string path = System.IO.Directory.GetCurrentDirectory();//相对路径
 FileInfo fi = new FileInfo(path + @"\Data\unity.txt");
  ////如果该文件存在需要删除重新写入
  if (fi.Exists)
 {
   fi.Delete(); 
  }

  StreamWriter sw = new StreamWriter(path + @"\Data\StreamingAssets\unity.txt");
   sw.WriteLine(需要写入的内容方法);
   sw.Close();
   MessageBox.Show("写入成功！ ^-^ ", "TaoHu", MessageBoxButtons.OK, MessageBoxIcon.None);