C# WinApp อยากทำ textbox ที่มี combobox อยู่ข้างในครับ แต่ไม่รู้จะสับเปลี่ยนกันยังไง

[ToolboxBitmap(typeof(TextBox))]
    public class TextBoxCombobox : TextBox
    {

        private readonly ComboBox _cmb;
       // private TextBox _TextBox { get; set; }
        public TextBoxCombobox()
        {
            _cmb = new  ComboBox
            {
                Cursor = Cursors.Hand,
                TabStop = false
            };
            _cmb.SizeChanged += (o, e) => { OnResize(e); };
            this.Controls.Add(_cmb);
            _cmb.Size = this.Size;
            _cmb.Location = this.Location;
           _cmb.Items.AddRange(new object[] { "TTT1", "TTT2", "TTT3", "TTT4", "TTT5", });
            this.ReadOnly = true;
        }
        protected override void OnResize(EventArgs e)
        {
            base.OnResize(e);
            _cmb.Size = this.Size;
            _cmb.Location = this.Location;
        }
        private void textBox1_KeyDown(object sender, System.Windows.Forms.KeyEventArgs e)
        {
            this.Hide();
            _cmb.Show();
        }
        private void TextBoxBase1_MouseClick(Object sender, MouseEventArgs e)
        {
            this.Hide();
            _cmb.Show();
        }
        
    }