How to make ComboBox drop-down list be *Narrower* than the ComboBox itself


Is it possible to make a ComboBox Drop-down list be <em>Narrower</em> than the Combobox itself?

There are plenty of examples setting the width using SendMessage(Handle, CB_SETDROPPEDWIDTH, 100, 200);

but the <em>minimum</em> value is taken from the combobox itself, regardless of what is specificed here.

All these examples make it bigger.


Before the drop-down list is to be painted, a <a href="https://msdn.microsoft.com/en-us/library/windows/desktop/bb761360(v=vs.85).aspx" rel="nofollow">WM_CTLCOLORLISTBOX</a> message is issued.

By overriding the combobox WindowProc it is possible to shrink the drop down list width.

The WM_CTLCOLORLISTBOX message is detected and since the message supplies the handle of the list window, we can grab the list bounds and call MoveWindow with a shrinked width.

type TMyForm = class(TForm) ... ComboBox1 : TComboBox; procedure FormCreate(Sender: TObject); ... private { Private declarations } ComboBox1WindowProcORIGINAL : TWndMethod; procedure ComboBox1WindowProc(var Message: TMessage); ... end; procedure TMyForm.ComboBox1WindowProc(var Message: TMessage); var lbr: TRect; begin //drawing the list box with combobox items if Message.Msg = WM_CTLCOLORLISTBOX then begin //list box rectangle GetWindowRect(Message.LParam, lbr); //Shrink window width MoveWindow( Message.LParam, lbr.Left, lbr.Top, 50, // New width lbr.Bottom-lbr.Top, false); end; ComboBox1WindowProcORIGINAL(Message); end; procedure TMyForm.FormCreate(Sender: TObject); begin //attach custom WindowProc for ComboBox1 ComboBox1WindowProcORIGINAL := ComboBox1.WindowProc; ComboBox1.WindowProc := ComboBox1WindowProc; end;

<a href="https://i.stack.imgur.com/41KMq.png" rel="nofollow"><img alt="enter image description here" class="b-lazy" data-src="https://i.stack.imgur.com/41KMq.png" data-original="https://i.stack.imgur.com/41KMq.png" src="https://etrip.eimg.top/images/2019/05/07/timg.gif" /></a> <a href="https://i.stack.imgur.com/JnPOV.png" rel="nofollow"><img alt="enter image description here" class="b-lazy" data-src="https://i.stack.imgur.com/JnPOV.png" data-original="https://i.stack.imgur.com/JnPOV.png" src="https://etrip.eimg.top/images/2019/05/07/timg.gif" /></a>

<hr />

You can make a small hack by creating an interposer class. Either put it in a separate unit and declare it after vcl.StdCtrls, or put it in your form unit.

type TComboBox = class(vcl.StdCtrls.TComboBox) private FDropDownWidth : Integer; function GetDropDownWidth : Integer; protected procedure WndProc(var Mess: TMessage); override; public Constructor Create( aOwner: TComponent ); override; property DropDownWidth : Integer read GetDropDownWidth write FDropDownWidth; end; constructor TComboBox.Create(aOwner: TComponent); begin inherited; DropDownWidth := -1; // Default state end; function TComboBox.GetDropDownWidth: Integer; begin if FDropDownWidth = -1 then // Just keep a default state Result := Self.Width else Result := FDropDownWidth; end; procedure TComboBox.WndProc(var Mess: TMessage); var lbr: TRect; begin if Mess.Msg = WM_CTLCOLORLISTBOX then begin //list box rectangle GetWindowRect(Mess.LParam, lbr); //Shrink window width MoveWindow( Mess.LParam, lbr.Left, lbr.Top, DropDownWidth, lbr.Bottom-lbr.Top, false); end else if Mess.Msg = CB_SETDROPPEDWIDTH then DropDownWidth := Mess.WParam; Inherited WndProc(Mess); end;

Either set the drop-down width with cb.Perform(CB_SETDROPPEDWIDTH,newWidth,0); or cb.DropDownWidth := newWidth;


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