C#

public partial class ComboEx : ComboBox
{
    internal ScrollbarEx vScrollBar;
    NativeListWindow listControl;
    public ComboEx()
    {
        InitializeComponent();
        DropDownHeight = 100;
        vScrollBar = new ScrollbarEx();
    }

    protected override void OnPaint(PaintEventArgs pe)
    {
        base.OnPaint(pe);
    }

    protected override void OnHandleCreated(EventArgs e)
    {
        base.OnHandleCreated(e);
        COMBOBOXINFO combInfo = new COMBOBOXINFO();
        combInfo.cbSize = Marshal.SizeOf(combInfo);
        Win32.GetComboBoxInfo( this.Handle, ref combInfo );

        listControl = new NativeListWindow(this, combInfo.hwndList);
    }

    protected override void WndProc(ref Message m)
    {
         if (m.Msg == (Win32.WM_REFLECT + Win32.WM_COMMAND))
        {
            if (Win32.HIWORD( (int)m.WParam ) == Win32.CBN_DROPDOWN)
            {
                COMBOBOXINFO combInfo = new COMBOBOXINFO();
                combInfo.cbSize = Marshal.SizeOf(combInfo);
                Win32.GetComboBoxInfo( this.Handle, ref combInfo );

                vScrollBar.Location = new Point( this.Width-23, 1 );
                vScrollBar.Size = new Size( 23, DropDownHeight );
                vScrollBar.Visible = true;
                Win32.SetParent(vScrollBar.Handle, combInfo.hwndList);
                Win32.ShowWindow(vScrollBar.Handle, ShowWindowCommands.Show);
                Win32.SetWindowPos(vScrollBar.Handle,HWND.TopMost, 155, 1, 23, 105, SetWindowPosFlags.SWP_SHOWWINDOW);
            }
        }
        base.WndProc(ref m);
    }

    // MyNativeWindow class to create a window given a class name.
    [System.Security.Permissions.PermissionSet(System.Security.Permissions.SecurityAction.Demand, Name = "FullTrust")]
    internal class NativeListWindow : NativeWindow
    {
        // Constant values were found in the "windows.h" header file.
        private const int WS_CHILD = 0x40000000,
                          WS_VISIBLE = 0x10000000,
                          WM_ACTIVATEAPP = 0x001C;

        private int windowHandle;

        private ComboEx parent;

        public NativeListWindow(ComboEx owner,IntPtr handle)
        {
            AssignHandle(handle);
            parent = owner;

        }

        // Listen to when the handle changes to keep the variable in sync
        [System.Security.Permissions.PermissionSet(System.Security.Permissions.SecurityAction.Demand, Name = "FullTrust")]
        protected override void OnHandleChange()
        {
            windowHandle = (int)this.Handle;
        }

        private void AdjustClientRect(ref RECT rect)
        {
            rect.right -= 23;
        }

        [System.Security.Permissions.PermissionSet(System.Security.Permissions.SecurityAction.Demand, Name = "FullTrust")]
        protected override void WndProc(ref Message message)
        {
            // Listen for messages that are sent to the button window. Some messages are sent 
            // to the parent window instead of the button's window. 

            switch (message.Msg)
            {
                case Win32.NCCALCSIZE:
                    {
                        if (message.WParam != IntPtr.Zero)
                        {
                            NCCALCSIZE_PARAMS rcsize = (NCCALCSIZE_PARAMS)Marshal.PtrToStructure(message.LParam, typeof(NCCALCSIZE_PARAMS));
                            AdjustClientRect(ref rcsize.rect0);
                            Marshal.StructureToPtr(rcsize, message.LParam, false);
                        }
                        else
                        {
                            RECT rcsize = (RECT)Marshal.PtrToStructure(message.LParam, typeof(RECT));
                            AdjustClientRect(ref rcsize);
                            Marshal.StructureToPtr(rcsize, message.LParam, false);
                        }
                        message.Result = new IntPtr(1);
                        return;
                        break;
                    }

