Hmmm...TextReader suddenly stopped working...

[carstenht]

I had this piece of code working just fine:

private void button1_Click(object sender, EventArgs e)
        {
            
            StreamReader sr = new StreamReader("file.txt");
            while (sr.Peek() >=0)

            {

                comboBox1.Items.Add(sr.ReadLine());
                sr.Close();
            }
 
            
        }

But then suddenly, it starts giving me this error:

Cannot read from a closed TextReader.

Why this all of a sudden???

Complete code:

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


namespace TrialAndError
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            
            StreamReader sr = new StreamReader("file.txt");
            while (sr.Peek() >=0)

            {

                comboBox1.Items.Add(sr.ReadLine());
                sr.Close();
            }
 
            
        }

        private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
        {
            MessageBox.Show("Selected text: " + comboBox1.Text);
        }

       
        private void button2_Click(object sender, EventArgs e)
        {
            StreamWriter sw = File.AppendText("file.txt");
            sw.Write(textBox1.Text);
            sw.Close();
            
        }
    }
}