System.Windows.Forms TreeView Drag-Drop with Dotted Box Around Drop Target

WinFormsTreeViewDragDropWithDottedBoxAroundTarget.cs

using System;
using System.Drawing;
using System.Windows.Forms;

namespace WinFormsApp1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();

            // These should be set in the designer, but I've put them here for reference.
            this.treeView1.AllowDrop = true;
            this.treeView1.ItemDrag += new System.Windows.Forms.ItemDragEventHandler(this.treeView1_ItemDrag);
            this.treeView1.DragDrop += new System.Windows.Forms.DragEventHandler(this.treeView1_DragDrop);
            this.treeView1.DragOver += new System.Windows.Forms.DragEventHandler(this.treeView1_DragOver);

            // These relate to drawing a dotted rectangle around the drop target.
            this.treeView1.DrawMode = System.Windows.Forms.TreeViewDrawMode.OwnerDrawText;
            this.treeView1.DrawNode += new System.Windows.Forms.DrawTreeNodeEventHandler(this.treeView1_DrawNode);
            this.treeView1.DragLeave += new System.EventHandler(this.treeView1_DragLeave);

        }

        private void Form1_Load(object sender, EventArgs e)
        {
            // Generate some nodes.
            Random rnd = new Random();
            void GenerateNodes(TreeNodeCollection parent, int count, float backoff)
            {
                var count_ = rnd.Next(count * 2);
                for (var i = 0; i < count_; i++)
                {
                    var node = parent.Add("Node " + i.ToString());
                    GenerateNodes(node.Nodes, (int)(count * backoff), backoff);
                }
            };
            GenerateNodes(treeView1.Nodes, 30, .5f);
        }

        private void treeView1_ItemDrag(object sender, ItemDragEventArgs e)
        {
            treeView1.DoDragDrop(
                e.Item,
                DragDropEffects.Copy | DragDropEffects.Move | DragDropEffects.Link);
        }

        private Tuple<TreeNode,TreeNode> GetDropSourceAndTarget(TreeView treeview, DragEventArgs e)
        {
            var source = e.Data.GetData(typeof(TreeNode)) as TreeNode;
            Point targetPoint = treeView1.PointToClient(new Point(e.X, e.Y));
            var target = treeView1.GetNodeAt(targetPoint);
            return Tuple.Create(source, target);
        }

        private void treeView1_DragOver(object sender, DragEventArgs e)
        {
            var source_target = GetDropSourceAndTarget(treeView1, e);
            var source = source_target.Item1;
            var target = source_target.Item2;

            bool IsSameOrDescendant(TreeNode parent, TreeNode descendant)
            {
                if (descendant == parent) return true;
                if (descendant.Parent == null) return false;
                return IsSameOrDescendant(parent, descendant.Parent);
            }

            if (target == null)
                e.Effect = DragDropEffects.None;
            else if (IsSameOrDescendant(target, source) || IsSameOrDescendant(source, target))
                e.Effect = DragDropEffects.None;
            else if ((e.KeyState & 8) != 0) // CTRL
                e.Effect = DragDropEffects.Copy;
            else if ((e.KeyState & 4) != 0) // SHIFT
                e.Effect = DragDropEffects.Link;
            else if ((e.KeyState & 32) != 0) // ALT
                e.Effect = DragDropEffects.None;
            else
                e.Effect = DragDropEffects.Move;

            // Update the dotted rectangle around the drop target.
            if (e.Effect == DragDropEffects.None)
                target = null;
            UpdateDragDropHoverTargetIndicator(target);
        }

        private void treeView1_DragDrop(object sender, DragEventArgs e)
        {
            // Clear the dotted rectangle around the drop target.
            UpdateDragDropHoverTargetIndicator(null);

            var source_target = GetDropSourceAndTarget(treeView1, e);
            var source = source_target.Item1;
            var target = source_target.Item2;
            MessageBox.Show(e.Effect.ToString() + " " + source.ToString() + " to " + target.ToString());
        }

        TreeNode dragDropHoverTarget;

        private void UpdateDragDropHoverTargetIndicator(TreeNode node)
        {
            // If the drop hover target has not changed, there is nothing to do.
            if (dragDropHoverTarget == node)
                return;

            // "Invalidate" the part of the screen where the previous drop target
            // was so that the rectangle is cleared and where the new drop target
            // is so that the new rectangle is drawn (but skip if either is null).
            if (dragDropHoverTarget != null)
                treeView1.Invalidate(dragDropHoverTarget.Bounds);
            if (node != null)
                treeView1.Invalidate(node.Bounds);

            // Update the target.
            dragDropHoverTarget = node;

            // Redraw the invalidated regions.
            treeView1.Update();
        }

        private void treeView1_DrawNode(object sender, DrawTreeNodeEventArgs e)
        {
            // DrawMode must be set to OwnerDrawText.

            if (e.Node == dragDropHoverTarget)
            {
                // Draw a dotted rectangle around the drop target.

                // Get the bounds of the node's drawing area.
                var bounds = e.Node.Bounds;
               
                // Clear the area for the node.
                e.Graphics.FillRectangle(new SolidBrush(treeView1.BackColor), bounds);

                // Draw the Node's text.
                // (On my computer, using 'bounds.Y - 1' was needed to prevent the text
                // from jumping, but on another computer it caused jumping. It may be
                // difficult to match text rendering exactly without rendering all
                // nodes ourselves.)
                // See https://stackoverflow.com/questions/63121949/positioning-and-highlighting-of-treeview-node-text-with-ownerdrawtext-mode.
                TextFormatFlags textFlags = TextFormatFlags.Left | TextFormatFlags.NoClipping
                           | TextFormatFlags.NoPrefix | TextFormatFlags.SingleLine;
                TextRenderer.DrawText(e.Graphics, e.Node.Text,
                    e.Node.NodeFont ?? treeView1.Font,
                    bounds,
                    treeView1.ForeColor, textFlags);

                // Draw the dotted rectangle.
                using (Pen pen = new Pen(new SolidBrush(treeView1.ForeColor)))
                {
                    // The right and bottom edges of the rectangle are drawn one pixel
                    // outside of the actual bounds of the node unless we bring it in.
                    bounds.Inflate(-1, -1);
                    pen.DashStyle = System.Drawing.Drawing2D.DashStyle.Dot;
                    e.Graphics.DrawRectangle(pen, bounds);
                }

                return;
            }

            // Let the platform draw other nodes.
            e.DrawDefault = true;
        }

        private void treeView1_DragLeave(object sender, EventArgs e)
        {
            // Clear the dotted rectangle around the drop target.
            // This event is fired if the drag is cancelled via pressing ESC,
            // in addition to drag leaving this control.
            UpdateDragDropHoverTargetIndicator(null);
        }
    }
}