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

Question

为什么在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;

Answer 1

它看起来像一个bug.

如果使用debug dcu进行编译(通常不要这样做,除非你想要失去理智!)你会看到对比较器的调用出错了.未设置比较函数的(可能是可选的)第三个值并导致访问冲突.

因此,您可能无法将方法指针放在通用列表中.

好的以下工作:

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;

你必须定义自己的比较器,可能有更多的智能;-).

Answer 2

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;