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

Yes, you'll have to create a custom text box so you can detect it scrolling. The trick is to pass the scroll message to the other text box so it will scroll in sync. This really only works well when that other text box is about the same size and has the same number of lines.

Add a new class to your project and paste the code shown below. Compile. Drop two of the new controls from the top of the toolbox onto your form. Set the Buddy property to the other control on both. Run, type some text in both of them and watch them scroll in sync as you drag the scrollbar.

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

class SyncTextBox : TextBox {
    public SyncTextBox() {
        this.Multiline = true;
        this.ScrollBars = ScrollBars.Vertical;
    }
    public Control Buddy { 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 (m.Msg == 0x115 && !scrolling && Buddy != null && Buddy.IsHandleCreated) {
            scrolling = true;
            SendMessage(Buddy.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);
}

Answer 2

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;
}

Answer 3

You can change this line:

if (m.Msg == 0x115) && !scrolling && Buddy != null && Buddy.IsHandleCreated)

to this:

if ((m.Msg == 0x115 || m.Msg==0x20a) && !scrolling && Buddy != null && Buddy.IsHandleCreated)

and it will support scrolling with the mouse wheel as well.

Answer 4

Here is what finally helped me to fix synchronization of multiple textboxes using mouse wheel.

I based it on very helpful Hans example.

int WM_MOUSEWHEEL   = 0x20a; // or 522
int WM_VSCROLL      = 0x115; // or 277

protected override void WndProc(ref Message m)
{
        //Trap WM_VSCROLL and WM_MOUSEWHEEL message and pass to buddy
        if (Buddies != null)
        {

            if (m.Msg == WM_MOUSEWHEEL)  //mouse wheel 
            {

                if ((int)m.WParam < 0)  //mouse wheel scrolls down
                    SendMessage(this.Handle, (int)0x0115, new IntPtr(1), new IntPtr(0)); //WParam: 1- scroll down, 0- scroll up
                else if ((int)m.WParam > 0)
                    SendMessage(this.Handle, (int)0x0115, new IntPtr(0), new IntPtr(0));



                return; //prevent base.WndProc() from messing synchronization up 
            }
            else if (m.Msg == WM_VSCROLL)
            {
                foreach (Control ctr in Buddies)
                {
                    if (ctr != this && !scrolling && ctr != null && ctr.IsHandleCreated)
                    {
                        scrolling = true;
                        SendMessage(ctr.Handle, m.Msg, m.WParam, m.LParam);
                        scrolling = false;
                    }

                }

            }
        }

    //do the usual
    base.WndProc(ref m);
}

Answer 5

Hans Passant's solution worked like a charm but I needed a RichTextBox with both horizontal and vertical scrollbars. If you extend a RichTextBox instead of a TextBox you'll need to change the ScrollBars property accordingly (I used RichTextBoxScrollBars.Both).

If you want to sync horizontal scrolling as well, look for (m.Msg == 0x115) || (m.Msg == 0x114).