为什么这个线程引发异常？

Question

我正在尝试在示例项目中创建一个线程，但我在示例项目代码中遇到了异常

unit Unit1;

interface

uses
  Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants,
  System.Classes, Vcl.Graphics,
  Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls;

type
  TForm1 = class(TForm)
    Button1: TButton;
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

type
  TURLDownload = class(TThread)
  private
    FURL: String;
    Fnameofimg: string;
    FPathImage: string;
    FFileNameImage: string;
    // Internal //
    ImageName: string;
    PathURL: string;
  protected
    procedure Execute; override;
  public
    constructor Create(const AUrl: String; Const AOutPathImages: string;
      Anameofimg: String); reintroduce;
    destructor Destroy; override;
    property URL: string read FURL write FURL;
    property PathImage: string read FPathImage;
    property FileNameImage: string read FFileNameImage;
  end;

var
  Form1: TForm1;
  th: TURLDownload;

implementation

{$R *.dfm}
{ TURLDownload }

procedure TURLDownload.reached;
begin
showmessage('done');
end;


    constructor TURLDownload.Create(const AUrl, AOutPathImages: string;
      Anameofimg: String);
    begin
      inherited Create(False);
      FreeOnTerminate := True;
      FURL := AUrl;
      Fnameofimg := Anameofimg;
      FPathImage := AOutPathImages;
    end;

    destructor TURLDownload.Destroy;
    begin

      inherited;
    end;

    procedure TURLDownload.Execute;
    begin
      synchronize(reached);
    end;

    procedure TForm1.Button1Click(Sender: TObject);
    begin
      th.Create('jgvjk', 'ngkj', 'jkgfjk');
    end;

    end.

当我点击 button1 开始创建线程时，我遇到了这条异常消息

First chance exception at [=12=]4C0384. Exception class $C0000005 with message 'access violation at 0x004c0384: read of address 0x0000003c'. Process Project1.exe (4060)

然后当我点击打破它 return 我到系统类线程内的文件创建在此代码

  FSuspended := not FExternalThread;

我做错了什么？我正在使用 Delphi xe7

Answer 1

您应该使用
创建线程对象 th := TURLDownload.Create('jgvjk', 'ngkj', 'jkgfjk');

其他问题：

在您的线程主体中，您使用 showmessage('Reached'); 调用 VCL window，但没有同步。如果没有某种同步，您不应该与 VCL 人员一起工作 - 使用同步或队列。

reintroduce 不需要非虚拟构造函数

inherited 在 Execute

中什么都不做

为什么这个线程引发异常？

Why this thread raise exception?

delphi

multithreading

delphi-xe7