This will enable appending to file using StreamWriter
using (StreamWriter stream = new StreamWriter("YourFilePath", true)) {...}
This is default mode, not append to file and create a new file.
using (StreamWriter stream = new StreamWriter("YourFilePath", false)){...}
or
using (StreamWriter stream = new StreamWriter("YourFilePath")){...}
Anyhow if you want to check if the file exists and then do other things,you can use
using (StreamWriter sw = (File.Exists(path)) ? File.AppendText(path) : File.CreateText(path))
{...}