How to Append Data to a Text File in a Windows Forms Application?

Question

In a Windows Forms application, data is read from a text file and displayed in a list box. The goal is to add a new item from a textbox to the same text file when a button is pressed, but the item is not being appended to the file as expected.

The button click event code is as follows:

private void btnAdd_Click(object sender, EventArgs e)
{
    string newItem = txtNewItem.Text.Trim(); // Get the new item from the TextBox
    if (!string.IsNullOrWhiteSpace(newItem))
    {
        listBoxFirmalar.Items.Add(newItem); // Add to the ListBox
        // Use StreamWriter to append the new item to the file
        try
        {
            using (StreamWriter writer = new StreamWriter(filePath, true, Encoding.UTF8)) // 'true' for appending
            {
                writer.WriteLine(newItem); // Write the new item to the file
            }
            MessageBox.Show("Item added to file successfully."); // Confirm success
        }
        catch (Exception ex)
        {
            MessageBox.Show($"Error writing to file: {ex.Message}", "Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
        }
        txtNewItem.Clear(); // Clear the input field
    }
    else
    {
        MessageBox.Show("Please enter a valid item.", "Invalid Input", MessageBoxButtons.OK, MessageBoxIcon.Warning);
    }
}

Answer 1

solved:

private readonly string filePath = Path.Combine(Application.StartupPath +"Data", "test.txt");

Answer 2

Consider the following extension methods to load/save to a unbound ListBox.

namespace WinFormsListBoxApp.Classes;

using System.Windows.Forms;

/// <summary>
/// Provides extension methods for the <see cref="ListBox.ObjectCollection"/> class,
/// enabling additional functionality such as saving to and loading from files.
/// </summary>
/// <remarks>
/// No exception handling provided as each developer may have different requirements like, log or other actions.
/// </remarks>
public static class ListBoxExtensions
{
    /// <summary>
    /// Saves the contents of the <see cref="ListBox.ObjectCollection"/> to a specified file.
    /// </summary>
    /// <param name="sender">The <see cref="ListBox.ObjectCollection"/> containing the items to save.</param>
    /// <param name="fileName">The path of the file where the contents will be saved.</param>
    /// <remarks>
    /// Each item in the collection is written as a separate line in the file.
    /// </remarks>
    public static void SaveToFile(this ListBox.ObjectCollection sender, string fileName)
    {
        File.WriteAllLines(fileName, sender.Cast<string>().Select(line => line).ToArray());
    }

    /// <summary>
    /// Loads the contents of a specified file into the <see cref="ListBox.ObjectCollection"/>.
    /// </summary>
    /// <param name="sender">The <see cref="ListBox.ObjectCollection"/> where the file contents will be loaded.</param>
    /// <param name="fileName">The path of the file to load the contents from.</param>
    /// <remarks>
    /// If the specified file does not exist, the method does nothing.
    /// Each line in the file is added as a separate item in the collection.
    /// </remarks>
    public static void LoadFromFile(this ListBox.ObjectCollection sender, string fileName)
    {
        if (!File.Exists(fileName)) return;
        sender.Clear();
        sender.AddRange(File.ReadAllLines(fileName).Cast<object>().ToArray());
    }
}

Form code

using Bogus;
using WinFormsListBoxApp.Classes;

namespace WinFormsListBoxApp;

public partial class Form1 : Form
{
    private string _fileName = Path.Combine(AppDomain.CurrentDomain.BaseDirectory, "companies.txt");
    public Form1()
    {

        InitializeComponent();

        if (!File.Exists(_fileName))
        {
            File.WriteAllLines(_fileName, new List<string>()
            {
                "Microsoft",
                "Apple"
            });
        }

        listBox1.Items.LoadFromFile(_fileName);
        listBox1.SelectedIndex = 0;

    }

    private void SaveButton_Click(object sender, EventArgs e)
    {
        var f = new Faker();
        listBox1.Items.Add(f.Company.CompanyName());
        listBox1.Items.SaveToFile(_fileName);
        listBox1.SelectedIndex = listBox1.Items.Count - 1;
    }

    private void LoadButton_Click(object sender, EventArgs e)
    {
        listBox1.Items.LoadFromFile(_fileName);
    }

    private void FreshStartButton_Click(object sender, EventArgs e)
    {
        
        listBox1.Items.Clear();

        File.WriteAllLines(_fileName, new List<string>()
        {
            "Microsoft",
            "Apple"
        });

        listBox1.Items.LoadFromFile(_fileName);
        listBox1.SelectedIndex = 0;

    }
}

Full source found here.