Concerning the (somewhat off topic I know but since it was brought up earlier) XNA vs OpenGL choice, it might be beneficial in several cases to go with OpenGL instead of XNA (and in other XNA instead of OpenGL...).
If you are going to run the applications on Linux or Mac using Mono, it might be a good choice to go with OpenGL. Also, which isn't so widely known I think, if you have customers that are going to run your applications in a Citrix environment, then DirectX/Direct3D/XNA won't be as economical a choice as OpenGL. The reason for this is that OpenGL applications can be co-hosted on a lesser number of servers (due to performance issues a single server cannot host an infinite number of application instances) than DirectX/XNA applications which demands dedicated virtual servers to run in hardware accelerated mode. Other requirements exists like supported graphics cards etc but I will keep to the XNA vs OpenGL issue. As an IT Architect, Software developer etc this will have to be considered before choosing between OpenGL and DirectX/XNA...
A side note is that WebGL is based on OpenGL ES3 afaik...
As a further note, these are not the only considerations, but they might make the choice easier for some...
You can OpenGL without a wrapper and use it natively in C#. Just as Jeff Mc said, you would have to import all the functions you need with DllImport.
What he left out is having to create context before you can use any of the OpenGL functions. It's not hard, but there are few other not-so-intuitive DllImports that need to be done.
I have created an example C# project in VS2012 with almost the bare minimum necessary to get OpenGL running on Windows box. It only paints the window blue, but it should be enough to get you started. The example can be found at http://www.glinos-labs.org/?q=programming-opengl-csharp. Look for the No Wrapper example at the bottom.
XNA 2.0 requires a minimum of a shader 1.1 card. While old tech, not everyone has one. Some newer laptops (in our experience Toshiba tablets with Intel graphics) have no shader 1.1 support. XNA simply wont run on these machines.
This is a significant issue for us and we have shifted to Tao and OpenGL. Plus with Tao we have bindings for audio & Lua support.
OpenTK is an improvement over the Tao API, as it uses idiomatic C# style with overloading, strongly-typed enums, exceptions, and standard .NET types:
GL.Begin(BeginMode.Points);
GL.Color3(Color.Yellow);
GL.Vertex3(Vector3.Up);
as opposed to Tao which merely mirrors the C API:
Gl.glBegin(Gl.GL_POINTS); // double "gl" prefix
Gl.glColor3ub(255, 255, 0); // have to pass RGB values as separate args
Gl.glVertex3f(0, 1, 0); // explicit "f" qualifier
This makes for harder porting but is incredibly nice to use.
As a bonus it provides font rendering, texture loading, input handling, audio, math...
Update 18th January 2016: Today the OpenTK maintainer has stepped away from the project, leaving its future uncertain. The forums are filled with spam. The maintainer recommends moving to MonoGame or SDL2#.
Update 30th June 2020: OpenTK has had new maintainers for a while now and has an active discord community. So the previous recommendation of using another library isn't necessarily true.
I think what @korona meant was since it's just a C API, you can consume it from C# directly with a heck of a lot of typing like this:
[DllImport("opengl32")]
public static extern void glVertex3f(float x, float y, float z);
You unfortunately would need to do this for every single OpenGL function you call, and is basically what Tao has done for you.
SharpGL is a project that lets you use OpenGL in your Windows Forms or WPF applications.
I would also recommend the Tao Framework. But one additional note:
Take a look at these tutorials: http://www.taumuon.co.uk/jabuka/
What would you like these support libraries to do? Just using OpenGL from C# is simple enough and does not require any additional libraries afaik.
Tao is supposed to be a nice framework.
From their site:
The Tao Framework for .NET is a collection of bindings to facilitate cross-platform media application development utilizing the .NET and Mono platforms.
Source: Stackoverflow.com