Question

I was trying to follow this tutorial to make an OpenGL application on C#. http://www.developerfusion.com/article/3823/opengl-in-c/2/

Later on, I tried to make the following changes:

class DrawObject : OpenGLControl
{
    public override void glDraw()
    {
        GL.glClear(GL.GL_COLOR_BUFFER_BIT | GL.GL_DEPTH_BUFFER_BIT);
        GL.glLoadIdentity();
        GL.glBegin(GL.GL_LINE);
        GL.glLineWidth(10.0f);
        GL.glVertex3f(-3.0f, 0.0f, 0.0f);
        GL.glVertex3f(3.0f, 0.0f, 0.0f);
        GL.glEnd();
        GL.glFlush();
    }

    protected override void InitGLContext()
    {
        GL.glShadeModel(GL.GL_SMOOTH);
        GL.glClearColor(1.0f, 1.0f, 1.0f, 0.0f);
        GL.glClearDepth(1.0f);
        GL.glEnable(GL.GL_DEPTH_TEST);
        GL.glDepthFunc(GL.GL_LEQUAL);
        GL.glHint(GL.GL_PERSPECTIVE_CORRECTION_HINT, GL.GL_NICEST);
    }

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

        Size s = Size;
        double aspect_ratio = (double)s.Width / (double)s.Height;
        GL.glMatrixMode(GL.GL_PROJECTION);
        GL.glLoadIdentity();

        GL.gluPerspective(45.0f, aspect_ratio, 0.1f, 100.0f);

        GL.glMatrixMode(GL.GL_MODELVIEW);
        GL.glLoadIdentity();
    }
}

public class DrawLine : Form
{
    DrawObject newObject = new DrawObject();

    public DrawLine()
    {
        Text = "Draw a line";
        newObject.Dock = DockStyle.Fill;
        Controls.Add(newObject);
    }

    public static void Main()
    {
        DrawLine drawLine = new DrawLine();
        Application.Run(drawLine);
    }
}

For some reason, I will then get an “invalid enumerant” error, at Application.Run(drawLine). Basically what I’m trying to do is to replace the part where it renders a dot on 3 specified vertices, with a part where it renders a line given 2 vertices. I do not know why the dot version does not throw this exception, but the line version does. I have referenced csgl.dll in my references, and added csgl.native.dll and made it to be published every time the solution is compiled (otherwise the whole thing wouldn’t run at all).

Answer 1

Don’t change state inside glBegin and glEnd (that is, don’t call glLineWidth). From the docs:

GL_INVALID_OPERATION is generated if glLineWidth is executed between 
the execution of glBegin and the corresponding execution of glEnd.

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Answer from JulianLee: glBegin must be called with GL_LINES, not GL_LINE. See comments.