ComboBox açılır pencere genişliğini değiştirmek

const
  PIXEL_WIDTH = 250;
begin
  ComboBox1.Perform(CB_SETDROPPEDWIDTH, PIXEL_WIDTH, 0);

ComboBox1.Perform(CB_SETDROPPEDWIDTH, 250, 0);

var
i, x, xx : integer;
begin
  with ComboBox1 do begin
  if Items.Count > 0 then begin
  xx := 0;
  for i := 0 to Items.Count -1 do begin
  x := Canvas.TextWidth(Items[i] +#32#32);
  if x > xx then
    xx := x;
  end;

  if Items.Count > DropDownCount then
    Inc(xx, GetSystemMetrics(SM_CYHSCROLL));

    if xx > Width then
      Perform(CB_SETDROPPEDWIDTH, xx, 0);
    end;
  end;
end;

unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;

type
  TForm1 = class(TForm)
    ComboBox1: TComboBox;
    ComboBox2: TComboBox;
    ComboBox3: TComboBox;
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;
  cwpHandle: THandle;
  SaveRight:Integer;

implementation

{$R *.dfm}

procedure AutoSetComboxDropDownWidth( AComboBox: TComboBox );
var
  i: Integer;
  MaxWidth: Integer;
  CurWidth: Integer;
  TempSize: SIZE;
  TempHDC: HDC;
  SaveFont: HFont;
begin
  MaxWidth := AComboBox.Width; //(GetSystemMetrics(SM_CXVSCROLL)*2);
  TempHDC := GetDC(0);
  try
    for i := 0 to AComboBox.Items.Count-1 do
  begin
    SaveFont := SelectObject( TempHDC, AComboBox.Font.Handle );
    Windows.GetTextExtentPoint32( TempHDC, PChar(AComboBox.Items[i]),
    Length(AComboBox.Items[i]),
    TempSize );
    SelectObject( TempHDC, SaveFont );

    //if scrollbar needed.
  if AComboBox.Items.Count>AComboBox.DropDownCount then
    //The 8 seems to provide a centering effect between left and right.
    CurWidth := TempSize.cx + (GetSystemMetrics(SM_CXVSCROLL)+8)
    else
      CurWidth := TempSize.cx+8;
    if CurWidth>MaxWidth then
    MaxWidth := CurWidth;
  end;
  finally
    ReleaseDC(0, TempHDC );
  end;
    SaveRight := AComboBox.ClientOrigin.X + AComboBox.Width;
    AComboBox.Perform( CB_SETDROPPEDWIDTH, MaxWidth, 0 );
end;

procedure MoveDropDownListIfNecessary( AComboBoxListHandle: LongInt );
var
  R: TRect;
begin
  GetWindowRect( AComboBoxListHandle, R );

  if R.Right>=Screen.Width then
  begin
    MoveWindow( AComboBoxListHandle, SaveRight-(R.Right-R.Left)-1, R.Top,
    R.Right-R.Left, R.Bottom-R.Top, True );
  end;
end;

function HookCallbackFunction(nCode: Integer; wParam: LongInt;
lParam: LongInt): Integer; stdcall;
var
  wNotifyCode: Integer;
  Control: TWinControl;
  ListHandle: LongInt;
begin
  Result := CallNextHookEx(cwpHandle,nCode,wParam,lParam);
  // Don't do anything when program is closing
  if (Application.Terminated) or (nCode<0) then
    Exit;

    if nCode=HC_ACTION then
    begin
      if PCWPStruct(LParam)^.message = WM_COMMAND then
      begin
        wNotifyCode := HIWORD(PCWPStruct(LParam)^.wParam);

        if wNotifyCode = CBN_DROPDOWN then
        begin
          Control := FindControl( PCWPStruct(LParam)^.lParam );

          if (Control<>nil) and (Control is TComboBox) then
          AutoSetComboxDropDownWidth( TComboBox( Control ) );
        end;
      end
      else if PCWPStruct(LParam)^.message = WM_CTLCOLORLISTBOX then
      begin
        ListHandle := PCWPStruct(LParam)^.lParam;
        MoveDropDownListIfNecessary( ListHandle );
    end;
  end;
end;

initialization
  cwpHandle := SetWindowsHookEx( WH_CALLWNDPROC, @HookCallbackFunction, 0, MainThreadID );

finalization
  if cwpHandle <> 0 then
  UnhookWindowsHookEx( cwpHandle );

end.