I recently had a problem when I tried to synchronize the scrolling between 2 RichTextBoxes - that was because these controls behave different than normal TextBoxes. However I managed to solve this…after some time, and I decided to post the solution. The idea is based on overriding one RichTextBox’s WndProc() and then sending its scroll position to the other one.

The advantage of this method is that the synchronization is based on the RichTextBox’s real position - not on the thumb/scrollbar position.

1.Fixed problems

These 2 problems are fixed if you use this method:

  1. If you press enter, while the caret is on the last line, the RichTextBox scrolls a little bit more than it should while the other one scrolls normally.
  2. If you delete the last lines in a RichTextBox, it doesn’t scroll down - but the other one will keep scrolling.

2. How to

As I said before, you need to create a custom RichTextBox, based on the original one, so you can override WndProc().

A short schema:

syncRichTextBox (object)
1) scroll
2) tell anotherRichTextBox to scroll to my position

anotherRichTextBox
1) idle - doing nothing but waiting for messages from syncRichTextBox

First, include this function, from user32.dll

1
2

[DllImport("user32.dll", CharSet = CharSet.Auto)]
private static extern int SendMessage(IntPtr hWnd, int wMsg, IntPtr wParam, ref Point lParam);
  • notice the lParam argument is a Point - this will store the scroll’s position.

Then, add these WM contants, you’ll need them when you synchronize the scrolling:

1
2
3
4
5
6
7
8
9

private const int WM_VSCROLL = 0x115;  //tells the control to scroll

private const int WM_GETDLGCODE = 0x87;   //sent when the caret is going out of the 'visible area' (so scroll is needed)

private const int WM_MOUSEFIRST = 0x200;  //scrolls if the mouse leaves the 'visible area' (example when you select text)

private const int EM_GETSCROLLPOS = 0x4DD;  //you send this message and the control returns it's scroll position

private const int EM_SETSCROLLPOS = 0x4DE;//this is used to set the control's scroll position

Now, you only have to create the custom RichTextBox - if its scroll position is changed, it will send a message containing its new position to the other RichTextBox (anotherRichTextBox) - the 2nd one doesn’t require overriding.

The code would look like this:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23

public class syncRichTextBox : RichTextBox
{
      public syncRichTextBox()
      {
              protected override void WndProc(ref Message m)
              {
                    base.WndProc(ref m);

                    if (m.Msg == WM_VSCROLL || m.Msg == WM_GETDLGCODE || m.Msg == WM_MOUSEFIRST)
                    {
                          Point p = new Point();

                          //the scroll position is returned in the variable p (point)
                          SendMessage(this.Handle, EM_GETSCROLLPOS, IntPtr.Zero, ref p);  

                          //sends the position to the other richtextbox (remember to replace its name)
                          SendMessage(anotherRichTextBox.Handle, EM_SETSCROLLPOS, IntPtr.Zero, ref p);  

                    }

              }
      }
}