Drawing a line in Winforms

Question

I am having trouble drawing a line within a group box in a simple windows form.

here is my code:

public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();                        
        }

        protected override void OnPaint(PaintEventArgs e)
        {
            base.OnPaint(e);            
            DrawLShapeLine(groupBox1.CreateGraphics(), 10, 10, 20, 40);
        }

        public void DrawLShapeLine(System.Drawing.Graphics g, int intMarginLeft, int intMarginTop, int intWidth, int intHeight)
        {
            Pen myPen = new Pen(Color.Black);
            myPen.Width = 2;
            // Create array of points that define lines to draw.
            int marginleft = intMarginLeft;
            int marginTop = intMarginTop;
            int width = intWidth;
            int height = intHeight;
            int arrowSize = 3;
            Point[] points =
             {
                new Point(marginleft, marginTop),
                new Point(marginleft, height + marginTop),
                new Point(marginleft + width, marginTop + height),
                // Arrow
                new Point(marginleft + width - arrowSize, marginTop + height - arrowSize),
                new Point(marginleft + width - arrowSize, marginTop + height + arrowSize),
                new Point(marginleft + width, marginTop + height)
             };

            g.DrawLines(myPen, points);
        }
    }

If I attach the DrawLShapeLine method to a button click event, it draws fine, but it does not draw on load of the form.

Please advice.

Answer 1

Hook up an event handler for the Paint event of the GroupBox and call DrawLShapeLine from within that event handler instead. You should then use the Graphics object supplied by in event arguments:

private void groupBox1_Paint(object sender, PaintEventArgs e)
{
    DrawLShapeLine(e.Graphics, 10, 10, 20, 40);
}

As your code looks now it will attempt to paint in the GroupBox when the form requires painting. The group box may be painted at any other occasion, which will the line you paint disappear.