1. Open your WinForms project in Visual Studio
2. Drag ComboBoxBase from Toolbox to Form
3. Drag ListBox from Toolbox to Form
4. In ComboBoxBase Properties, set ListControl = listBox1

using Syncfusion.Windows.Forms.Tools;
using System.Windows.Forms;

public partial class Form1 : Form
{
    private ComboBoxBase comboBoxBase1;
    private ListBox listBox1;
    
    public Form1()
    {
        InitializeComponent();
        SetupComboBoxBase();
    }
    
    private void SetupComboBoxBase()
    {
        // Create ComboBoxBase and ListBox
        comboBoxBase1 = new ComboBoxBase();
        listBox1 = new ListBox();
        
        // Configure ComboBoxBase
        comboBoxBase1.Size = new Size(200, 25);
        comboBoxBase1.Location = new Point(20, 20);
        comboBoxBase1.ListControl = listBox1; // Connect ListBox
        
        // Add items to ListBox
        listBox1.Items.AddRange(new object[] {
            "Option 1",
            "Option 2",
            "Option 3",
            "Option 4"
        });
        
        // Add to form
        this.Controls.Add(listBox1);
        this.Controls.Add(comboBoxBase1);
    }
}

Imports Syncfusion.Windows.Forms.Tools
Imports System.Windows.Forms

Public Class Form1
    Private comboBoxBase1 As ComboBoxBase
    Private listBox1 As ListBox
    
    Public Sub New()
        InitializeComponent()
        SetupComboBoxBase()
    End Sub
    
    Private Sub SetupComboBoxBase()
        ' Create ComboBoxBase and ListBox
        comboBoxBase1 = New ComboBoxBase()
        listBox1 = New ListBox()
        
        ' Configure ComboBoxBase
        comboBoxBase1.Size = New Size(200, 25)
        comboBoxBase1.Location = New Point(20, 20)
        comboBoxBase1.ListControl = listBox1 ' Connect ListBox
        
        ' Add items to ListBox
        listBox1.Items.AddRange(New Object() {
            "Option 1",
            "Option 2",
            "Option 3",
            "Option 4"
        })
        
        ' Add to form
        Me.Controls.Add(listBox1)
        Me.Controls.Add(comboBoxBase1)
    End Sub
End Class

// Create data source
List<string> states = new List<string> 
{ 
    "California", "Texas", "Florida", "New York" 
};

// Set DataSource on ListBox (not ComboBoxBase)
listBox1.DataSource = states;
comboBoxBase1.ListControl = listBox1;

// Use CheckedListBox for multi-select
CheckedListBox checkedListBox1 = new CheckedListBox();
checkedListBox1.Items.AddRange(new object[] {
    "Item 1", "Item 2", "Item 3", "Item 4"
});

comboBoxBase1.ListControl = checkedListBox1;

// Handle DropDownCloseOnClick to prevent premature closing
comboBoxBase1.DropDownCloseOnClick += (sender, e) => {
    e.Cancel = true; // Keep dropdown open while selecting
};

// Update text with checked items
checkedListBox1.ItemCheck += (sender, e) => {
    // Build text from checked items
    var checkedItems = checkedListBox1.CheckedItems.Cast<object>();
    comboBoxBase1.TextBox.Text = string.Join(", ", checkedItems);
};

public class Employee
{
    public int Id { get; set; }
    public string Name { get; set; }
    public override string ToString() => Name;
}

// Create employee list
List<Employee> employees = new List<Employee>
{
    new Employee { Id = 1, Name = "John Doe" },
    new Employee { Id = 2, Name = "Jane Smith" },
    new Employee { Id = 3, Name = "Bob Johnson" }
};

listBox1.DataSource = employees;
listBox1.DisplayMember = "Name"; // Display property
listBox1.ValueMember = "Id";     // Value property

comboBoxBase1.ListControl = listBox1;