It is an oversight in the RichTextBox class. Other controls, like ListBox, support the BeginUpdate and EndUpdate methods to suppress painting. Those methods generate the WM_SETREDRAW message. RTB in fact supports this message, but they forgot to add the methods.
Just add them yourself. Project + Add Class, paste the code shown below. Compile and drop the control from the top of the toolbox onto your form.
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;
}
Or P/Invoke SendMessage directly before/after you update the text.