Example to hook up an SDL2 window to a Panel for use in a System.Windows.Forms window

FNAWindowExample.cs

#region License
/* FNA GameWindow for System.Windows.Forms Example
 * Written by Ethan "flibitijibibo" Lee
 * http://www.flibitijibibo.com/
 *
 * Released under public domain.
 * No warranty implied; use at your own risk.
 */
#endregion

#region Using Statements
using System;
using System.Threading;
using System.Runtime.InteropServices;

/* Bad awful Win32 Stuff */
using System.Drawing;
using System.Windows.Forms;
using Point = System.Drawing.Point;

/* Good nice FNA Stuff */
using SDL2;
using Microsoft.Xna.Framework;
using Color = Microsoft.Xna.Framework.Color;
#endregion

public class FNAWindowExample : Form
{
	#region FNA Game

	class ExampleGame : Game
	{
		public Color ClearColor;

		GraphicsDeviceManager graphics;

		public ExampleGame() : base()
		{
			graphics = new GraphicsDeviceManager(this);
			graphics.PreferredBackBufferWidth = 640;
			graphics.PreferredBackBufferHeight = 480;
			ClearColor = new Color(255, 255, 255, 255);
		}

		protected override void Draw(GameTime gameTime)
		{
			GraphicsDevice.Clear(ClearColor);
		}
	}

	#endregion

	#region Private FNA Window/Control Variables

	// These are the variables you care about.
	private static ExampleGame game;
	private Panel gamePanel;
	private bool windowAttached = false;

	#endregion

	#region Private Variables

	private Random random = new Random();

	#endregion

	#region WinAPI Entry Points

	[DllImport("user32.dll")]
	private static extern IntPtr SetWindowPos(
		IntPtr handle,
		IntPtr handleAfter,
		int x,
		int y,
		int cx,
		int cy,
		uint flags
	);
	[DllImport("user32.dll")]
	private static extern IntPtr SetParent(IntPtr child, IntPtr newParent);
	[DllImport("user32.dll")]
	private static extern IntPtr ShowWindow(IntPtr handle, int command);

	#endregion

	#region Form Constructor

	public FNAWindowExample()
	{
		// This is what we're going to attach the SDL2 window to
		gamePanel = new Panel();
		gamePanel.Size = new Size(640, 480);
		gamePanel.Location = new Point(80, 10);

		// Make the WinForms window
		Size = new Size(800, 600);
		FormClosing += new FormClosingEventHandler(WindowClosing);
		Button button = new Button();
		button.Text = "Whatever";
		button.Location = new Point(
			(Size.Width / 2) - (button.Size.Width / 2),
			gamePanel.Location.Y + gamePanel.Size.Height + 10
		);
		button.Click += new EventHandler(ClickedButton);
		Controls.Add(button);
		Controls.Add(gamePanel);
	}

	#endregion

	#region Button Event Method

	private void ClickedButton(object sender, EventArgs e)
	{
		if (!windowAttached)
		{
			// Make the Game, give it time to start up
			new Thread(GameThread).Start();
			Thread.Sleep(1000);

			// Get the Win32 HWND from the FNA window
			SDL.SDL_SysWMinfo info = new SDL.SDL_SysWMinfo();
			SDL.SDL_GetWindowWMInfo(game.Window.Handle, ref info);
			IntPtr winHandle = info.info.win.window;

			// Move the SDL2 window to 0, 0
			game.Window.IsBorderlessEXT = true;
			SetWindowPos(
				winHandle,
				Handle,
				0,
				0,
				0,
				0,
				0x0401 // NOSIZE | SHOWWINDOW
			);

			// Attach the SDL2 window to the panel
			SetParent(winHandle, gamePanel.Handle);
			ShowWindow(winHandle, 1); // SHOWNORMAL

			// We out.
			windowAttached = true;
		}

		game.ClearColor = new Color(
			(float) random.NextDouble(),
			(float) random.NextDouble(),
			(float) random.NextDouble(),
			1.0f
		);
	}

	#endregion

	#region Window Close Method

	private void WindowClosing(object sender, FormClosingEventArgs e)
	{
		game.Exit();
	}

	#endregion

	#region Game Thread

	// Run the Game in its own thread, to prevent GL ugliness
	private static void GameThread()
	{
		using (game = new ExampleGame())
		{
			game.Run();
		}
	}

	#endregion

	#region Program Entry Point

	[STAThread]
	static void Main()
	{
		Application.Run(new FNAWindowExample());
	}

	#endregion
}

SDL2WindowExample.cs

#region License
/* SDL2 Window for System.Windows.Forms Example
 * Written by Ethan "flibitijibibo" Lee
 * http://www.flibitijibibo.com/
 *
 * Released under public domain.
 * No warranty implied; use at your own risk.
 */
#endregion

