Full Screen

Most games have the ability to switch between full screen mode and windowed mode. A lot of times this is implemented as a menu option. It used to be standard to toggle fullscreen mod with the Alt+Enter shortcut, however that trent has been slowly dying out.

How you want to implement your fullscreen toggle logic is up to you, i'm just going to show you how to actually toggle. If you end up accidentally going into fullscreen mode press Alt+Tab to bring up the program selection toolbox. Navigate to Visual studio (Keep alt pressed down while tapping the tab button to move between programs) and use the red square to terminate the application.

API

We can switch our game to fullscreen mode by setting a few variables inherited from OpenTK.GameWindow.

First, we need to set the WindowBorder variable. It is an enum of type WindowBorder. The values that interest us are WindowBorder.Resizable for a regular window and WindowBorder.Hidden for a fullscreen window.

Next, we want to set the WindowState variable. It is an enum of type WindowState. The values that we care about are WindowState.Normal for windowed mode and WindowState.Fullscreen for fullscreen mode.

One last thing, when going from fullscreen to windowed mode you need to reset the client size the same way you did when we first created the window:

ClientSize = new Size(800, 600);

Failure to do so will result in undefined behaviour. On my machine the window shrinks by 1 pixel every frame until it has a sie of 0

Implementation

The first thing we need to do is add a new boolean to the MainGameWindow class to keep track of the current window mode (windowed or full screen).

using System;
using OpenTK;
using System.Drawing;
using OpenTK.Graphics.OpenGL;
using GameFramework;

namespace GameApplication {
    class MainGameWindow : OpenTK.GameWindow {
        // reference to OpenTK window
        public static OpenTK.GameWindow Window = null;

        // keep track of window mode
        public static bool IsFullscreen = false;

Next we implement the actual toggle function

        public static void ToggleFullscreen() {
            if (IsFullscreen) {
                Window.WindowBorder = WindowBorder.Resizable;
                Window.WindowState = WindowState.Normal;
                Window.ClientSize = new Size(800, 600);
            }
            else {
                Window.WindowBorder = WindowBorder.Hidden;
                Window.WindowState = WindowState.Fullscreen;
            }
            IsFullscreen = !IsFullscreen;
        }

        // ... Rest of MainGameWindow class

Keep in mind, when exiting fullscreen mode you MUST reset the window to its original size, otherwise the behaviour is undefined.

That's it. You can now call the ToggleFullscreen function from a menu, or hook it up to Alt+Enter your call.