How can I sync the scrolling of two multiline textboxes?

Question

How can I sync the scrolling of two multiline textboxes in C# (WinForms)?

When you scroll up/down a line in TextBox A, TextBox B should scroll up/down too. The same

Answer 1

Hans Passant's solution was awesome. However I needed to sync three text boxes not just two.

So I modified it a little - but all credence should go to Hans, there's no way I would have even got close without his work - I thought I would post it back here in case others need the same.

SyncBox class:

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

class SyncTextBox : TextBox
{
    public SyncTextBox()
    {
        this.Multiline = true;
        this.ScrollBars = ScrollBars.Vertical;
    }

    public Control[] Buddies { get; set; }

    private static bool scrolling;   // In case buddy tries to scroll us
    protected override void WndProc(ref Message m)
    {
        base.WndProc(ref m);
        // Trap WM_VSCROLL message and pass to buddy
        if (Buddies != null)
        {
            foreach (Control ctr in Buddies)
            {
                if (ctr != this)
                {
                    if ((m.Msg == 0x115 || m.Msg == 0x20a) && !scrolling && ctr.IsHandleCreated)
                    {
                        scrolling = true;
                        SendMessage(ctr.Handle, m.Msg, m.WParam, m.LParam);
                        scrolling = false;
                    }
                }
            }
        }
    }
    [DllImport("user32.dll", CharSet = CharSet.Auto)]
    private static extern IntPtr SendMessage(IntPtr hWnd, int msg, IntPtr wp, IntPtr lp);
}

Then in the form initilizer:

// add the required controls into scroll sync
Control[] syncedCtrls = new Control[] { ctrl1, ctrl2, ..., ctrln };
foreach (SyncTextBox ctr in syncedCtrls)
{
    ctr.Buddies = syncedCtrls;
}