#region Using Statements
using System;
using System.Drawing;
using System.Runtime.InteropServices;
using System.Windows.Forms;

using SDL2;
#endregion

public class SDL2WindowExample : Form
{
	#region Private SDL2 Window/Control Variables

	// These are the variables you care about.
	private Panel gamePanel;
	private IntPtr gameWindow; // For FNA, this is Game.Window.Handle

	#endregion

	#region Private GL Variables

	// IGNORE MEEEEE
	private Random random;
	private IntPtr glContext;
	private delegate void Viewport(int x, int y, int width, int height);
	private delegate void ClearColor(float r, float g, float b, float a);
	private delegate void Clear(uint flags);
	private Viewport glViewport;
	private ClearColor glClearColor;
	private Clear glClear;

	#endregion

	#region WinAPI Entry Points

	[DllImport("user32.dll")]
	private static extern IntPtr SetWindowPos(
		IntPtr handle,
		IntPtr handleAfter,
		int x,
		int y,
		int cx,
		int cy,
		uint flags
	);
	[DllImport("user32.dll")]
	private static extern IntPtr SetParent(IntPtr child, IntPtr newParent);
	[DllImport("user32.dll")]
	private static extern IntPtr ShowWindow(IntPtr handle, int command);

	#endregion

	#region Form Constructor

	public SDL2WindowExample()
	{
		// This is what we're going to attach the SDL2 window to
		gamePanel = new Panel();
		gamePanel.Size = new Size(640, 480);
		gamePanel.Location = new Point(80, 10);

		// Make the WinForms window
		Size = new Size(800, 600);
		FormClosing += new FormClosingEventHandler(WindowClosing);
		Button button = new Button();
		button.Text = "Whatever";
		button.Location = new Point(
			(Size.Width / 2) - (button.Size.Width / 2),
			gamePanel.Location.Y + gamePanel.Size.Height + 10
		);
		button.Click += new EventHandler(ClickedButton);
		Controls.Add(button);
		Controls.Add(gamePanel);

		// IGNORE MEEEEE
		random = new Random();
		SDL.SDL_Init(SDL.SDL_INIT_VIDEO);
		gameWindow = SDL.SDL_CreateWindow(
			String.Empty,
			0,
			0,
			gamePanel.Size.Width,
			gamePanel.Size.Height,
			SDL.SDL_WindowFlags.SDL_WINDOW_BORDERLESS | SDL.SDL_WindowFlags.SDL_WINDOW_OPENGL
		);
		glContext = SDL.SDL_GL_CreateContext(gameWindow);
		glViewport = (Viewport) Marshal.GetDelegateForFunctionPointer(
			SDL.SDL_GL_GetProcAddress("glViewport"),
			typeof(Viewport)
		);
		glClearColor = (ClearColor) Marshal.GetDelegateForFunctionPointer(
			SDL.SDL_GL_GetProcAddress("glClearColor"),
			typeof(ClearColor)
		);
		glClear = (Clear) Marshal.GetDelegateForFunctionPointer(
			SDL.SDL_GL_GetProcAddress("glClear"),
			typeof(Clear)
		);
		glViewport(0, 0, gamePanel.Size.Width, gamePanel.Size.Height);
		glClearColor(1.0f, 0.0f, 0.0f, 1.0f);
		glClear(0x4000);
		SDL.SDL_GL_SwapWindow(gameWindow);

		// Get the Win32 HWND from the SDL2 window
		SDL.SDL_SysWMinfo info = new SDL.SDL_SysWMinfo();
		SDL.SDL_GetWindowWMInfo(gameWindow, ref info);
		IntPtr winHandle = info.info.win.window;

		// Move the SDL2 window to 0, 0
		SetWindowPos(
			winHandle,
			Handle,
			0,
			0,
			0,
			0,
			0x0401 // NOSIZE | SHOWWINDOW
		);

		// Attach the SDL2 window to the panel
		SetParent(winHandle, gamePanel.Handle);
		ShowWindow(winHandle, 1); // SHOWNORMAL
	}

	#endregion

	#region Button Event Method

	private void ClickedButton(object sender, EventArgs e)
	{
		glClearColor(
			(float) random.NextDouble(),
			(float) random.NextDouble(),
			(float) random.NextDouble(),
			1.0f
		);
		glClear(0x4000); // GL_COLOR_BUFFER_BIT
		SDL.SDL_GL_SwapWindow(gameWindow);
	}

	#endregion

	#region Window Close Method

	private void WindowClosing(object sender, FormClosingEventArgs e)
	{
		glClear = null;
		glClearColor = null;
		glViewport = null;
		SDL.SDL_GL_DeleteContext(glContext);
		glContext = IntPtr.Zero;
		SDL.SDL_DestroyWindow(gameWindow);
		gameWindow = IntPtr.Zero;
		SDL.SDL_Quit();
	}

	#endregion

	#region Program Entry Point

	[STAThread]
	static void Main()
	{
		Application.Run(new SDL2WindowExample());
	}

	#endregion
}