private void Form1_Load(object sender, EventArgs e)
{
    // Check if directory exists
    if (Directory.Exists(@"C:UsersrsDesktopTest)"))
    {
        // Do nothing
    }

    else
    {
        Directory.CreateDirectory(@"C:UsersrsDesktopTest");
    }

    if (File.Exists(@"C:UsersrsDesktopTesttest.txt"))
    {
        // Do nothing
    }

    else
    {
        File.Create(@"C:UsersrsDesktopTesttest.txt");
    }

    StreamReader sr = new StreamReader(@"C:UsersrsDesktopTesttest.txt");
    while (sr.Peek() >= 0)
    {
        combox_Name2.Items.Add(sr.ReadLine());
    }
    sr.Close();
}

private void btn_Save_Click(object sender, EventArgs e)
{

    StreamWriter writer = new StreamWriter(@"C:UsersrsDesktopTesttest.txt", true);

    try
    {
        Process.Start(combox_Name2.Text, txt_Name.Text);

        if (combox_Name2.Items.Contains(txt_Name.Text))
        {
            // Do nothing
        }

        else
        {
            writer.WriteLine(combox_Name2.Text);
            writer.Flush();
            writer.Dispose();
            combox_Name2.Items.Add(txt_Name.Text);
        }
    }

    catch
    {
        MessageBox.Show("The file '" + txt_Name.Text + "' could not be located", "File could not be located");
    }
}

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;
using System.Diagnostics;

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

    private void Form1_Load(object sender, EventArgs e)
    {
        // Check if directory exists
        if (Directory.Exists(@"C:UsersrsDesktopTest)"))
        {
            // Do nothing
        }

        else
        {
            Directory.CreateDirectory(@"C:UsersrsDesktopTest");
        }

        if (File.Exists(@"C:UsersrsDesktopTesttest.txt"))
        {
            // Do nothing
        }

        else
        {
            File.Create(@"C:UsersrsDesktopTesttest.txt");
        }

        using (StreamReader writer = new StreamReader(@"C:UsersrsDesktopTesttest.txt"))
        {
            while (writer.Peek() >= 0)
            {
                combox_Name2.Items.Add(writer.ReadLine());
            }
            writer.Close();
        }

    }

    private void btn_Save_Click(object sender, EventArgs e)
    {
        using (StreamWriter writer = new StreamWriter(@"C:UsersrsDesktopTesttest.txt", true))
        {
            try
            {
                if (combox_Name2.Items.Contains(txt_Name.Text))
                {
                    MessageBox.Show("The task '" + txt_Name.Text + "' already exist in list", "Task already exists");
                }

                else
                {
                    combox_Name2.Items.Add(txt_Name.Text);
                    writer.WriteLine(txt_Name.Text);
                    writer.Flush();
                    writer.Dispose();
                }
            }

            catch
            {
                MessageBox.Show("The file '" + txt_Name.Text + "' could not be located", "File could not be located");
            }

        }
    }
}
}

using(IDisposable object goes here)
{
    // Do something.
}

using(StreamWriter writer = new StreamWriter(@"C:UsersrsDesktopTesttest.txt", true))
{
    // do your thing here.

}

StreamWriter writer = new StreamWriter(@"C:UsersrsDesktopTesttest.txt", true);
try
{
    // do your thing here.

}
finally
{
    writer.Dispose();
}

combox_Name2.Items.Clear();
if (File.Exists(@"C:UsersrsDesktopTesttest.txt"))
{
    using(StreamReader sr = new StreamReader(@"C:UsersrsDesktopTesttest.txt"))
    {
        while (sr.Peek() >= 0)
        {
            combox_Name2.Items.Add(sr.ReadLine());
        }
    }
}