How can I speed up custom rendering in WinForms?

Question

.NET

I am working on a WinForms application in which I need to implement spell checking.
To redraw the form, I use the Windows message interception.

/// <summary>
        /// Processes Windows messages.
        /// </summary>
        /// <param name="msg">
        /// A Windows Message object.
        /// </param>
        protected override void WndProc(ref Message msg)
        {
            switch (msg.Msg)
            {
                case WM_PAINT:
                    Invalidate();
                    base.WndProc(ref msg);

                    if (IsSpellingAutoEnabled)
                    {
                        CustomPaint();
                    }

                    break;

            ...

                default:
                    base.WndProc(ref msg);
                    break;
            }
        }

The CustomPaint() method looks like this:

private void CustomPaint()
        {
            Bitmap tempBitmap;
            Graphics bufferGraphics;

            tempBitmap = new Bitmap(Width, Height);
            bufferGraphics = Graphics.FromImage(tempBitmap);
            bufferGraphics.Clip = new Region(ClientRectangle);

            _textBoxGraphics = CreateGraphics();
            bufferGraphics.Clear(Color.Transparent);

            foreach (var wordStartIndex in UnderlinedSections.Keys)
            {
                UnderlineWords(wordStartIndex, bufferGraphics);
            }

            _textBoxGraphics.DrawImageUnscaled(tempBitmap, 0, 0);
        }

In UnderlineWords, lines are added to the clipboard to be drawn with help. this method:

private void DrawWave(Graphics bufferGraphics, Point startOfLine, Point endOfLine)
        {
            startOfLine.Y--;
            endOfLine.Y--;

            // kvv: Определяем цвет линий подчеркивания.
            var pen = Pens.Red;

            if ((endOfLine.X - startOfLine.X) >= 4)
            {
                var points = new ArrayList();
                for (int i = startOfLine.X; i <= (endOfLine.X - 2); i += 4)
                {
                    points.Add(new Point(i, startOfLine.Y));
                    points.Add(new Point(i + 2, startOfLine.Y + 2));
                }

                var p = (Point[])points.ToArray(typeof(Point));
                bufferGraphics.DrawLines(pen, p);
            }
        }

When this code is running on text larger than 500 characters, strong brakes and application freezes begin. How can you get rid of this? In which direction to dig?

Answer 1

You don't have to do that. You have a lot of logic to draw, of course, it will be wildly slow to work.
Take a standard RichTextBox and do something like this:

richTextBox1.Text = "blah blah meeh";
richTextBox1.SelectionStart = 5;
richTextBox1.SelectionLength = 4;
richTextBox1.SelectionFont = new Font(richTextBox1.SelectionFont, FontStyle.Underline);
// еще варианты:
//richTextBox1.SelectionColor = Color.Red;
//richTextBox1.SelectionBackColor = Color.Yellow;
//повторить для все слов с ошибками

If it is critical that the display be standard (red waveline under black text), then expand the RichTextBox: geekswithblogs.net/pvidler/archive/2003/10/15/188.aspx