最近做Winform项目需要用到类似ComboBox的TreeView控件。

虽然各种第三方控件很多，但是存在各种版本不兼容问题。所以自己写了个简单的ComboTreeView控件。

下图是实现效果：

目前实现的比较简单，能满足我项目中的需求。

此处是项目中的代码简化后的版本，供大家参考。

using System;
using System.Collections.Generic;
using System.Windows.Forms;

namespace CustomControl.Tree
{
    public abstract class ComboTreeView<T> : ComboBox where T : class
    {
        protected const int WM_LBUTTONDOWN = 0x0201, WM_LBUTTONDBLCLK = 0x0203;

        protected TreeView treeView;
        protected ToolStripControlHost treeViewHost;
        protected ToolStripDropDown dropDown;
        protected bool dropDownOpen = false;
        protected TreeNode selectedNode;
        protected T toBeSelected;

        public ComboTreeView(TreeView internalTreeView)
        {
            if (null == internalTreeView)
            {
                throw new ArgumentNullException("internalTreeView");
            }
            this.InitializeControls(internalTreeView);
        }

        public event TreeNodeChangedEventHandler TreeNodeChanged;

        protected virtual void InitializeControls(TreeView internalTreeView)
        {
            this.treeView = internalTreeView;
            this.treeView.BorderStyle = BorderStyle.FixedSingle;
            this.treeView.Margin = new Padding(0);
            this.treeView.Padding = new Padding(0);
            this.treeView.AfterExpand += new TreeViewEventHandler(this.WhenAfterExpand);

            this.treeViewHost = new ToolStripControlHost(this.treeView);
            this.treeViewHost.Margin = new Padding(0);
            this.treeViewHost.Padding = new Padding(0);
            this.treeViewHost.AutoSize = false;

            this.dropDown = new ToolStripDropDown();
            this.dropDown.Margin = new Padding(0);
            this.dropDown.Padding = new Padding(0);
            this.dropDown.AutoSize = false;
            this.dropDown.DropShadowEnabled = true;
            this.dropDown.Items.Add(this.treeViewHost);
            this.dropDown.Closed += new ToolStripDropDownClosedEventHandler(this.OnDropDownClosed);

            this.DropDownWidth = this.Width;
            base.DropDownStyle = ComboBoxStyle.DropDownList;
            base.SizeChanged += new EventHandler(this.WhenComboboxSizeChanged);
        }

        public new ComboBoxStyle DropDownStyle
        {
            get { return base.DropDownStyle; }
            set { base.DropDownStyle = ComboBoxStyle.DropDownList; }
        }

        public virtual TreeNode SelectedNode
        {
            get { return this.selectedNode; }
            private set { this.treeView.SelectedNode = value; }
        }

        public virtual T SelectedNodeData
        {
            get { return (T)base.SelectedItem; }
            set
            {
                this.toBeSelected = value;
                this.UpdateComboBox(value);
            }
        }

        protected new int SelectedIndex
        {
            get { return base.SelectedIndex; }
            set { base.SelectedIndex = value; }
        }

        protected new object SelectedItem
        {
            get { return base.SelectedItem; }
            set { base.SelectedItem = value; }
        }

        public virtual string DisplayMember { get; set; } = "Name";

        /// <summary>Gets the internal TreeView control.</summary>
        public virtual TreeView TreeView => this.treeView;

        /// <summary>Gets the collection of tree nodes that are assigned to the tree view control.</summary>
        /// <returns>A <see cref="T:System.Windows.Forms.TreeNodeCollection" /> that represents the tree nodes assigned to the tree view cont