3D and 2D Mixed

using System;
using System.Collections.Generic;
using System.Drawing;
using OpenTK;
using OpenTK.Graphics.OpenGL;
using OpenTK.Input;

namespace OpenGLSwapTest
{
    public class TestingWindow : GameWindow
    {

        const float RotationSpeed = 180.0f;
        float _angle;


        protected override void OnLoad(EventArgs e)
        {
            base.OnLoad(e);

            GL.ClearColor(Color.MidnightBlue);
            GL.Enable(EnableCap.DepthTest);
        }


        protected override void OnResize(EventArgs e)
        {
            base.OnResize(e);
            SwitchTo3D();
        }

        protected override void OnUpdateFrame(FrameEventArgs e)
        {
            base.OnUpdateFrame(e);
            KeyboardState keyboard = OpenTK.Input.Keyboard.GetState();
            if (keyboard[Key.Escape])
            {
                this.Exit();
            }
        }

        private void SwitchTo3D()
        {
            GL.Viewport(0, 0, Width, Height);
            double aspectRatio = Width / (double)Height;
            Matrix4 perspective = Matrix4.CreatePerspectiveFieldOfView(MathHelper.PiOver4, (float)aspectRatio, 1, 64);
            GL.MatrixMode(MatrixMode.Projection);
            GL.LoadMatrix(ref perspective);
        }


        protected override void OnRenderFrame(FrameEventArgs e)
        {
            base.OnRenderFrame(e);
            GL.Clear(ClearBufferMask.ColorBufferBit | ClearBufferMask.DepthBufferBit);

            //SwitchTo3D();
            Matrix4 lookat = Matrix4.LookAt(0, 5, 5, 0, 0, 0, 0, 1, 0);
            GL.MatrixMode(MatrixMode.Modelview);
            GL.LoadMatrix(ref lookat);
            _angle += RotationSpeed * (float)e.Time;
            GL.Rotate(_angle, 0.0f, 1.0f, 0.0f);
            DrawCube();
            GL.DepthMask(false);
            Draw2DOver3D();
            GL.DepthMask(true);
            this.SwapBuffers();
        }

        private void Draw2DOver3D()
        {
            //
            GL.PushMatrix();
            GL.LoadIdentity();
            Matrix4 orthoProjection = Matrix4.CreateOrthographicOffCenter(0, Width, Height, 0, -1, 1);
            GL.MatrixMode(MatrixMode.Projection);
            GL.PushMatrix();//
            GL.LoadMatrix(ref orthoProjection);
            //
            int width = 400;
            int height = 400;
            GL.Begin(PrimitiveType.Quads);

            GL.TexCoord2(0, 0); GL.Vertex2(0, 0);
            GL.TexCoord2(1, 0); GL.Vertex2(width, 0);
            GL.TexCoord2(1, 1); GL.Vertex2(width, height);
            GL.TexCoord2(0, 1); GL.Vertex2(0, height);

            GL.End();

            //
            GL.PopMatrix();
            GL.MatrixMode(MatrixMode.Modelview);
            GL.PopMatrix();
        }

        private void DrawCube()
        {
            GL.Begin(PrimitiveType.Quads);

            GL.Color3(Color.Silver);
            GL.Vertex3(-1.0f, -1.0f, -1.0f);
            GL.Vertex3(-1.0f, 1.0f, -1.0f);
            GL.Vertex3(1.0f, 1.0f, -1.0f);
            GL.Vertex3(1.0f, -1.0f, -1.0f);

            GL.Color3(Color.Honeydew);
            GL.Vertex3(-1.0f, -1.0f, -1.0f);
            GL.Vertex3(1.0f, -1.0f, -1.0f);
            GL.Vertex3(1.0f, -1.0f, 1.0f);
            GL.Vertex3(-1.0f, -1.0f, 1.0f);

            GL.Color3(Color.Moccasin);

            GL.Vertex3(-1.0f, -1.0f, -1.0f);
            GL.Vertex3(-1.0f, -1.0f, 1.0f);
            GL.Vertex3(-1.0f, 1.0f, 1.0f);
            GL.Vertex3(-1.0f, 1.0f, -1.0f);

            GL.Color3(Color.IndianRed);
            GL.Vertex3(-1.0f, -1.0f, 1.0f);
            GL.Vertex3(1.0f, -1.0f, 1.0f);
            GL.Vertex3(1.0f, 1.0f, 1.0f);
            GL.Vertex3(-1.0f, 1.0f, 1.0f);

            GL.Color3(Color.PaleVioletRed);
            GL.Vertex3(-1.0f, 1.0f, -1.0f);
            GL.Vertex3(-1.0f, 1.0f, 1.0f);
            GL.Vertex3(1.0f, 1.0f, 1.0f);
            GL.Vertex3(1.0f, 1.0f, -1.0f);

            GL.Color3(Color.ForestGreen);
            GL.Vertex3(1.0f, -1.0f, -1.0f);
            GL.Vertex3(1.0f, 1.0f, -1.0f);
            GL.Vertex3(1.0f, 1.0f, 1.0f);
            GL.Vertex3(1.0f, -1.0f, 1.0f);

            GL.End();
        }



    }
}