MainWindow.xaml.cs

private void Button_Click_1(object sender, RoutedEventArgs e)
        {
            int countThreads = 10;
            int sleepTime = 1000;
            for (int i = 0; i < countThreads; i++)
            {
                Thread thread = new Thread(setDataInTextFileLine);
                thread.Name = i.ToString();
                thread.Start();
                Thread.Sleep(sleepTime / countThreads);
            }

        }
        private void setDataInTextFileLine()
        {
            int sleepTime = 1000;
            for (int i = 0; i < 10000; i++)
            {
                using (StreamWriter streamWriter = new StreamWriter("text1.txt", true))
                {
                    streamWriter.WriteLine(Thread.CurrentThread.Name + ":" + i.ToString());
                }
                Thread.Sleep(sleepTime);
            }
        }