Question

I am using Visual Studio 2010 (C#) and a Windows Forms application.

I have two treeviews side by side, and I have figured out how to synchronise the scrolling using the up/down buttons on the scrollbar, but when I use the slider it does not move the other treeview. I have taken a listview example that works, but the same code does not work for treeviews.

So far I have, in the main form:

    [DllImport("User32.dll")]
    public static extern int SendMessage(IntPtr hWnd, uint Msg, uint wParam, uint lParam);

    private void myListBox1_Scroll(ref Message m)
    {
        SendMessage(myListBox2.Handle, (uint)m.Msg, (uint)m.WParam, (uint)m.LParam);
    }

I have created a control:

public partial class MyTreeView : TreeView
{
    public MyTreeView()
    {
        InitializeComponent();
    }

    public event ScrollEventHandler Scroll;
    public delegate void ScrollEventHandler(ref Message m);


    private const int WM_VSCROLL = 0x115;

    protected override void WndProc(ref System.Windows.Forms.Message m)
    {
        if (m.Msg == WM_VSCROLL)
            if (Scroll != null)
            {
                Scroll(ref m);
            }

        base.WndProc(ref m);
    }

}

which I add two of to the form.

I can use the same code to have a listivew control the treeview and that will work if you drag the slider, but in reverse it only works with the up down buttons.

Answer 1

Rather than use SendMessage and mark your DLL as unsafe you can use the GetScrollPos and SetScrollPos functions from user32.dll.

I’ve wrapped the code up into your MyTreeView class so it’s nicely encapsulated.

You just need to call the AddLinkedTreeView method like so:

treeView1.AddLinkedTreeView(treeView2);

Here’s the source for the MyTreeView class.

public partial class MyTreeView : TreeView
{
    public MyTreeView() : base()
    {
    }

    private List<MyTreeView> linkedTreeViews = new List<MyTreeView>();

    /// <summary>
    /// Links the specified tree view to this tree view.  Whenever either treeview
    /// scrolls, the other will scroll too.
    /// </summary>
    /// <param name="treeView">The TreeView to link.</param>
    public void AddLinkedTreeView(MyTreeView treeView)
    {
        if (treeView == this)
            throw new ArgumentException("Cannot link a TreeView to itself!", "treeView");

        if (!linkedTreeViews.Contains(treeView))
        {
            //add the treeview to our list of linked treeviews
            linkedTreeViews.Add(treeView);
            //add this to the treeview's list of linked treeviews
            treeView.AddLinkedTreeView(this);

            //make sure the TreeView is linked to all of the other TreeViews that this TreeView is linked to
            for (int i = 0; i < linkedTreeViews.Count; i++)
            {
                //get the linked treeview
                var linkedTreeView = linkedTreeViews[i];
                //link the treeviews together
                if (linkedTreeView != treeView)
                    linkedTreeView.AddLinkedTreeView(treeView);
            }
        }
    }

    /// <summary>
    /// Sets the destination's scroll positions to that of the source.
    /// </summary>
    /// <param name="source">The source of the scroll positions.</param>
    /// <param name="dest">The destinations to set the scroll positions for.</param>
    private void SetScrollPositions(MyTreeView source, MyTreeView dest)
    {
        //get the scroll positions of the source
        int horizontal = User32.GetScrollPos(source.Handle, Orientation.Horizontal);
        int vertical = User32.GetScrollPos(source.Handle, Orientation.Vertical);
        //set the scroll positions of the destination
        User32.SetScrollPos(dest.Handle, Orientation.Horizontal, horizontal, true);
        User32.SetScrollPos(dest.Handle, Orientation.Vertical, vertical, true);
    }

    protected override void WndProc(ref Message m)
    {
        //process the message
        base.WndProc(ref m);

        //pass scroll messages onto any linked views
        if (m.Msg == User32.WM_VSCROLL || m.Msg == User32.WM_MOUSEWHEEL)
        {
            foreach (var linkedTreeView in linkedTreeViews)
            {
                //set the scroll positions of the linked tree view
                SetScrollPositions(this, linkedTreeView);
                //copy the windows message
                Message copy = new Message
                {
                    HWnd = linkedTreeView.Handle,
                    LParam = m.LParam,
                    Msg = m.Msg,
                    Result = m.Result,
                    WParam = m.WParam
                };
                //pass the message onto the linked tree view
                linkedTreeView.RecieveWndProc(ref copy);
            }                               
        }
    }

    /// <summary>
    /// Recieves a WndProc message without passing it onto any linked treeviews.  This is useful to avoid infinite loops.
    /// </summary>
    /// <param name="m">The windows message.</param>
    private void RecieveWndProc(ref Message m)
    {
        base.WndProc(ref m);
    }

    /// <summary>
    /// Imported functions from the User32.dll
    /// </summary>
    private class User32
    {
        public const int WM_VSCROLL = 0x115;
        public const int WM_MOUSEWHEEL = 0x020A;  

        [DllImport("user32.dll", CharSet = CharSet.Auto)]
        public static extern int GetScrollPos(IntPtr hWnd, System.Windows.Forms.Orientation nBar);

        [DllImport("user32.dll")]
        public static extern int SetScrollPos(IntPtr hWnd, System.Windows.Forms.Orientation nBar, int nPos, bool bRedraw);
    }
}

Edit: Added forwarding of the WM_MOUSEWHEEL message as per MinnesotaFat’s suggestion.