c# - StreamWriter replace line with a new text

Question

Is it possible to replace the text in a text file with a new text without erasing the other data, here is my sample code, but its not working, I know there's a problem with it but I can't figure out, thanks,

private void button1_Click_1(object sender, EventArgs e)
{
    StreamReader sr = new StreamReader("test10101.txt");
    List<string> lines = new List<string>();
    while (!sr.EndOfStream)
        lines.Add(sr.ReadLine());
    output = Convert.ToInt32(textBox1.Text);
    newbal = Convert.ToInt32(lines[0]) - output;
    MessageBox.Show("Please get your cash....

Your new balance is: $" + newbal);
    sr.Close();
    {
        string linetoreplace = lines[0];
        int newlinevalue = newbal;
        string contents = sr.ReadToEnd();

        StreamWriter sw = new StreamWriter("test10101.txt" + ".tmp");
        //contents = Regex.Replace(contents, linetoreplace, newlinevalue.ToString());
        contents = contents.Replace(linetoreplace, newlinevalue.ToString());
        sw.WriteLine(contents);
        sw.Close();

    }

I'm wondering if I use the Regex or directly replace the line,

Answer 1

You could do it a lot more easily:

        string[] lines = System.IO.File.ReadAllLines("test");
        lines[0] = /* replace with whatever you need */
        System.IO.File.WriteAllLines("test", lines);

hope this helps

also I'd suggest using int.TryParse if you don't want an exception to be raised in your portion of code in case the first line of the file or the textbox values aren't numeric

if you really want to use the streamwriter you could go with this, also a simpler way:

line[0] = newbal.ToString();
foreach(string s in lines)
    sw.WriteLine(s);