How Can I Optimize RichTextBox Repainting for Real-Time Syntax Highlighting?

Disabling Repaint for Real-Time RichTextBox Syntax Highlighting

In certain programming scenarios, you might need to dynamically highlight keywords or specific words in a RichTextBox as the user types. However, constant repainting can cause flickering and discomfort during input.

To improve the user experience, you can disable the RichTextBox's automatic repaint while you're editing its text. Unfortunately, the standard RichTextBox class doesn't provide built-in methods for this.

Customizing RichTextBox with an External Class

One solution is to create a custom RichTextBox class that adds the missing functionality. Here's an example:

using System;
using System.Windows.Forms;
using System.Runtime.InteropServices;

class MyRichTextBox : RichTextBox {
    public void BeginUpdate() {
        SendMessage(this.Handle, WM_SETREDRAW, (IntPtr)0, IntPtr.Zero);
    }
    public void EndUpdate() {
        SendMessage(this.Handle, WM_SETREDRAW, (IntPtr)1, IntPtr.Zero); 
        this.Invalidate();
    }
    [DllImport("user32.dll")]
    private static extern IntPtr SendMessage(IntPtr hWnd, int msg, IntPtr wp, IntPtr lp);
    private const int WM_SETREDRAW = 0x0b;
}

You can then use the BeginUpdate() and EndUpdate() methods in your custom function that highlights keywords in real time:

void HighlightKeywords(MyRichTextBox richTextBox) {
    richTextBox.BeginUpdate();

    // Highlight keywords and bad words

    richTextBox.EndUpdate();
}

Directly Controlling Repaint Using P/Invoke

Alternatively, you can bypass using a custom class and directly control repaint using the SendMessage method with the WM_SETREDRAW message.

Before updating the RichTextBox's text:

[DllImport("user32.dll")]
static extern IntPtr SendMessage(IntPtr hWnd, int msg, IntPtr wp, IntPtr lp);
const int WM_SETREDRAW = 0x0b;

// (Disable repainting)
SendMessage(richTextBox.Handle, WM_SETREDRAW, (IntPtr)0, IntPtr.Zero);

After updating the RichTextBox's text:

// (Enable repainting)
SendMessage(richTextBox.Handle, WM_SETREDRAW, (IntPtr)1, IntPtr.Zero);
richTextBox.Invalidate();

This approach allows you to achieve the same result without modifying the standard RichTextBox class.

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