我按照书上的例子编了一个简单的绘图程序，可是运行时提示出错：List index out of bounds(0)

Question

我按照书上的例子编了一个简单的绘图程序，可是运行时提示出错：
Project Project1.exe rasied exception clss ElistError with message 'List index out of bound(0)!'Process stopped.Use Step or Run to continue.
再运行提示List index out of bounds(0) 后，可以执行。
这是什么原因呢？

Answer 1

代码比较长，是这样的：unit Unit1;interfaceuses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, ExtCtrls, ComCtrls, ToolWin, ImgList, Menus, ColorGrd, StdCtrls;type
  TForm1 = class(TForm)
    StatusBar1: TStatusBar;
    Panel1: TPanel;
    Panel2: TPanel;
    CoolBar1: TCoolBar;
    ToolBar1: TToolBar;
    ToolBar2: TToolBar;
    ToolBar3: TToolBar;
    ToolBar4: TToolBar;
    line: TToolButton;
    pencil: TToolButton;
    eraser: TToolButton;
    square: TToolButton;
    fsquare: TToolButton;
    rsquare: TToolButton;
    frsquare: TToolButton;
    circle: TToolButton;
    fcircle: TToolButton;
    ImageList1: TImageList;
    ColorGrid1: TColorGrid;
    PaintBox1: TPaintBox;
    PopupMenu1: TPopupMenu;
    N1: TMenuItem;
    SaveDialog1: TSaveDialog;
    procedure LineClick(Sender: TObject);
    procedure ColorGrid1Change(Sender: TObject);
    procedure PaintBox1MouseDown(Sender: TObject; Button: TMouseButton;
      Shift: TShiftState; X, Y: Integer);
    procedure PaintBox1MouseUp(Sender: TObject; Button: TMouseButton;
      Shift: TShiftState; X, Y: Integer);
    procedure PaintBox1MouseMove(Sender: TObject; Shift: TShiftState; X,
      Y: Integer);
    procedure FormCreate(Sender: TObject);
    procedure displayhint(sender:Tobject);
    procedure N1Click(Sender:Tobject);
  private
  IsDown:boolean;
  startP,endP:Tpoint;
    { Private declarations }
  public
    { Public declarations }
  end;var
  Form1: TForm1;implementation{$R *.dfm}procedure SetButtonValid;
var
 i:integer;
begin
  for i:=0 to form1.ComponentCount -1 do
  begin
    if form1.Components[i] is TToolbutton then
    (form1.Components [i] as TToolbutton).Down :=false;
  end;
end;procedure TForm1.LineClick(Sender: TObject);
begin
 if sender is Ttoolbutton then
   begin
      setbuttonvalid;  //引用自定义过程setbuttonvalid
      with sender as Ttoolbutton do        down:=true;
   end;
end;
procedure TForm1.ColorGrid1Change(Sender: TObject);
begin
paintbox1.Canvas .pen.Color :=colorgrid1.ForegroundColor ;
end;
procedure TForm1.PaintBox1MouseDown(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
       IsDown:=true;          //将鼠标按下的标志设置为真，表示开始画
       if pencil.down then                    //如果用铅笔画
          paintbox1.Canvas .MoveTo(x,y);
       if line.Down then                      //如果画直线
       begin
         startp.x:=x;                    //设置开始和结束点坐标
         startp.y:=y;
         endp.x:=x;
         endp.y:=y;
       end;
       if square.down or rsquare.down or circle.down then //如果是绘制空心矩形
                                                  //或是空心圆角矩形，空心椭圆
       begin
          startp.x:=x;                    //设置开始和结束点坐标
          startp.y:=y;
          endp.x:=x;
          endp.y:=y;
       end;       if Fsquare.down or Frsquare.down or Fcircle.down then //如果是绘制
                           //实心矩形或是实心圆角矩形，实心椭圆
       begin
          paintbox1.Canvas .brush.Color :=colorgrid1.BackgroundColor ;
                                           //设置填充颜色          startp.x:=x;                    //设置开始和结束点坐标
          startp.y:=y;
          endp.x:=x;
          endp.y:=y;
       end;
end;procedure TForm1.PaintBox1MouseUp(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  IsDown:=false;
end;

