delphi自动调整combobox下拉列表宽度（PostMessage CB_SETDROPPEDWIDTH）

在combobox所在的窗口的Formshow事件里调用即可

procedure SetComboBoxListWidth( AComboBox: TComboBox );
var
  i: Integer;
  nMaxLen, nMinWidth: integer;
  nFontWidth: Integer;
  nCboLeft: integer;
  ctlCustom: TControl;
begin
  nCboLeft := AComboBox.Left;
  ctlCustom := AComboBox;
  with AComboBox do
  begin
    nFontWidth := Round( Abs(Font.Height / 2 ) );
    nMaxLen:= 0;
    for i:=0 to Items.Count-1 do
    begin
      if length(Items[i])* nFontWidth > nMaxLen then
        nMaxlen:= length(Items[i])* nFontWidth+5;
    end;
    if Items.Count > DropDownCount then
      nMaxLen := nMaxLen + 20;
    if nMaxLen > Width then
    begin
      if Items.Count>DropDownCount then
      begin
        SendMessage( Handle, CB_SETHORIZONTALEXTENT, nMaxLen+5, 0 );

        {解决分辨率小导致的ComboboxList的宽度超出屏幕}
        //while 部分是用来取出combobox控件相对与窗体的横坐标
        while not (ctlCustom.Parent is TForm) do
        begin
          nCboLeft := nCboLeft + ctlCustom.Parent.Left;
          ctlCustom := ctlCustom.Parent;
        end;
        nMinWidth := Min(400, nMaxLen);                      // 使用不大于nMaxLen的数做比较
        if (nCboLeft + nMinWidth) > Screen.Width-25 then     // 不超出屏幕，并保留窗口滚动条宽度，约25
          nMinWidth := Screen.Width-25-nCboLeft;
        nMinWidth := Max(nMinWidth, Width);                  // 不小于控件自身宽度

        PostMessage(Handle, CB_SETDROPPEDWIDTH, nMinWidth, 0);
      end
      else
        PostMessage(Handle, CB_SETDROPPEDWIDTH, nMaxLen , 0);

      ShowHint := True;
    end
    else
    begin
      SendMessage( Handle, CB_SETHORIZONTALEXTENT, 0, 0 );
      PostMessage(Handle, CB_SETDROPPEDWIDTH, Width , 0);
    end;
  end;
end;

http://blog.csdn.net/youthon/article/details/8179348