Since I tried OpenTK, I decided to give SlimDX a try as well. So, here’s a simple getting started app:

using System;
using System.Drawing;
using System.Windows.Forms;
using SlimDX;
using SlimDX.Direct3D9;
using SlimDX.Windows;

namespace SlimDXApp1
{
	public partial class SlimDXApp1Form : RenderForm
	{
		struct Vertex
		{
			public Vector4 Position;
			public int Color;
		}

		Device device;
		VertexDeclaration vertexDeclaration;

		public SlimDXApp1Form()
			: base("SlimDXApp1")
		{
			this.ClientSize = new Size(800, 600);

			this.device = new Device(new Direct3D(), 0, DeviceType.Hardware, this.Handle, CreateFlags.HardwareVertexProcessing, new PresentParameters()
            {
                BackBufferWidth = this.ClientSize.Width,
                BackBufferHeight = this.ClientSize.Height
            });

			this.vertexDeclaration = new VertexDeclaration(this.device, new[] {
        		new VertexElement(0, 0, DeclarationType.Float4, DeclarationMethod.Default, DeclarationUsage.PositionTransformed, 0),
        		new VertexElement(0, 16, DeclarationType.Color, DeclarationMethod.Default, DeclarationUsage.Color, 0),
				VertexElement.VertexDeclarationEnd
        	});
		}

		public void Run()
		{
			MessagePump.Run(this, () =>
			{
				this.device.Clear(ClearFlags.Target | ClearFlags.ZBuffer, Color.Black, 1.0f, 0);
				this.device.BeginScene();

				this.device.VertexDeclaration = this.vertexDeclaration;

				this.device.DrawUserPrimitives(PrimitiveType.TriangleList, 1, new[] {
					new Vertex() { Color = Color.Red.ToArgb(), Position = new Vector4(400.0f, 100.0f, 0.5f, 1.0f) },
					new Vertex() { Color = Color.Blue.ToArgb(), Position = new Vector4(650.0f, 500.0f, 0.5f, 1.0f) },
					new Vertex() { Color = Color.Green.ToArgb(), Position = new Vector4(150.0f, 500.0f, 0.5f, 1.0f) }
				});

				this.device.EndScene();
				this.device.Present();
			});
		}

		[STAThread]
		static void Main()
		{
			SlimDXApp1Form form = new SlimDXApp1Form();
			form.Run();

			// Cleans up COM handles
			foreach(var item in ObjectTable.Objects)
				item.Dispose();
		}
	}
}