Question

I’m trying to move line in a text file up by one then rewrite it back to the original file, but getting error for some reason, can’t seem to figure it out.

using (StreamReader reader = new StreamReader("file.txt"))
{
    string line;
    int Counter = 0;

    while ((line = reader.ReadLine()) != null)
    {
        string filepath = "file.txt";
        int i = 5;
        string[] lines = File.ReadAllLines(filepath);

        if (lines.Length >= i)
        {
            string tmp = lines[i];
            lines[i] = lines[i-1];
            lines[i-1] = tmp;
            File.WriteAllLines(filepath, lines);
        }   
    }
    Counter++;
}

Answer 1

I assume that you actually want to swap each line(?) in the file, because of this code snippet:

string tmp = lines[i];
lines[i] = lines[i-1];
lines[i-1] = tmp;

So here’s an approach that should work:

String[] lines = System.IO.File.ReadAllLines(path);
List<String> result = new List<String>();
for (int l = 0; l < lines.Length; l++)
{
    String thisLine = lines[l];
    String nextLine = lines.Length > l+1 ? lines[l + 1] : null;
    if (nextLine == null)
    {
        result.Add(thisLine);
    }
    else
    {
        result.Add(nextLine);
        result.Add(thisLine);
        l++;
    }
}
System.IO.File.WriteAllLines(path, result);

Edit: Here’s the slightly modified version that swaps only one line with it’s previous line, since you’ve commented that this is your requirement:

String[] lines = System.IO.File.ReadAllLines(path);
List<String> result = new List<String>();
int swapIndex = 5;
if (swapIndex < lines.Length && swapIndex > 0)
{
    for (int l = 0; l < lines.Length; l++)
    {
        String thisLine = lines[l];
        if (swapIndex == l + 1) // next line must be swapped with this
        {
            String nextLine = lines[l + 1];
            result.Add(nextLine);
            result.Add(thisLine);
            l++;
        }
        else
        {
            result.Add(thisLine);
        }
    }
}
System.IO.File.WriteAllLines(path, result);