C# 9&Win API システムメニューを表示する

システムメニューを表示するサンプルコードです。

#nullable enable

using System;
using System.Drawing;
using System.Runtime.InteropServices;
using System.Windows.Forms;

static class Program
{
    [STAThread]
    static void Main()
    {
        Application.SetHighDpiMode(HighDpiMode.SystemAware);
        Application.EnableVisualStyles();
        Application.SetCompatibleTextRenderingDefault(false);
        Application.Run(new Form1());
    }
}

sealed class Form1 : Form
{
    public Form1() : base()
    {
        Controls.AddRange(new[] {
            new Label(){
                Text="右クリックでシステムメニューが表示されます。",
                AutoSize=true
            }
        });
    }

    protected override void OnMouseUp(MouseEventArgs e)
    {
        if (e.Button == MouseButtons.Right)
        {
            this.ShowSystemMenu(Cursor.Position);
        }
        else
        {
            base.OnMouseUp(e);
        }
    }
}

public static class FormSystemMenuExtensions
{
    /// <summary>
    /// ウィンドウのシステムメニューを表示します。
    /// </summary>
    /// <param name="form">システムメニューを表示するフォーム。</param>
    /// <param name="location">システムメニューを表示する画面上の位置。</param>
    /// <param name="flags">フラグ。</param>
    /// <returns></returns>
    public static int ShowSystemMenu(this Form form, Point location, TrackPopupMenuFlags flags = 0)
    {
        const uint TPM_RETURNCMD = 0x0100;
        const int WM_SYSCOMMAND = 0x0112;

        var sysMenuHandle = NativeMethods.GetSystemMenu(form.Handle, false);
        var id = NativeMethods.TrackPopupMenu(sysMenuHandle,
            (uint)flags | TPM_RETURNCMD, location.X, location.Y, 0, form.Handle);
        NativeMethods.SendMessage(form.Handle, WM_SYSCOMMAND, id, 0);
        return id;
    }

    private static class NativeMethods
    {
        [DllImport("user32.dll")]
        public static extern nint SendMessage(IntPtr hwnd, int msg, nint wParam, nint lParam);

        [DllImport("user32.dll")]
        public static extern IntPtr GetSystemMenu(
            IntPtr hWnd,
            [MarshalAs(UnmanagedType.Bool)] bool bRevert);

        [DllImport("user32.dll")]
        public static extern int TrackPopupMenu(
            IntPtr hMenu,
            uint uFlags,
            int x,
            int y,
            int nReserved,
            IntPtr hWnd,
            IntPtr prcRect = default);
    }

    public enum TrackPopupMenuFlags : uint
    {
        LButton = 0x0000,
        RButton = 0x0002,
        Leftn = 0x0000,
        Center = 0x0004,
        Right = 0x0008,
        Top = 0x0000,
        Middle = 0x0010,
        Bottom = 0x0020,

        Hozirontal = 0x0000,
        Vertical = 0x0040,
        NoNotify = 0x0080,
        //TPM_RETURNCMD = 0x0100,
        Recurse = 0x0001,
        AnimationLR = 0x0400,
        AnimationRL = 0x0800,
        AnimationTB = 0x1000,
        AnimationBT = 0x2000,
        NoAnimation = 0x4000,
        LayoutRTL = 0x8000,
        WorkArea = 0x10000,
    }
}