why it fails at SetScriptSite

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C++

#pragma once
#include <objbase.h>

//#define LOCALE_SCRIPT_DEFAULT	(MAKELCID(LANG_ENGLISH_NEUTRAL, SORT_DEFAULT))

class CScriptHost : public IActiveScriptSite,
	public IActiveScriptSiteWindow
{
public:
	CScriptHost(LPUNKNOWN lpUnkCtrl, LPCOLESTR pNamedItem, HWND hWnd);
	virtual ~CScriptHost();

	HRESULT CreateScriptEngine();
	HRESULT ParseFile(const char* pszFileName,LPCOLESTR pstrItemName);
public:
	//IUnknown members
	STDMETHOD(QueryInterface)(REFIID riid,void** ppvObj);
	STDMETHOD_(unsigned long,AddRef)(void);
	STDMETHOD_(unsigned long,Release)();

	//IActiveScriptSite members
	STDMETHOD(GetLCID)(LCID  *plcid) ;
	STDMETHOD(GetItemInfo)(LPCOLESTR pstrName,DWORD dwReturnMask,
		IUnknown  * *ppiunkItem,ITypeInfo  * *ppti) ;
	STDMETHOD(GetDocVersionString)(BSTR  *pbstrVersion) ;
	STDMETHOD(OnScriptTerminate)(const VARIANT  *pvarResult,const EXCEPINFO  *pexcepinfo) ;
	STDMETHOD(OnStateChange)(SCRIPTSTATE ssScriptState) ;
	STDMETHOD(OnScriptError)(IActiveScriptError  *pscripterror) ;
	STDMETHOD(OnEnterScript)(void) ;
	STDMETHOD(OnLeaveScript)(void) ;
	//IActiveScriptSiteWindow members
	STDMETHOD(GetWindow)(HWND  *phwnd) ;
	STDMETHOD(EnableModeless)(BOOL fEnable);
public:
	IActiveScript* m_ps;
	IActiveScriptParse* m_psp;
private:
	LONG m_cRef;
	CLSID m_clsidEngine;
	LPUNKNOWN m_lpUnkCtrl;
	LPOLESTR m_pNamedItem;
	HWND m_Wnd;
};


#include "StdAfx.h"
#include "ScriptHost.h"

#include <sys/stat.h>

//类CScriptHost通过多继承的方法实现了两个接口，并负责脚本引擎的创建和维护工作。其数据成员m_ps和m_psp用于保存引擎的IActiveScript和IActiveScriptParse接口指针；数据成员m_clsidEngine记录了引擎的类ID；m_Wnd记录了应用系统提供给引擎的主窗口；m_lpUnkCtrl记录了应用系统的唯一的一个受控对象，m_pNamedItem记录了受控对象的名字。在CScriptHost类的构造函数中初始设置m_lpUnkCtrl、m_pNamedItem和m_Wnd成员变量。构造函数和析构函数代码如下：
CScriptHost::CScriptHost(LPUNKNOWN lpUnkCtrl, LPCOLESTR pNamedItem , HWND hWnd) 
: m_ps(NULL),m_psp(NULL),m_cRef(0)
{
	m_lpUnkCtrl = lpUnkCtrl;
	m_pNamedItem = const_cast<LPOLESTR>(pNamedItem);
	m_Wnd = hWnd;
	// the clsid of VBScript Engine
	static CLSID const clsid = 
	{0xb54f3741, 0x5b07, 0x11cf, {0xa4, 0xb0, 0x0, 0xaa, 0x0, 0x4a, 0x55, 0xe8}};

	// Default to VBScript
	m_clsidEngine = clsid;
}

CScriptHost::~CScriptHost()
{
	if(m_psp)
		m_psp->Release();
	// we must first close the script engine
	if(m_ps)
	{
		m_ps->Close();
		m_ps->Release();
	}
}

HRESULT CScriptHost::CreateScriptEngine()
{
	HRESULT hr = S_OK;

	hr = ::CoCreateInstance(m_clsidEngine, NULL, CLSCTX_INPROC_SERVER, 
		IID_IActiveScript,(void**)&m_ps);
	if ( SUCCEEDED( hr ) )
	{
		// QI the IActiveScriptParse pointer
		hr = m_ps->QueryInterface(IID_IActiveScriptParse, (void**)&m_psp);
		if (FAILED(hr) )
		{
			m_ps->Release();
			return hr;
		}
		// set the script site
		IActiveScriptSite* Iass = static_cast<IActiveScriptSite*>(this);

		hr = m_ps->SetScriptSite(Iass);
		if ( FAILED( hr ) )
			return hr;
		m_ps->SetScriptState(SCRIPTSTATE_INITIALIZED);
		// initiate the script engine
		hr = m_psp->InitNew();
		if ( FAILED( hr ) )
			return hr;
		hr = m_ps->AddNamedItem(m_pNamedItem,
			SCRIPTITEM_ISVISIBLE  |  SCRIPTITEM_ISSOURCE);

