为什么 TList.Remove() 会产生 EAccessViolation 错误？

发布于 2024-07-08 17:26:29 字数 453 浏览 14 评论 0原文

为什么执行下面的代码时会引发 EAccessViolation？

uses
  Generics.Collections;
  ...

var
  list: TList<TNotifyEvent>;
  ...

begin
  list := TList<TNotifyEvent>.Create();
  try
    list.Add(myNotifyEvent);
    list.Remove(myNotifyEvent);  // EAccessViolation at address...
  finally
    FreeAndNil(list);
  end;
end;

procedure myNotifyEvent(Sender: TObject);
begin
  OutputDebugString('event');  // nebo cokoliv jineho
end;

原文

Why EAccessViolation is raised when executing the code below?

uses
  Generics.Collections;
  ...

var
  list: TList<TNotifyEvent>;
  ...

begin
  list := TList<TNotifyEvent>.Create();
  try
    list.Add(myNotifyEvent);
    list.Remove(myNotifyEvent);  // EAccessViolation at address...
  finally
    FreeAndNil(list);
  end;
end;

procedure myNotifyEvent(Sender: TObject);
begin
  OutputDebugString('event');  // nebo cokoliv jineho
end;

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深陷 2024-07-15 17:26:29

它看起来像一个错误。

如果您使用 debug dcu 进行编译（通常不要这样做，除非您想失去理智！）您会发现对比较器的调用出错了。比较函数的第三个值（可能是可选的）未设置并导致访问冲突。

因此，您可能无法将方法指针放入通用列表中。

好的，以下工作有效：

uses
  Generics.Defaults;

type
  TForm4 = class(TForm)
    ...
  private
    procedure myNotifyEvent(Sender: TObject);
  end;

TComparer<T> = class (TInterfacedObject, IComparer<T>)
public
  function Compare(const Left, Right: T): Integer;
end;

implementation

uses
  Generics.Collections;

var
  list: TList<TNotifyEvent>;
begin
  list := TList<TNotifyEvent>.Create(TComparer<TNotifyEvent>.Create);
  try
    list.Add(myNotifyEvent);
    list.Remove(myNotifyEvent);
  finally
    FreeAndNil(list);
  end;
end;

procedure TForm4.myNotifyEvent(Sender: TObject);
begin
  ShowMessage('event');
end;

{ TComparer<T> }

function TComparer<T>.Compare(const Left, Right: T): Integer;
begin
  Result := 0;
end;

您必须定义自己的比较器，并可能具有更多智能;-)。

It looks like a bug.

If you compile with debug dcu's (normally don't do that unless you want to loose your sanity!) you see that a call to the comparer went wrong. A (possibly optional) third value of a compare function is not set and causes the access violation.

So possibly you can't put method pointers in a generic list.

Ok the following works:

uses
  Generics.Defaults;

type
  TForm4 = class(TForm)
    ...
  private
    procedure myNotifyEvent(Sender: TObject);
  end;

TComparer<T> = class (TInterfacedObject, IComparer<T>)
public
  function Compare(const Left, Right: T): Integer;
end;

implementation

uses
  Generics.Collections;

var
  list: TList<TNotifyEvent>;
begin
  list := TList<TNotifyEvent>.Create(TComparer<TNotifyEvent>.Create);
  try
    list.Add(myNotifyEvent);
    list.Remove(myNotifyEvent);
  finally
    FreeAndNil(list);
  end;
end;

procedure TForm4.myNotifyEvent(Sender: TObject);
begin
  ShowMessage('event');
end;

{ TComparer<T> }

function TComparer<T>.Compare(const Left, Right: T): Integer;
begin
  Result := 0;
end;

You have to define your own comparer, with possiby some more intelligence ;-).

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等风也等你 2024-07-15 17:26:29

访问冲突是由于缺少比较器引起的。我怀疑这已在补丁中修复，但如果您使用 TObjectList，问题仍然存在（至少在 Delphi 2009 中），所以我只是用最简单的解决方案进行更新：

TList<TNotifyEvent>.Create(TComparer<TNotifyEvent>.Default);

或者就我而言

TObjectList<TNotifyEvent>.Create(TComparer<TNotifyEvent>.Default);

Access Violation is caused by missing comparer. I suspect this was fixed in a patch but the problem still persists (at least in Delphi 2009) if you use a TObjectList so I'm just updating with the simplest solution:

TList<TNotifyEvent>.Create(TComparer<TNotifyEvent>.Default);

or in my case

TObjectList<TNotifyEvent>.Create(TComparer<TNotifyEvent>.Default);

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甜中书 2024-07-15 17:26:29

是否可以将自定义比较器传递给 TList？我面前没有D2009，所以无法尝试。

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鱼窥荷 2024-07-15 17:26:29

上面的代码在TForm1中使用...

uses 
  Generics.Collections;

procedure TForm1.Button1Click(Sender: TObject);
var
  list: TList<TNotifyEvent>;
begin
  list := TList<TNotifyEvent>.Create();
  try
    list.Add(myNotifyEvent);
    list.Remove(myNotifyEvent);  // EAccessViolation at address...
  finally
    FreeAndNil(list);
  end;
end;
procedure TForm1.myNotifyEvent(Sender: TObject);
begin
  OutputDebugString('event');  // nebo cokoliv jineho
end;

the above code is used in TForm1 ...

uses 
  Generics.Collections;

procedure TForm1.Button1Click(Sender: TObject);
var
  list: TList<TNotifyEvent>;
begin
  list := TList<TNotifyEvent>.Create();
  try
    list.Add(myNotifyEvent);
    list.Remove(myNotifyEvent);  // EAccessViolation at address...
  finally
    FreeAndNil(list);
  end;
end;
procedure TForm1.myNotifyEvent(Sender: TObject);
begin
  OutputDebugString('event');  // nebo cokoliv jineho
end;

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