Simple GLSL shader using OpenTK doesn't work

See more:

C#

OpenGL

GLSL

Background: I've started playing around with raytracing and thought it'll be a good idea to write the raytracer using GLSL shaders as it'll optimize the performance a lot.

Problem: I've written (more of a assemble from parts of code on the internet) a simple program to test GLSL shaders but it isn't working. It runs but just shows the clear color. I'm using OpenTK and C#. My video card supports OpenGL 2.1 and I've seen some GLSL shaders running on it (though I don't have their source code).

Here is my code.

The Shader.cs file can be found here.
My main class is this (It's the OpenTK's Game.cs file with some code added:

using System;

using OpenTK;
using OpenTK.Graphics;
using OpenTK.Graphics.OpenGL;
using OpenTK.Audio;
using OpenTK.Audio.OpenAL;
using OpenTK.Input;

namespace GLSLTest
{
    class Game : GameWindow
    {

        public Shader Shader;

        /// <summary>Creates a 800x600 window with the specified title.</summary>
        public Game()
            : base(800, 600, GraphicsMode.Default, "OpenTK Quick Start Sample")
        {
            VSync = VSyncMode.On;
            Shader = new Shader(
                "void main(void){ gl_Position = gl_Vertex;}",
                "void main(void) {"
                + "vec2 pos = mod(gl_FragCoord.xy, vec2(50.0)) - vec2(25.0);"
                + "float dist_squared = dot(pos, pos);"

                + "gl_FragColor = (dist_squared < 400.0) "
                + "? vec4(.90, .90, .90, 1.0)"
                + ": vec4(.20, .20, .40, 1.0);"
                + "}");
            Shader.Activate();
        }

        /// <summary>Load resources here.</summary>
        /// <param name="e">Not used.</param>
        protected override void OnLoad(EventArgs e)
        {
            base.OnLoad(e);

            GL.ClearColor(0.1f, 0.2f, 0.5f, 0.0f);
            GL.Enable(EnableCap.DepthTest);
        }

        /// <summary>
        /// Called when your window is resized. Set your viewport here. It is also
        /// a good place to set up your projection matrix (which probably changes
        /// along when the aspect ratio of your window).
        /// </summary>
        /// <param name="e">Not used.</param>
        protected override void OnResize(EventArgs e)
        {
            base.OnResize(e);

            GL.Viewport(ClientRectangle.X, ClientRectangle.Y, ClientRectangle.Width, ClientRectangle.Height);

            Matrix4 projection = Matrix4.CreatePerspectiveFieldOfView((float)Math.PI / 4, Width / (float)Height, 1.0f, 64.0f);
            GL.MatrixMode(MatrixMode.Projection);
            GL.LoadMatrix(ref projection);
        }

        /// <summary>
        /// Called when it is time to setup the next frame. Add you game logic here.
        /// </summary>
        /// <param name="e">Contains timing information for framerate independent logic.</param>
        protected override void OnUpdateFrame(FrameEventArgs e)
        {
            base.OnUpdateFrame(e);

            if (Keyboard[Key.Escape])
                Exit();
        }

        /// <summary>
        /// Called when it is time to render the next frame. Add your rendering code here.
        /// </summary>
        /// <param name="e">Contains timing information.</param>
        /// 

        protected override void OnRenderFrame(FrameEventArgs e)
        {
            base.OnRenderFrame(e);

            GL.Clear(ClearBufferMask.ColorBufferBit | ClearBufferMask.DepthBufferBit);

            Matrix4 modelview = Matrix4.LookAt(Vector3.Zero, Vector3.UnitZ, Vector3.UnitY);
            GL.MatrixMode(MatrixMode.Modelview);
            GL.LoadMatrix(ref modelview);

            SwapBuffers();
        }

        /// <summary>
        /// The main entry point for the application.
        /// </summary>
        [STAThread]
        static void Main()
        {
            // The 'using' idiom guarantees proper resource cleanup.
            // We request 30 UpdateFrame events per second, and unlimited
            // RenderFrame events (as fast as the computer can handle).
            using (Game game = new Game())
            {
                game.Run(30.0);
            }
        }
    }
}