Open existing file, append a single line

Question

You can use File.AppendAllText for that:

File.AppendAllText(@"c:\path\file.txt", "text content" + Environment.NewLine);

using (StreamWriter w = File.AppendText("myFile.txt"))
{
  w.WriteLine("hello");
}

Choice one! But the first is very simple. The last maybe util for file manipulation:

//Method 1 (I like this)
File.AppendAllLines(
    "FileAppendAllLines.txt", 
    new string[] { "line1", "line2", "line3" });

//Method 2
File.AppendAllText(
    "FileAppendAllText.txt",
    "line1" + Environment.NewLine +
    "line2" + Environment.NewLine +
    "line3" + Environment.NewLine);

//Method 3
using (StreamWriter stream = File.AppendText("FileAppendText.txt"))
{
    stream.WriteLine("line1");
    stream.WriteLine("line2");
    stream.WriteLine("line3");
}

//Method 4
using (StreamWriter stream = new StreamWriter("StreamWriter.txt", true))
{
    stream.WriteLine("line1");
    stream.WriteLine("line2");
    stream.WriteLine("line3");
}

//Method 5
using (StreamWriter stream = new FileInfo("FileInfo.txt").AppendText())
{
    stream.WriteLine("line1");
    stream.WriteLine("line2");
    stream.WriteLine("line3");
}

Or you could use File.AppendAllLines(string, IEnumerable<string>)

File.AppendAllLines(@"C:\Path\file.txt", new[] { "my text content" });

Might want to check out the TextWriter class.

//Open File
TextWriter tw = new StreamWriter("file.txt");

//Write to file
tw.WriteLine("test info");

//Close File
tw.Close();

File.AppendText will do it:

using (StreamWriter w = File.AppendText("textFile.txt")) 
{
    w.WriteLine ("-------HURRAY----------");
    w.Flush();
}

The technically best way is probably this here:

private static async Task AppendLineToFileAsync([NotNull] string path, string line)
{
    if (string.IsNullOrWhiteSpace(path)) 
        throw new ArgumentOutOfRangeException(nameof(path), path, "Was null or whitepsace.");

    if (!File.Exists(path)) 
        throw new FileNotFoundException("File not found.", nameof(path));

    using (var file = File.Open(path, FileMode.Append, FileAccess.Write))
    using (var writer = new StreamWriter(file))
    {
        await writer.WriteLineAsync(line);
        await writer.FlushAsync();
    }
}