TInterlocked.CompareExchange是否支持接口对象？多线程单例疑问

问题背景 #

在ISAPI多线程DLL环境下实现接口单例的懒加载，初始代码存在线程安全风险，使用AtomicCmpExchange重构后，发现创建的对象会因超出作用域被销毁。

初始线程不安全代码 #

...
interface

var
  FHIRResourceFactory: function: IFHIRResourceFactory;

implementation
...
// global var and function
var
  _FHIRFactory: IFHIRResourceFactory;

function GetFHIRResourceFactory: IFHIRResourceFactory;
begin
  if (_FHIRFactory = nil) then
    { The object doesn't exist yet. Create one. }
    _FHIRFactory := TFHIRResourceFactory.Create;
  Result := _FHIRFactory;
end;

initialization

FHIRResourceFactory := GetFHIRResourceFactory;
end.

重构后的线程安全尝试代码 #

...
interface

var
  FHIRResourceFactory: function: IFHIRResourceFactory;

implementation
...
// global var and function
var
  _FHIRFactory: IFHIRResourceFactory;

function GetFHIRResourceFactory: IFHIRResourceFactory;
var
  newFHIRFactory: IFHIRResourceFactory;
begin
  if (_FHIRFactory = nil) then
  begin
    { The object doesn't exist yet. Create one. }
    newFHIRFactory := TFHIRResourceFactory.Create;
    { It's possible another thread also created one.
      Only one of us will be able to set the _FHIRFactory singleton variable }
    if AtomicCmpExchange(Pointer(_FHIRFactory), Pointer(newFHIRFactory), nil) <> nil then
    begin
      { The other beat us. Destroy our newly created object and use theirs. }
      newFHIRFactory := nil;
    end;
// if I don't add this line the object will fall out of scope and be destroyed
//      _FHIRFactory._AddRef
  end;
  Result := _FHIRFactory;
end;

initialization

FHIRResourceFactory := GetFHIRResourceFactory;
end.

解决方案与问题解析 #

核心问题原因 #

你遇到的对象销毁问题，本质是接口引用计数在原子操作中未被正确维护：

• 当把接口类型转换为Pointer传入AtomicCmpExchange时，不会自动触发接口的_AddRef操作。
• 若你的线程在竞态中成功设置_FHIRFactory，newFHIRFactory出作用域时会执行_Release，此时_FHIRFactory指向的对象引用计数会减至0，导致对象被销毁。

是否有支持接口的TInterlocked.CompareExchange？ #

Delphi没有提供直接针对接口类型的TInterlocked.CompareExchange重载，因为接口的内部结构包含指针和虚表，原子操作仅能针对指针部分，引用计数的维护必须手动处理。以下是两种可靠的替代方案：

方案一：手动维护引用计数的原子操作

用指针变量存储对象实例，配合原子操作手动管理引用计数：

var
  _FHIRFactoryPtr: Pointer; // 存储对象指针，而非接口变量

function GetFHIRResourceFactory: IFHIRResourceFactory;
var
  newObj: TFHIRResourceFactory;
  existingPtr: Pointer;
begin
  if _FHIRFactoryPtr = nil then
  begin
    newObj := TFHIRResourceFactory.Create;
    try
      // 提前给新对象加引用，避免后续转换为接口时的计数问题
      newObj._AddRef;
      // 原子比较交换指针
      existingPtr := AtomicCmpExchange(_FHIRFactoryPtr, Pointer(newObj), nil);
      if existingPtr <> nil then
      begin
        // 竞态失败，释放自己创建的对象
        newObj._Release;
        // 将已有指针转为接口返回
        Result := IFHIRResourceFactory(existingPtr);
      end
      else
      begin
        // 竞态成功，将新对象转为接口返回
        Result := newObj;
      end;
    except
      // 创建失败时直接释放对象
      newObj.Free;
      raise;
    end;
  end
  else
  begin
    // 将已有指针转为接口，自动触发_AddRef
    Result := IFHIRResourceFactory(_FHIRFactoryPtr);
  end;
end;

方案二：双重检查锁定（临界区实现）

这种方式在单例创建完成后，后续调用不会进入临界区，性能损失可忽略，且完全避免引用计数问题：

type
  TFHIRResourceFactory = class(TInterfacedObject, IFHIRResourceFactory)
  public
    class function Instance: IFHIRResourceFactory; static;
    // 接口方法声明
  end;

var
  _Instance: IFHIRResourceFactory;
  _InstanceLock: TRTLCriticalSection;

class function TFHIRResourceFactory.Instance: IFHIRResourceFactory;
begin
  if _Instance = nil then
  begin
    EnterCriticalSection(_InstanceLock);
    try
      // 二次检查确保未被其他线程创建
      if _Instance = nil then
        _Instance := TFHIRResourceFactory.Create;
    finally
      LeaveCriticalSection(_InstanceLock);
    end;
  end;
  Result := _Instance;
end;

initialization
  InitializeCriticalSection(_InstanceLock);
finalization
  DeleteCriticalSection(_InstanceLock);

内容的提问来源于stack exchange，提问作者David Moorhouse