TransformFeedback problem

It looks correct. Though, you don't need fragment shader as you disable the rasterizer.

This can be easily a driver bug.
ATI fixed their Catalyst to support TF properly only 1-2 months ago.
Try latest NV drivers (even beta ones).

Which version of OpenTK are you using and what is your OS language settings for non-Unicode applications? (You can find this in "Control Panel" -> "Regional and Language Options", it should be the last tab).

Thanks. I've tested this on my Linux laptop (Quadro NVS135M, 190.42 drivers, Mono 2.4.3 / english codepage) and the code does not fail at that point. Instead, it fails with an "InvalidOperation" OpenGL error in the first GL.DrawArrays call. Unfortunately, I don't have access to a windows machine with transform-feedback-capable drivers right now, so I cannot test in a configuration more similar to yours.

This looks like a marshaling issue but the cause is not immediately obvious (it could be similar to #1332: 2byte character may causes a bug with GL.ShaderSource .).

Suggestion: try importing the function manually.

// Code was wrong, see below for corrected code.

We need to find out what works, so we can add this to OpenTK.

Edit: Corrected code here, http://www.opentk.com/node/1389#comment-7477

Indeed, but you have to take into account that string-related functions are affected by your regional settings. I'm not saying that this is the problem here (it could be, but that's for from certain at this point - hence the code in my previous post), but it has caused problems before so it's something to keep an eye out for.

Ugh, that's what you get for writing code directly in the browser. :)

Corrected code below:

using System.Runtime.InteropServices;
 
// Note: this code assumes you are using GameWindow. The same code can be used with
// GLControl - the only limitation is that you need a valid GraphicsContext prior to calling
// GraphicsContext.CurrentContext.
class Game : GameWindow
{
    // Edit, this should be IntPtr[] varyings
    delegate void glTransformFeedbackVaryings(int program, int count, IntPtr varyings, TransformFeedbackMode bufferMode);
 
    static glTransformFeedbackVaryings TransformFeedbackVaryings;
 
    protected override void OnLoad(object sender, EventArgs e)
    {
        base.OnLoad(sender, e);
 
        TransformFeedbackVaryings = (glTransformFeedbackVaryings)Marshal.GetDelegateForFunctionPointer(
            (GraphicsContext.CurrentContext as IGraphicsContextInternal).GetAddress("glTransformFeedbackVaryings"),,
            typeof(glTransformFeedbackVaryings));
    }
}

Afterwards, simply use TransformFeedbackVaryings instead of GL.TransformFeedbackVaryings.

If it still crashes, please post the exception message, this could be useful.

Edit: this code also has an error, see below.