private void btnclr_Click(object sender, EventArgs e)
        {
            Label lbl;
            int num;
            string tag;
            try
            {
                Enabled = false;
                this.Cursor = Cursors.WaitCursor;
                panel.SuspendLayout();
                foreach (Control cntrl in panel.Controls)
                {
                    lbl = cntrl as Label;
                    if (lbl != null)
                    {
                        tag = lbl.Tag as string;
                        num = 1 + Convert.ToInt32(tag.Split('|')[1]);
                        lbl.Text = num.ToString();
                    }
                }
                for (int i = 0; i < 9; i++)
                    for (int j = 0; j < 9; j++)
                        _mat[i, j] = i;
            }
            finally
            {
                this.Cursor = Cursors.Default;
                Enabled = true;
                panel.ResumeLayout(true);
            }
        }

lbl.ForeColor = lbl.AllowDrop ? Color.Blue : Color.Black; // Set hard numbers Black, and user numbers Blue
//could you explain how does it work? is it like if allowdrop=true then -->blue else black? I've never seen this kind of thing, so simple and beautiful!
 
                            passes = 0;                                 // Going to test to see if this is a good value. But limit it to 9 tries.
                            while (passes++ < 9 && !PassChecks())       // Check to see if this number in this position is okay
                            {
                                value = rd.Next(1, 9);                  // Now we want a good number so min is 1, and max is 9
                                lbl.Text = value.ToString();            // Set the value as text
                                _mat[row, col] = value;                 // set the Matrix value, go back and test, if we fall out of the loop
                            }                                           // then either it passed,
                            if (passes >= 9)                            // or failed, in which case we set it to nothing and the matrix to 0
                            {
                                lbl.Text = string.Empty;
                                _mat[row, col] = 0;
                            }
                        }
 
//comment after the code:

private void panel_Paint(object sender, PaintEventArgs e)
        {
            int rowLineOffset = panel.Height / 3;
            int colLineOffset = panel.Width / 3;
            Pen pen = new Pen(Brushes.Red);
            pen.Width = 3;
            pen.DashStyle = System.Drawing.Drawing2D.DashStyle.Dot;
            for (int i = 1; i < 3; i++)
            {
                e.Graphics.DrawLine(pen, new Point(3, i * rowLineOffset - i * 2), new Point(panel.Width -3, i * rowLineOffset - i * 2));
                e.Graphics.DrawLine(pen, new Point(i * colLineOffset - i * 2, 3), new Point(i * colLineOffset - i * 2, panel.Height -3));
            }
            pen.Dispose();
        }