// 1. Drag ComboDropDown and TreeView onto form
// 2. Add nodes to TreeView (in designer or code)
// 3. Set TreeView.HideSelection = false
// 4. Set ComboDropDown.PopupControl = treeView1

// Event handlers for interaction
private void comboDropDown1_DropDown(object sender, EventArgs e)
{
    // Before dropdown shown: select TreeNode matching combo text
    if (!string.IsNullOrEmpty(this.comboDropDown1.Text))
    {
        TreeNode matchedNode = FindNode(this.treeView1.Nodes, this.comboDropDown1.Text);
        this.treeView1.SelectedNode = matchedNode;
    }
}

private void treeView1_DoubleClick(object sender, EventArgs e)
{
    // Transfer selected node text to combo
    if (this.treeView1.SelectedNode != null)
    {
        this.comboDropDown1.Text = this.treeView1.SelectedNode.Text;
    }
    
    // Close the dropdown
    this.comboDropDown1.PopupContainer.HidePopup(PopupCloseType.Done);
}

private TreeNode FindNode(TreeNodeCollection nodes, string text)
{
    // Recursively find matching node
    foreach (TreeNode child in nodes)
    {
        if (child.Text == text) return child;
    }
    foreach (TreeNode child in nodes)
    {
        TreeNode matched = FindNode(child.Nodes, text);
        if (matched != null) return matched;
    }
    return null;
}

using Syncfusion.Windows.Forms.Tools;

// Create ComboDropDown
ComboDropDown comboDropDown1 = new ComboDropDown();
comboDropDown1.Location = new Point(20, 20);
comboDropDown1.Size = new Size(200, 25);

// Create and configure TreeView
TreeView treeView1 = new TreeView();
treeView1.HideSelection = false;
treeView1.Nodes.Add("Root");
treeView1.Nodes[0].Nodes.Add("Child 1");
treeView1.Nodes[0].Nodes.Add("Child 2");

// Associate TreeView with ComboDropDown
comboDropDown1.PopupControl = treeView1;

// Wire up events
comboDropDown1.DropDown += comboDropDown1_DropDown;
treeView1.DoubleClick += treeView1_DoubleClick;

// Add to form
this.Controls.Add(comboDropDown1);

// Setup
comboDropDown1.PopupControl = treeView1;
treeView1.HideSelection = false;

// Sync text → TreeView selection (before dropdown)
comboDropDown1.DropDown += (s, e) => {
    var node = FindNode(treeView1.Nodes, comboDropDown1.Text);
    treeView1.SelectedNode = node;
};

// Sync TreeView selection → text (on double-click)
treeView1.DoubleClick += (s, e) => {
    if (treeView1.SelectedNode != null)
    {
        comboDropDown1.Text = treeView1.SelectedNode.Text;
        comboDropDown1.PopupContainer.HidePopup(PopupCloseType.Done);
    }
};

// Host a GridList control for multi-column data
GridListControl gridList = new GridListControl();
// Configure grid columns and data...

comboDropDown1.PopupControl = gridList;

// Sync selection to combo text
gridList.SelectedIndexChanged += (s, e) => {
    if (gridList.SelectedIndex >= 0)
    {
        comboDropDown1.Text = gridList.SelectedItem.ToString();
    }
};

// Make dropdown control respond to mouse immediately
comboDropDown1.PopupContainer.Popup += (s, e) => {
    // Give focus to hosted control after dropdown shown
    treeView1.Focus();
};