System themed context menu for .NET Core and .NET 5.0 based Windows Forms applications

SystemContextMenu.cs

using System.Collections.Generic;
using System.Drawing;
using System.Linq;
using System.Runtime.InteropServices;

namespace System.Windows.Forms
{
	[Flags]
	public enum SystemContextMenuItemFlags : uint
	{
		Default = 0x0000,
		GrayedAndDisabled = 0x0001,
		Disabled = 0x0002,
		Bitmap = 0x0004,
		Checked = 0x0008,
		OwnerDraw = 0x0100,
		IsSeperator = 0x0800
	}

	[Flags]
	public enum SystemContextMenuTrackPopupMenuFlags : uint
	{
		Default = 0x0000,

		LeftAlign = 0x0000,
		HorizontalCenterAlign = 0x0004,
		RightAlign = 0x0008,

		TopAlign = 0x0000,
		VerticalCenterAlign = 0x0010,
		BottomAlign = 0x0020,

		TrackLeftButton = 0x0000,
		TrackRightButton = 0x0002,

		LeftToRightAnimation = 0x0400,
		RightToLeftAnimation = 0x0800,
		TopToBottomAnimation = 0x1000,
		BottomToTopAnimation = 0x2000,
		NoAnimation = 0x4000,

		ReturnResult = 0x0100
	}

	public class SystemContextMenuItem
	{
		public SystemContextMenuItemFlags MenuItemFlags;
		public Bitmap Image;
		public string Text;
		public Action OnClick;
		public int ID;

		public SystemContextMenuItem(string text, Action onClick, SystemContextMenuItemFlags flags)
		{
			Text = text;
			OnClick = onClick;
			MenuItemFlags = flags;
		}

		public SystemContextMenuItem(string text, Action onClick, SystemContextMenuItemFlags flags, Image image)
		{
			if (!flags.HasFlag(SystemContextMenuItemFlags.Bitmap))
				flags = flags | SystemContextMenuItemFlags.Bitmap;

			Text = text;
			OnClick = onClick;
			MenuItemFlags = flags;
			Image = new Bitmap(image);
		}
	}

	public class SystemContextMenu
	{
		[DllImport("user32.dll")]
		static extern IntPtr CreatePopupMenu();
		[DllImport("user32.dll", CharSet = CharSet.Auto)]
		static extern bool AppendMenu(IntPtr hMenu, SystemContextMenuItemFlags uFlags, int uIDNewItem, string lpNewItem);
		[DllImport("user32.dll", CharSet = CharSet.Auto)]
		static extern bool AppendMenu(IntPtr hMenu, SystemContextMenuItemFlags uFlags, int uIDNewItem, IntPtr lpNewItem);
		[DllImport("user32.dll")]
		static extern bool DeleteMenu(IntPtr hMenu, int uPosition, uint uFlags);
		[DllImport("user32.dll")]
		static extern int TrackPopupMenuEx(IntPtr hmenu, SystemContextMenuTrackPopupMenuFlags fuFlags, int x, int y, IntPtr hwnd, IntPtr lptpm);

		public IntPtr MenuHandle;
		public List<SystemContextMenuItem> Items = new();

		private int nextItemId = 0;

		public SystemContextMenu()
		{
			MenuHandle = CreatePopupMenu();
		}

		public void AddItem(SystemContextMenuItem item)
		{
			Items.Add(item);
			item.ID = nextItemId;
			if (item.MenuItemFlags.HasFlag(SystemContextMenuItemFlags.Bitmap))
				AppendMenu(MenuHandle, item.MenuItemFlags, nextItemId++, item.Image.GetHbitmap());
			else
				AppendMenu(MenuHandle, item.MenuItemFlags, nextItemId++, item.Text);
		}

		public void RemoveItem(SystemContextMenuItem item)
		{
			if (Items.Contains(item))
			{
				Items.Remove(item);
				DeleteMenu(MenuHandle, item.ID, 0x0000);
			}
		}

		public void Show(IntPtr windowHandle, int x, int y)
		{
			int id = TrackPopupMenuEx(MenuHandle, SystemContextMenuTrackPopupMenuFlags.ReturnResult, x, y, windowHandle, IntPtr.Zero);
			PerformAction(id);
		}

		public void PerformAction(int itemId)
		{
			IEnumerable<SystemContextMenuItem> items = Items.Where(a => a.ID == itemId);
			if (items.Count() > 0)
			{
				SystemContextMenuItem item = items.First();
				item.OnClick();
			}
		}

		public static SystemContextMenu FromContextMenuStrip(ContextMenuStrip contextMenuStrip)
		{
			SystemContextMenu systemContextMenu = new();
			foreach (ToolStripItem item in contextMenuStrip.Items)
			{
				if (item is ToolStripMenuItem menuItem)
				{
					SystemContextMenuItemFlags flags = SystemContextMenuItemFlags.Default;
					if (menuItem.Checked)
						flags |= SystemContextMenuItemFlags.Checked;
					if (!menuItem.Enabled)
						flags |= SystemContextMenuItemFlags.Disabled;

					if (menuItem.Image != null)
					{
						flags |= SystemContextMenuItemFlags.Bitmap;
						SystemContextMenuItem systemContextMenuItem = new(menuItem.Text, menuItem.PerformClick, flags, menuItem.Image);
						systemContextMenu.AddItem(systemContextMenuItem);
					}
					else
					{
						SystemContextMenuItem systemContextMenuItem = new(menuItem.Text, menuItem.PerformClick, flags);
						systemContextMenu.AddItem(systemContextMenuItem);
					}
				}
				else if (item is ToolStripSeparator seperator)
					systemContextMenu.AddItem(new SystemContextMenuItem("", new Action(() => { }), SystemContextMenuItemFlags.IsSeperator));
			}
			return systemContextMenu;
		}
	}
}