Efficient way to delete a line from a text file

Question

I need to delete a certain line from a text file. What is the most efficient way of doing this? File can be potentially large(over million records).

UPDATE: below is the code I'm currently using, but I'm not sure if it is good.

internal void DeleteMarkedEntries() {
    string tempPath=Path.GetTempFileName();
    using (var reader = new StreamReader(logPath)) {
        using (var writer = new StreamWriter(File.OpenWrite(tempPath))) {
            int counter = 0;
            while (!reader.EndOfStream) {
                if (!_deletedLines.Contains(counter)) {
                    writer.WriteLine(reader.ReadLine());
                }
                ++counter;
            }
        }
    }
    if (File.Exists(tempPath)) {
        File.Delete(logPath);
        File.Move(tempPath, logPath);
    }
}

Answer 1

The most straight forward way of doing this is probably the best, write the entire file out to a new file, writing all lines except the one(s) you don't want.

Alternatively, open the file for random access.

Read to the point where you want to "delete" the line. Skip past the line to delete, and read that number of bytes (including CR + LF - if necessary), write that number of bytes over the deleted line, advance both locations by that count of bytes and repeat until end of file.

Hope this helps.

EDIT - Now that I can see your code

if (!_deletedLines.Contains(counter)) 
{                            
    writer.WriteLine(reader.ReadLine());                        
}

Will not work, if its the line you don't want, you still want to read it, just not write it. The above code will neither read it or write it. The new file will be exactly the same as the old.

You want something like

string line = reader.ReadLine();
if (!_deletedLines.Contains(counter)) 
{                            
    writer.WriteLine(line);                        
}

Answer 2

Text files are sequential, so when deleting a line, you'll have to move all the following lines up. You can use file mapping (a win32 api that you can call through PInvoke) to make this operation a bit less painfull, but you surelly should considere using a non sequential structure for you file so that you can mark a line as deleted without realy removing it from the file... Especially if it should happen frenquently.

If I've remember File Mapping Api should be added to .Net 4.

Answer 3

     try{
     Scanner reader = new Scanner(new File("D:/seenu.txt")); 
     System.out.println("Enter serial number:");
     String sl1=bufRead.readLine();
     System.out.print("Please Enter The ServerName:");
     String name=bufRead.readLine();
     System.out.println("Please Enter The IPAddress");
     String ipa=bufRead.readLine();

    System.out.println("Line Deleted.");
     PrintWriter writer = new PrintWriter(new FileWriter(new File("D:/user.txt")),true); 
     //for(int w=0; w<n; w++)
       writer.write(reader.nextLine()); 
     reader.nextLine(); 
     while(reader.hasNextLine())
       writer.write(reader.nextLine());
     } catch(Exception e){
       System.err.println("Enjoy the stack trace!");
       e.printStackTrace();
     }