How to delete last line in a text file?

If you want to delete last N lines from a file without loading all into memory:

int numLastLinesToIgnore = 10;
string line = null;
Queue<string> deferredLines = new Queue<string>();
using (TextReader inputReader = new StreamReader(inputStream))
using (TextWriter outputReader = new StreamWriter(outputStream))
{
    while ((line = inputReader.ReadLine()) != null)
    {
        if (deferredLines.Count() == numLastLinesToIgnore)
        {
            outputReader.WriteLine(deferredLines.Dequeue());
        }

        deferredLines.Enqueue(line);
    }
    // At this point, lines still in Queue get lost and won't be written
}

What happens is that you buffer each new line in a Queue with dimension numLastLinesToIgnore and pop from it a line to write only when Queue is full. You actually read-ahead the file and you are able to stop numLastLinesToIgnore lines before the end of file is reached, without knowing the total number of lines in advance.

Note that if text has less than numLastLinesToIgnore, result is empty.

I came up with it as a mirror solution to this: Delete specific line from a text file?

Use this method to remove the last line of the file:

public static void DeleteLastLine(string filepath)
{
    List<string> lines = File.ReadAllLines(filepath).ToList();

    File.WriteAllLines(filepath, lines.GetRange(0, lines.Count - 1).ToArray());

}

Edit: Realized the lines variable didn't exist previously, so I updated the code.

How about something like :

var lines = System.IO.File.ReadAllLines("...");
System.IO.File.WriteAllLines("...", lines.Take(lines.Length - 1).ToArray());

Explanation:

Technically, you don't delete a line from a file. You read the contents of a file and write them back excluding the content you want deleted.

What this code does is read all the lines into an array, and write these lines back to the file excluding only the last line. (Take() method (Part of LINQ) takes number of lines specified which in our case, is length - 1). Here, var lines can be read as String[] lines.