	}

	return hr;
}

HRESULT CScriptHost::ParseFile(const char * pszFileName, LPCOLESTR pstrItemName)
{
	HRESULT hr = S_OK;

	struct _stat stat;
	size_t cch;
	EXCEPINFO ei;
	FILE *pfile; 
	if(::_stat(pszFileName,&stat))
		return E_FAIL;
	cch = stat.st_size;
	char* pszAlloc = new char[cch + 1];
	pszAlloc[cch] = '\0';// this is vitally important
	if(pszAlloc == NULL)
		return E_OUTOFMEMORY;
	memset(pszAlloc,0,cch);
	// get the script text into a memory block
	pfile = fopen(pszFileName,"rb");
	if ( !pfile ) 
	{
		hr = E_FAIL;
		return hr;
	}
	fread(pszAlloc,cch,1,pfile);
	fclose(pfile);
	LPOLESTR pwszCode;

	int CharCount = MultiByteToWideChar(CP_ACP,0,pszAlloc,-1,NULL,0);
	pwszCode = new WCHAR[CharCount];
	MultiByteToWideChar(CP_ACP,0,pszAlloc,-1,pwszCode,CharCount);
	size_t t = wcslen(pwszCode);
	hr = m_psp->ParseScriptText(pwszCode, pstrItemName,NULL, NULL, 0,0,0L,NULL,&ei);
	delete []pwszCode;
	delete []pszAlloc;
	return hr;
}

STDMETHODIMP CScriptHost::GetItemInfo(LPCOLESTR pstrName,DWORD dwReturnMask,
									  IUnknown  **ppiunkItem, ITypeInfo  **ppti) 
{
	HRESULT hr = S_OK;
	// initialize the sent-in pointers
	if(dwReturnMask & SCRIPTINFO_ITYPEINFO)
	{
		if(ppti == NULL)
			return E_INVALIDARG;
		*ppti = NULL;
	}
	if(dwReturnMask & SCRIPTINFO_IUNKNOWN)
	{  
		if(ppiunkItem == NULL)
			return E_INVALIDARG;
		*ppiunkItem = NULL;
	}
	if(!_wcsicmp(m_pNamedItem, pstrName))
	{
		if(dwReturnMask & SCRIPTINFO_IUNKNOWN)
		{
			// give out the object's IUnknown pointer
			*ppiunkItem = m_lpUnkCtrl;
			static_cast<IUnknown*>(*ppiunkItem)->AddRef();
		}

		if(dwReturnMask & SCRIPTINFO_ITYPEINFO)
		{
			IProvideClassInfo* pClsInfo = NULL;
			hr = m_lpUnkCtrl->QueryInterface(IID_IProvideClassInfo, (void**)&pClsInfo);

			if(pClsInfo != NULL)
			{
				hr = pClsInfo->GetClassInfo(ppti);
				pClsInfo->Release();
			}
		}
	}
	return hr;
}


HRESULT CScriptHost::GetWindow(HWND  *phwnd)
{
	if (m_Wnd != NULL)
	{
		*phwnd = m_Wnd;
		return S_OK;
	} 
	else
	{
		return E_FAIL;
	}
}

STDMETHODIMP CScriptHost::GetLCID(LCID  *plcid) 
{
	//*plcid = LOCALE_SCRIPT_DEFAULT;	// Default LCID: english-neutral
	return E_NOTIMPL;
}

STDMETHODIMP CScriptHost::GetDocVersionString(BSTR  *pbstrVersion)
{
	return E_NOTIMPL;
}

STDMETHODIMP CScriptHost::OnScriptTerminate(const VARIANT  *pvarResult,const EXCEPINFO  *pexcepinfo)
{
	return S_OK;
}

STDMETHODIMP CScriptHost::OnStateChange(SCRIPTSTATE ssScriptState)
{
	return S_OK;
}

STDMETHODIMP CScriptHost::OnScriptError(IActiveScriptError  *pscripterror)
{
	return S_OK;
}

STDMETHODIMP CScriptHost::OnEnterScript(void)
{
	return S_OK;

}
	
STDMETHODIMP CScriptHost::OnLeaveScript(void)
{
	return S_OK;

}


STDMETHODIMP CScriptHost::EnableModeless(BOOL fEnable)
{
	return S_OK;
}

ULONG CScriptHost::AddRef()
{
	::InterlockedIncrement(&m_cRef);
	return m_cRef;
}


ULONG CScriptHost::Release()
{
	::InterlockedDecrement(&m_cRef);
	return m_cRef;
}

STDMETHODIMP CScriptHost::QueryInterface(REFIID riid,void** ppvObj)
{

	return S_OK;
}

When function "SetScriptSite" is called in function "CreateScriptEngine", error occurs:
Unhandled exception at 0x7327c96b in CScriptTest.exe: 0xC0000005: Access violation reading location 0x00000000.

I have just got to ActiveX Scripting, i have googled a lot, still i can't solve the problem.

Who can tell me where it is wrong?