Answer 2

procedure TForm1.PaintBox1MouseMove(Sender: TObject; Shift: TShiftState; X,
  Y: Integer);
begin
       if Isdown then                    //如鼠标被按下，则进行分析
       begin
            if pencil.down then          //如使用铅笔，直接连线到新的点
               paintbox1.Canvas.LineTo(x,y) ;    
            if line.down then               //如是绘制直线
            begin
             with paintbox1 do
             begin                         //对Paintbox的canvas进行设置，包括
               canvas.Pen.color:=clwhite;  //画笔颜色、起始坐标点位置
               canvas.MoveTo (startp.x,startp.y);
               canvas.LineTo (endp.X,endp.Y );
               canvas.pen.Color :=colorgrid1.ForegroundColor;
               canvas.MoveTo(startp.x,startP.y);
               canvas.LineTo (x,y);
               endp.x:=x;
               endp.y:=y;
             end;
           end;           if eraser.down then              //如用橡皮
           begin
              with paintbox1.Canvas do
              begin
                 brush.Color :=clwhite;   //设置橡皮擦除后面板的颜色及
                 FillRect(rect(x-10,y-10,x+15,y+15)); //擦除的图案区域
              end;
           end;           if square.down then           //如选择绘制矩形
           begin
              with paintbox1.Canvas  do
              begin
                brush.Color :=clwhite;
                framerect(rect(startp.x,startp.Y,endp.X,endp.y));//设置画刷
                endp.X:=x;                                    //颜色和绘制
                endp.Y:=y;                                   //矩形框的区域
                brush.Color:=colorgrid1.ForegroundColor;
                framerect(rect(startp.x,startp.Y,endp.X,endp.y));
              end;
            end;            if Fsquare.down then           //如选择绘制带有颜色填充矩形
            begin
              with paintbox1.Canvas  do
              begin
                brush.Color :=clwhite;
                fillrect(rect(startp.x,startp.Y,endp.X,endp.y));//设置绘制
                endp.X:=x;                                    // 矩形框的区域
                endp.Y:=y;                                   //和内部填充颜色
                brush.Color:=colorgrid1.BackgroundColor;
                fillrect(rect(startp.x,startp.Y,endp.X,endp.y));
                brush.Color:=colorgrid1.ForegroundColor;
                framerect(rect(startp.x,startp.Y,endp.X,endp.y));
              end;
            end;
            if Rsquare.down then           //如选择绘制圆角矩形
            begin
              with paintbox1.Canvas  do
              begin
                brush.Color :=clwhite;           //设置圆角矩形区域
                pen.color:=clwhite;
                     roundrect(startp.x,startp.y,endp.x,endp.y,(startp.x-endp.x) div 2,(startp.y-endp.y) div 2);
                     endp.x:=x;
                       endp.y:=y;
                       pen.color:=colorgrid1.ForegroundColor; //设置画笔的颜色
                       roundrect(startp.x,startp.y,endp.x,endp.y,(startp.x-endp.x) div 2,(startp.y-endp.y) div 2) ;               end;
            end;            if FRsquare.down then           //如是用色彩填充圆角矩形
            begin
              with paintbox1.Canvas  do
              begin
                brush.Color :=clwhite;
                pen.color:=clwhite;
                     roundrect(startp.x,startp.y,endp.x,endp.y,(startp.x-endp.x) div 2,(startp.y-endp.y) div 2);
                endp.x:=x;                             //设置圆角矩形的区域
                endp.y:=y;                             // 和内部填充颜色
                pen.color:=colorgrid1.ForegroundColor;
                brush.Color:=colorgrid1.BackgroundColor;
                    roundrect(startp.x,startp.y,endp.x,endp.y,(startp.x-endp.x) div 2,(startp.y-endp.y) div 2);
              end;
            end;            if circle.down then            //如选择绘制椭圆
            begin
              with paintbox1.Canvas do
              begin
                pen.color:=clwhite;
                arc(startp.x,startp.y,endp.x,endp.y,startp.x,startp.y,startp.x,startp.y);
                                         //设置画笔的颜色和绘制椭圆的区域
                endp.x:=x;
                endp.y:=y;
                pen.color:=colorgrid1.ForegroundColor;
                arc(startp.x,startp.y,endp.x,endp.y,startp.x,startp.y,startp.x,startp.y);
              end;
            end;            if Fcircle.down then         //如选择绘制带有彩色填充的椭圆
            begin
               with paintbox1.Canvas do
               begin
                  brush.color:=clwhite;
                  pen.color:=clwhite;
                  ellipse(startp.x,startp.y,endp.x,endp.y);//设置画笔颜色、
                  endp.x:=x;                            //椭圆内部填充颜色、
                  endp.y:=y;                            //椭圆形的区域
                  pen.color:=colorgrid1.ForegroundColor;
                  brush.Color:=Colorgrid1.BackgroundColor;
                  ellipse(startp.x,startp.y,endp.x,endp.y);
                  arc(startp.x,startp.y,endp.x,endp.y,startp.x,startp.y,startp.x,startp.y);
               end;
            end;
      end;end;
procedure TForm1.FormCreate(Sender: TObject);
begin
   application.onhint:=displayhint;
end;procedure TForm1.displayhint(sender:Tobject);
begin
   statusbar1.panels[0].text:=application.hint;
   statusbar1.refresh;
end;procedure TForm1.N1Click(Sender:Tobject);
var
    rect: Trect;
    filename: string;
begin
   if savedialog1.Execute then
     rect.left :=0;
     rect.top:=0;
     rect.right:=paintbox1.width;
     rect.bottom:=paintbox1.height;
     with Tbitmap.create do
     begin
        try
         width:=rect.right;
          height:=rect.bottom;
          canvas.Copyrect(rect,paintbox1.canvas,rect) ;
          filename:=savedialog1.FileName;
          if extractfileext(filename)=''then
          Filename:=filename+'.bmp';
          saveTofile(filename);
        finally
        free;
        end;
      end;
end;end.

Answer 3

运行后的程序基本可以执行，只是好像有一点bug.凡是鼠标按下后移动的位置，就会被清除

Answer 4

这个很正常，paintbox不会保存你画的东东，一旦form有刷新，就没了。你可以改用timage控件来实现