C# WinForms и WPF — простые окна — WinForms — полный `Program.cs`

using System.Drawing;
using System.Windows.Forms;

namespace ButtonDemo;

static class Program
{
    [STAThread]
    static void Main()
    {
        ApplicationConfiguration.Initialize();

        var form = new Form
        {
            Text = "Кнопка",
            ClientSize = new Size(320, 180),
            StartPosition = FormStartPosition.CenterScreen
        };

        var button = new Button
        {
            Text = "Нажми меня",
            Location = new Point(20, 20),
            AutoSize = true
        };

        // Click += подписка на событие; лямбда вызывается ТОЛЬКО по клику
        button.Click += (_, _) => MessageBox.Show(
            "Кнопка нажата!",
            "Сообщение",
            MessageBoxButtons.OK,
            MessageBoxIcon.Information);

        form.Controls.Add(button);
        Application.Run(form);
    }
}

using System.Drawing;
using System.Windows.Forms;

namespace GreetApp;

static class Program
{
    [STAThread]
    static void Main()
    {
        ApplicationConfiguration.Initialize();

        var form = new Form
        {
            Text = "Приветствие",
            ClientSize = new Size(360, 160),
            StartPosition = FormStartPosition.CenterScreen,
            FormBorderStyle = FormBorderStyle.FixedDialog,
            MaximizeBox = false
        };

        var nameLabel = new Label
        {
            Text = "Ваше имя:",
            AutoSize = true,
            Location = new Point(20, 24)
        };

        var entry = new TextBox
        {
            Location = new Point(120, 20),
            Width = 200
        };

        var greetButton = new Button
        {
            Text = "Приветствовать",
            Location = new Point(20, 60),
            AutoSize = true
        };

        greetButton.Click += (_, _) =>
        {
            var name = entry.Text.Trim();
            if (string.IsNullOrEmpty(name))
            {
                MessageBox.Show("Введите имя", "Пусто",
                    MessageBoxButtons.OK, MessageBoxIcon.Warning);
                return;
            }
            MessageBox.Show($"Здравствуй, {name}!", "Привет");
        };

        form.Controls.Add(nameLabel);
        form.Controls.Add(entry);
        form.Controls.Add(greetButton);
        entry.Focus();

        Application.Run(form);
    }
}

using System.Drawing;
using System.Globalization;
using System.Windows.Forms;

namespace TempConverter;

static class Program
{
    [STAThread]
    static void Main()
    {
        ApplicationConfiguration.Initialize();

        var form = new Form
        {
            Text = "Конвертер температуры",
            ClientSize = new Size(340, 180),
            StartPosition = FormStartPosition.CenterScreen,
            FormBorderStyle = FormBorderStyle.FixedDialog,
            MaximizeBox = false
        };

        var tempLabel = new Label
        {
            Text = "Температура (°C):",
            AutoSize = true,
            Location = new Point(20, 24)
        };

        var entry = new TextBox { Location = new Point(160, 20), Width = 80 };

        var resultLabel = new Label
        {
            Text = "—",
            AutoSize = true,
            Location = new Point(20, 60),
            Font = new Font("Segoe UI", 12f)
        };

        var convertButton = new Button
        {
            Text = "Перевести",
            Location = new Point(20, 100),
            AutoSize = true
        };

        convertButton.Click += (_, _) =>
        {
            var raw = entry.Text.TrimReplace(',', '.');
            if (!double.TryParse(raw, NumberStyles.Any, CultureInfo.InvariantCulture, out var celsius))
            {
                MessageBox.Show("Введите число, например 25", "Ошибка");
                return;
            }
            var fahrenheit = celsius * 9 / 5 + 32;
            resultLabel.Text = $"{celsius:F1} °C = {fahrenheit:F1} °F";
        };

        entry.KeyDown += (_, e) =>
        {
            if (e.KeyCode == Keys.Enter)
                convertButton.PerformClick();
        };

        form.Controls.Add(tempLabel);
        form.Controls.Add(entry);
        form.Controls.Add(resultLabel);
        form.Controls.Add(convertButton);
        entry.Focus();

        Application.Run(form);
    }
}

using System.Drawing;
using System.Windows.Forms;

namespace TodoList;

static class Program
{
    [STAThread]
    static void Main()
    {
        ApplicationConfiguration.Initialize();

        var form = new Form
        {
            Text = "Список задач",
            ClientSize = new Size(320, 280),
            StartPosition = FormStartPosition.CenterScreen
        };

        var entry = new TextBox { Location = new Point(12, 12), Width = 280 };
        var addButton = new Button { Text = "Добавить", Location = new Point(12, 44), AutoSize = true };
        var removeButton = new Button { Text = "Удалить", Location = new Point(100, 44), AutoSize = true };
        var listBox = new ListBox { Location = new Point(12, 80), Size = new Size(280, 160) };

        void AddTask()
        {
            var text = entry.Text.Trim();
            if (text.Length == 0) return;
            listBox.Items.Add(text);
            entry.Clear();
        }

        void RemoveTask()
        {
            if (listBox.SelectedIndex >= 0)
                listBox.Items.RemoveAt(listBox.SelectedIndex);
        }

        addButton.Click += (_, _) => AddTask();
        removeButton.Click += (_, _) => RemoveTask();
        entry.KeyDown += (_, e) => { if (e.KeyCode == Keys.Enter) AddTask(); };

        form.Controls.AddRange(new Control[] { entry, addButton, removeButton, listBox });
        entry.Focus();

        Application.Run(form);
    }
}