【C++】CustomItemHost类中使用 ApplyCustomItem (ItemTypeCR itemType) const函数修改元素属性时无法成功

各位老师好，添加一个新埋件，使用ApplyCustomItem 函数第一次写埋件属性可以成功，但第二次选中该埋件进行修改属性时不成功，在CustomItemHost itemHost(eeh, true); // true = schedule changed
DgnECInstancePtr pInstance = itemHost.ApplyCustomItem(*itemType);时pInstance 为空，查看eeh正常，请问是什么原因呢，以下为函数部分代码。请老师指点。

void SetElemGraphItemTypeValue(EditElementHandle& eeh, string Solidname, string Solidtype, WString libName, WString typeName, DgnModelRefP model)
{

	WString WSolidname(Solidname.c_str());
	WString WSolidtype(Solidtype.c_str());
	//ElementRefP oldref = eeh.GetElementRef();
	DgnFileP dgnfile = model->GetDgnFileP();
	ItemTypeLibraryPtr itemTypeLibrary = ItemTypeLibrary::FindByName(libName.c_str(), *dgnfile);
	if (itemTypeLibrary == NULL)
	{
		itemTypeLibrary = ItemTypeLibrary::Create(libName.c_str(), *dgnfile);
	}

	ItemTypeP itemType = itemTypeLibrary->GetItemTypeByName(typeName.GetWCharCP());

	if (itemType == NULL)
	{
		itemType = itemTypeLibrary->AddItemType(typeName.GetWCharCP());
	}
	WString strName;
	strName = L"Name";
	CustomPropertyP Custype2 = itemType->GetPropertyByName(strName.GetWCharCP());
	if (Custype2 == NULL)
	{
		Custype2 = itemType->AddProperty(strName.GetWCharCP());
		Custype2->SetType(CustomProperty::Type::String);
	}
	strName = L"Type";
	CustomPropertyP Custype3 = itemType->GetPropertyByName(strName.GetWCharCP());
	if (Custype3 == NULL)
	{
		Custype3 = itemType->AddProperty(strName.GetWCharCP());
		Custype3->SetType(CustomProperty::Type::String);
	}
	itemTypeLibrary->Write();
	CustomItemHost itemHost(eeh, true);  // true = schedule changed
	DgnECInstancePtr pInstance = itemHost.ApplyCustomItem(*itemType);
	if (pInstance == nullptr)
	{
		return;
	}
	...
	}