Hi Morris,

When there're more items in a ListBox that can show in the ListBox, a
vertical scroll bar will appear automatically. This is a default behavior.

In fact, we could use the Win32 API function SetWindowLong to remove the
vertical scroll bar, if present, within a ListBox.

However, even if we use the SetWindowLong function to ensure a ListBox
doesn't have a vertical scroll bar at present, the vertical scroll bar
would appear again if the user navigates the items by pressing the down
arrow key. So we need to remove the veritcal scroll bar in the ListBox's
SelectedIndexChanged event handler or in the override
OnSelectedIndexChanged method.

In addition, I bind a ComboBox and a ListBox to a same data source in my
test and I find that if I select the last item from the ComboBox so that
the selected item in the ListBox changes also, the vertical scroll bar
appears instantaneously in the ListBox.

I use Spy++ to monitor the Windows messages that the ListBox receives in
the above case and I find two messages: WM_PRINT and WM_PRINTCLIENT. If I
filter these two Windows messages, the vertical scroll bar never appears in
this case.

The following is a sample code for you.

using System;
using System.Windows.Forms;
using System.Runtime.InteropServices;

class MyListBox:ListBox
   {
       int WM_PRINT = 0x0317;
       int WM_PRINTCLIENT = 0x0318;

       int WS_VSCROLL = 0x00200000;
       int GWL_STYLE = -16;
       int SWP_FRAMECHANGED = 0x0020;
       int SWP_NOMOVE = 0x0002;
       int SWP_NOSIZE = 0x0001;
       int SWP_NOZORDER = 0x0004;

       [DllImport("user32.dll")]
       static extern int GetWindowLong(IntPtr hWnd, int nIndex);
       [DllImport("user32.dll")]
       static extern int SetWindowLong(IntPtr hWnd, int nIndex, int
dwNewLong);
       [DllImport("user32.dll")]
       static extern bool SetWindowPos(IntPtr hWnd, IntPtr
hWndInsertAfter, int X, int Y, int cx, int cy, int uFlags);

       protected override void WndProc(ref Message m)
       {
           if (m.Msg == WM_PRINT || m.Msg == WM_PRINTCLIENT)
           { }
           else
           {
               base.WndProc(ref m);
           }          
      }

       private void SetNonVerScrollbar()
       {
           int style = GetWindowLong(this.Handle, GWL_STYLE);
           style = style & ~WS_VSCROLL;
           SetWindowLong(this.Handle, GWL_STYLE, style);
           SetWindowPos(this.Handle, IntPtr.Zero, 0, 0, 0, 0, SWP_NOMOVE |
SWP_NOSIZE | SWP_NOZORDER | SWP_FRAMECHANGED);
       }
       
       protected override void OnSelectedIndexChanged(EventArgs e)
       {  
           SetNonVerScrollbar();
           base.OnSelectedIndexChanged(e);      
           
       }
   }

Hope this helps.
If you have any question, please feel free to let me know.

Sincerely,
Linda Liu
Microsoft Online Community Support

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