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using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.IO;

namespace Mail_Footer_Frontend
{
    public partial class Form4 : Form
    {
        public Form4()
        {
            InitializeComponent();

            StreamReader tempread = new StreamReader("templ-aalen.txt", System.Text.Encoding.UTF7, true);
            {
                string line;
                //Überprüfung auf Leerzeilen
                while ((line = tempread.ReadLine()) != null)
                {
                    //Ausgabe der Liste in der ListBox
                    fBox.Items.Add(line);
                }
            }
            tempread.Close();

            StreamReader mread = new StreamReader("mitarbeiter.csv", System.Text.Encoding.UTF7, true);
            {
                string line;
                while ((line = mread.ReadLine()) != null)
                {
                    miBox.Items.Add(line);
                }
            }
            mread.Close();

            StreamReader eread = new StreamReader("events.csv", System.Text.Encoding.UTF7, true);
            {
                string line;
                while ((line = eread.ReadLine()) != null)
                {
                    evBox.Items.Add(line);
                }
            }
            eread.Close();


        }

        private void btn1_Click(object sender, EventArgs e)
        {
            
            
        }

        private void btn2_Click(object sender, EventArgs e)
        {
            string line2 = Convert.ToString(miBox.Text);
           
            StreamWriter tempOne = new StreamWriter("tempone.txt", true);
            tempOne.Write(line2);
            tempOne.WriteLine();
            tempOne.Close();
        }
    }
}					

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using (var sw = new StreamWriter(@"d:\foo.txt", true))
                sw.WriteLine(textBox1.Text);