split('\r\n');
var csvLines = line.Where(l => l.Count(',') == 2);
// now csvLines contains only the lines you are after

List<string> info = new List<string>();
int counter = 0;
// Open the file to read from.
info = System.IO.File.ReadAllLines(path).ToList();
// Find the lines up until (& including) the empty one
foreach (string s in info)
{
counter++;
if(string.IsNullOrEmpty(s))
break; //exit from the loop
}
// Remove the lines including the blank one.
info.RemoveRange(0,counter);
Something like this should work, you should probably put some tests in to make sure counter is not > length and other tests to handle errors.
You could adapt this code so that it just finds the empty line number using linq or something, but I don't like the overhead of linq (Yeah ironic considering I'm using c#).
Regards,
Slipoch

Read specified line from external text file

I'm using C# and writing a Windows Form application for Windows 7 or Windows 8.
I need to know how to get my app to read a specific line which should be assigned to a specific text box or combo box.
This is the code I have so far.
using (StreamReader QNARead = new StreamReader(TestPath))
if (QNARead.Peek() >= 0)
{
string Line1 = QNARead.ReadLine();
QuestionText1.Text = Line1;
string Line2 = QNARead.ReadLine();
QuestionText2.Text = Line2;
string Line3 = QNARead.ReadLine();
AlternativesList1.Items.Add(Line3);
}
But with this method I'll have to read a lot of lines, because the line could be the 500th in the text file. (I do know the exact line number, eg. 54).

I used once this to read a specific line in a text file:
string filename = "test.txt";
if (File.Exists(filename))
{
string[] lines = File.ReadAllLines(filename);
Console.WriteLine(lines[5]);
}
where 5 can be replaced with any line number (zero based).

Since a file is just a list of bytes, you need to know how far into it to read, to get the line you want.
Unless the lines are fixed (or predictable) lengths, or you have some kind of separate index, you need to read every line up to the one you want. From a code point of view you can make this fairly tidy, for example in Framework 4.0:
string line54 = File.ReadLines(FileName).Skip(53).Take(1).First();
Note that at this point, you're not taking advantage of reading the earlier lines being skipped over, so a more efficient way would be to keep reading in a loop until you reach the last line needed.

Adding a Line to the Middle of a File with .NET

Hello I am working on something, and I need to be able to be able to add text into a .txt file. Although I have this completed I have a small problem. I need to write the string in the middle of the file more or less. Example:
Hello my name is Brandon,
I hope someone can help, //I want the string under this line.
Thank you.
Hopefully someone can help with a solution.
Edit Alright thanks guys, I'll try to figure it out, probably going to just rewrite the whole file. Ok well the program I am making is related to the hosts file, and not everyone has the same hosts file, so I was wondering if there is a way to read their hosts file, and copy all of it, while adding the string to it?

With regular files there's no way around it - you must read the text that follows the line you wish to append after, overwrite the file, and then append the original trailing text.
Think of files on disk as arrays - if you want to insert some items into the middle of an array, you need to shift all of the following items down to make room. The difference is that .NET offers convenience methods for arrays and Lists that make this easy to do. The file I/O APIs offer no such convenience methods, as far as I'm aware.
When you know in advance you need to insert in the middle of a file, it is often easier to simply write a new file with the altered content, and then perform a rename. If the file is small enough to read into memory, you can do this quite easily with some LINQ:
var allLines = File.ReadAllLines( filename ).ToList();
allLines.Insert( insertPos, "This is a new line..." );
File.WriteAllLines( filename, allLines.ToArray() );

This is the best method to insert a text in middle of the textfile.
string[] full_file = File.ReadAllLines("test.txt");
List<string> l = new List<string>();
l.AddRange(full_file);
l.Insert(20, "Inserted String");
File.WriteAllLines("test.txt", l.ToArray());

one of the trick is file transaction. first you read the file up to the line you want to add text but while reading keep saving the read lines in a separate file for example tmp.txt and then add your desired text to the tmp.txt (at the end of the file) after that continue the reading from the source file till the end. then replace the tmp.txt with the source file. at the end you got file with added text in the middle :)

Check out File.ReadAllLines(). Probably the easiest way.
string[] full_file = File.ReadAllLines("test.txt");
List<string> l = new List<string>();
l.AddRange(full_file);
l.Insert(20, "Inserted String");
File.WriteAllLines("test.txt", l.ToArray());

If you know the line index use readLine until you reach that line and write under it.
If you know exactly he text of that line do the same but compare the text returned from readLine with the text that you are searching for and then write under that line.
Or you can search for the index of a specified string and writ after it using th escape sequence \n.

As others mentioned, there is no way around rewriting the file after the point of the newly inserted text if you must stick with a simple text file. Depending on your requirements, though, it might be possible to speed up the finding of location to start writing. If you knew that you needed to add data after line N, then you could maintain a separate "index" of the offsets of line numbers. That would allow you to seek directly to the necessary location to start reading/writing.

How do you specify where to start reading in a file when using StreamReader?

How do you specify where to start reading in a file when using StreamReader?
I have created a streamreader object, along with a file stream object. After both objects are created, how would I go upon controlling where I want the StreamReader to start reading from a file?
Let's say the file's contents are as follows,
// song list.
// junk info.
1. Song Name
2. Song Name
3. Song Name
4. Song Name
5. Song Name
6. Song Name
How would I control the streamreader to read from let's say #2? Also, how could I also control where to make it stop reading by a similar delimiter like at #5?
Edit: By delimiter I mean, a way to make StreamReader start reading from ('2.')

Are you trying to deserialize a file into some in-memory object? If so, you may want to simply parse the entire file in using ReadLine or something similar, store each line, and then access it via a data structure such as a KeyValuePair<int, string>.
Update: Ok... With the new info, I think you have two options. If you're looking at reading until you find a match, you can Peek(), check to see if the character is the one you're looking for, and then Read(). Alternatively, if you're looking for a set position, you can simply Read() that many characters and throw away the return value.
If you're looking for complex delimiter, you can read the entire line or even the entire file into memory and use Regular Expressions.
Hope that helps...

If the file contains new line delimiters you can use ReadLine to read a line at a time.
So to start reading at line #2, you would read the first line and discard and then read lines until line #5.

Well if the content is just plain text like that, you should use the StreamReader's ReadLine method.
http://msdn.microsoft.com/en-us/library/system.io.streamreader.readline.aspx
-Oisin