How Can I Disable Repainting in a RichTextBox for Real-Time Syntax Highlighting?

Disabling Repaint for Real-Time Syntax Highlighting in RichTextBox

You need to highlight keywords and bad words in a RichTextBox while the user types, but the excessive flickering caused by constant repainting is a concern. However, the standard method of overriding the "WndProc" function and intercepting the repaint message requires subclassing, which isn't feasible.

Fortunately, you can address this issue by extending the functionality of the RichTextBox class yourself. Here's how:

1. Create a custom RichTextBox class:

   • Add a new class named "MyRichTextBox" to your project.
   • Copy and paste the following code into the class definition:

   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;
   }       

2. Use the custom control:

   • Add the "MyRichTextBox" control to your toolbox from the "Custom Controls" section.
   • Drag and drop the control onto your form and use it as you would a regular RichTextBox.

3. Disable repainting:

   • Before performing any text editing or formatting within the custom RichTextBox, call the "BeginUpdate" method:

   myRichTextBox1.BeginUpdate();

4. Enable repainting:

   • After completing your text editing and formatting, call the "EndUpdate" method:

   myRichTextBox1.EndUpdate();

By using the custom RichTextBox control, you can now highlight keywords and bad words in real time without the distracting flickering caused by automatic repainting.

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