Question

Okay so in my DragDropManager I have set:

DragDropEffects.Scroll

as an allowed effect but for some reason it’s not working properly. It still is not scrolling at all when I am trying to scroll. Could someone please explain to me properly about this.

I am trying to use this while dragging my categories around.

I have checked the MSDN but it is really uninformative on what the feature actually does and how to use it.

I am sorry if this question is silly but I hope someone can help me out.

Answer 1

Two solutions can be found here. One is using SendMessage, the other is pure .NET code.

http://www.dotnet247.com/247reference/msgs/16/81341.aspx

(the site seems to be down, archived page can be found here)

Related windows message codes are described here:

ListView onScroll event

My resulting code is below. It is incomplete though in that aspect that I would like it to have ability to detect the existence of vertical and horizontal scrollbars and adjust the bottom and right scroll-trigger areas accordingly.

private void treeView_DragOver(object sender, DragEventArgs e)
{
    TreeView senderTree = sender as TreeView;

    TreeNode destinationNode;
    TreeNode newNode = GetDragTreeNode(out destinationNode, sender, e);

    if (newNode == null)
    {
        e.Effect = DragDropEffects.None;
        return;
    }

    int scrollDetectRangeU = 10;
    int scrollDetectRangeL = 10;
    int scrollDetectRangeB = 10 + 20;
    int scrollDetectRangeR = 10 + 20;
    Point treeCoord = CoordToTreeCoord(senderTree, e);

    if (treeCoord.Y < scrollDetectRangeU 
        && (destinationNode == null || destinationNode.PrevVisibleNode != null))
    {
        SendMessage(senderTree.Handle, WindowsMessages.WM_VSCROLL, ScrollBarCommands.SB_LINEUP, IntPtr.Zero);    //scroll up
        Thread.Sleep(10);   //slow down the scrolling a bit
    }
    else if (treeCoord.Y >= senderTree.Height - scrollDetectRangeB 
        && (destinationNode == null || destinationNode.NextVisibleNode != null))
    {
        SendMessage(senderTree.Handle, WindowsMessages.WM_VSCROLL, ScrollBarCommands.SB_LINEDOWN, IntPtr.Zero);    //scroll down
        Thread.Sleep(10);   //slow down the scrolling a bit
    }

    if (treeCoord.X < scrollDetectRangeL)
    {
        SendMessage(senderTree.Handle, WindowsMessages.WM_HSCROLL, ScrollBarCommands.SB_LINELEFT, IntPtr.Zero);     //scroll left
    }
    else if (treeCoord.X >= senderTree.Width - scrollDetectRangeR)
    {
        SendMessage(senderTree.Handle, WindowsMessages.WM_HSCROLL, ScrollBarCommands.SB_LINERIGHT, IntPtr.Zero);     //scroll right
    }

}

private static TreeNode GetDragTreeNode(out TreeNode destinationNode, object sender, DragEventArgs e)
{
    TreeNode newNode = null;

    if (e.Data.GetDataPresent("System.Windows.Forms.TreeNode", false))  
    {
        newNode = (TreeNode)e.Data.GetData("System.Windows.Forms.TreeNode");
    }

    if (newNode != null)
    {
        destinationNode = CoordToTreeNode(sender, e);    
    }
    else
    {
        destinationNode = null;
    }

    return newNode;
}

public static Point CoordToTreeCoord(object senderTree, DragEventArgs e)
{
    return CoordToTreeCoord(senderTree as TreeView, e.X, e.Y, true);
}

public static Point CoordToTreeCoord(TreeView senderTree, int x, int y, bool usePointToClient)
{
    Point pt = usePointToClient ? senderTree.PointToClient(new Point(x, y)) : new Point(x, y);

    return pt;
}

public static TreeNode CoordToTreeNode(object senderTree, DragEventArgs e)
{
    return CoordToTreeNode(senderTree as TreeView, e.X, e.Y, true);
}

public static TreeNode CoordToTreeNode(TreeView senderTree, int x, int y, bool usePointToClient)
{
    Point pt = CoordToTreeCoord(senderTree, x, y, usePointToClient);
    TreeNode destinationNode = senderTree.GetNodeAt(pt);

    return destinationNode;
}

[DllImport("user32.dll")]
static extern IntPtr SendMessage(IntPtr hWnd, Int32 Msg, ScrollBarCommands wParam, IntPtr lParam);  

public enum ScrollBarCommands : int
{
    SB_LINEUP = 0,
    SB_LINELEFT = 0,
    SB_LINEDOWN = 1,
    SB_LINERIGHT = 1,
    SB_PAGEUP = 2,
    SB_PAGELEFT = 2,
    SB_PAGEDOWN = 3,
    SB_PAGERIGHT = 3,
    SB_THUMBPOSITION = 4,
    SB_THUMBTRACK = 5,
    SB_TOP = 6,
    SB_LEFT = 6,
    SB_BOTTOM = 7,
    SB_RIGHT = 7,
    SB_ENDSCROLL = 8
}

public static class WindowsMessages
{
    // Windows messages 
    public const int WM_PAINT = 0x000F;
    public const int WM_HSCROLL = 0x0114;
    public const int WM_VSCROLL = 0x0115;
    public const int WM_MOUSEWHEEL = 0x020A;
    public const int WM_KEYDOWN = 0x0100;
    public const int WM_LBUTTONUP = 0x0202;

    // ScrollBar types 
    public const int SB_HORZ = 0;
    public const int SB_VERT = 1;

    // ScrollBar interfaces 
    public const int SIF_TRACKPOS = 0x10;
    public const int SIF_RANGE = 0x01;
    public const int SIF_POS = 0x04;
    public const int SIF_PAGE = 0x02;
    public const int SIF_ALL = SIF_RANGE | SIF_PAGE | SIF_POS | SIF_TRACKPOS;

    // ListView messages 
    public const uint LVM_SCROLL = 0x1014;
    public const int LVM_FIRST = 0x1000;
    public const int LVM_SETGROUPINFO = (LVM_FIRST + 147);

}