                    case Win32.WM_NCMOUSEMOVE:
                    {
                        base.WndProc(ref message);
                        Win32.SendMessage(parent.vScrollBar.Handle, (uint)message.Msg, message.WParam, message.LParam);
                        break;
                        break;
                    }
                    case Win32.WM_NCLBUTTONDOWN:
                    {
                        base.WndProc(ref message);
                        Win32.SendMessage(parent.vScrollBar.Handle, (uint)message.Msg, message.WParam, message.LParam);
                        break;
                    }
                    case Win32.WM_NCACTIVATE:
                    {
                        base.WndProc(ref message);
                        Win32.SendMessage(parent.vScrollBar.Handle, (uint)message.Msg, message.WParam, message.LParam);
                        break;
                    }
                    case Win32.WM_NCMOUSELEAVE:
                    {
                        base.WndProc(ref message);
                        Win32.SendMessage(parent.vScrollBar.Handle, (uint)message.Msg, message.WParam, message.LParam);
                        break;
                    }
                    case Win32.WM_NCLBUTTONUP:
                    {
                        base.WndProc(ref message);
                        Win32.SendMessage(parent.vScrollBar.Handle, (uint)message.Msg, message.WParam, message.LParam);
                        break;
                    }
                    case Win32.WM_NCHITTEST:
                    {
                        base.WndProc(ref message);
                        Win32.SendMessage(parent.vScrollBar.Handle, (uint)message.Msg, message.WParam, message.LParam);
                        break;
                    }
                    case Win32.WM_MOUSEMOVE:
                    {
                        base.WndProc(ref message);
                        if ((int)message.LParam > 0)
                        {
                            int x = Win32.LOWORD((int)message.LParam);
                            int y = Win32.HIWORD((int)message.LParam);
                            RECT rect = new RECT(); ;
                            Win32.GetWindowRect(new HandleRef(parent.vScrollBar, parent.vScrollBar.Handle),out rect);
                            Rectangle rc = new Rectangle(parent.vScrollBar.Location.X, parent.vScrollBar.Location.Y,
                                (rect.right - rect.left), (rect.bottom - rect.top));
                            if (rc.Contains(new Point(x, y)))
                            {
                                Win32.SetFocus(parent.vScrollBar.Handle);
                                Win32.SetWindowPos(parent.vScrollBar.Handle, HWND.TopMost, 155, 1, 23, 105, SetWindowPosFlags.SWP_SHOWWINDOW);
                                Win32.SendMessage(parent.vScrollBar.Handle, (uint)message.Msg, message.WParam, message.LParam);
                            }
                        }
                        break;
                    }
            }
            base.WndProc(ref message);
        }
    }
}

if (e.KeyCode == Keys.Enter && (sender as TextEdit).Text.Trim() != "")
{
    this.Cursor = Cursors.WaitCursor;

    is_form_exists = false;

    foreach (Form search_resul_form in this.MdiChildren)
    {
        frmSearchResult search_form = search_resul_form as frmSearchResult;

        if ((string)search_form.Tag == "BROWSE_PATIENTS")
        {
            // search_form.Activated(null, null);
            search_form.find_what = "FILE";
            search_form.find_text1 = (sender as TextEdit).Text.Trim();
            search_form.find_text2 = null;
            search_form.browse_result();
            search_form.Activate();
            is_form_exists = true;
            break;
        }
    }

    if (is_form_exists == false)
    {
        frmSearchResult SearchResultForm = new frmSearchResult(public_var);
        SearchResultForm.Tag = "BROWSE_PATIENTS";
        SearchResultForm.MdiParent = this;
        SearchResultForm.Dock = DockStyle.Fill;
        SearchResultForm.find_what = "FILE";
        SearchResultForm.find_text1 = (sender as TextEdit).Text.Trim();
        SearchResultForm.find_text2 = null;
        SearchResultForm.Show();
    }

    (sender as TextEdit).Text = "";

    this.Cursor = Cursors.Default;
}