delphi - Manually scrolling through a TComboBox -

I do not understand that due to programming drop down, manually scrolling through a TComboBox component Why I'm unable to select the list that is showing here is the code I am using:

  SendMessage (combo box 1.handle, cbi shohddraffen, integer (true), 0);   
 This causes the drop down pick list to show However, if I try to use my keyboard and use up and down arrow keys, then drop down The pickup list immediately closes. How do I keep a list with automatic shutdown when using the keyboard up / down navigation keys?   
 
 
 activate combo box and you want to use the  DroppedDown  property better , Which also denies Cambodia to reflect the change.  
  ActiveControl: = combobox1; Combo box 1. Dropdown: = True;