一個link 2019的錯誤，一直沒有解決思路，最后解決掉，把思路記錄下
error msg:

UpDlgView.obj : error LNK2019: unresolved external symbol "__declspec(dllimport) public: __thiscall CMarkup::~CMarkup(void)" (__imp_??1CMarkup@@QAE@XZ) referenced in function "public: void __thiscall CUpDlgView::OnButtonadd(void)" (?OnButtonadd@CUpDlgView@@QAEXXZ)

UpDlgView.obj : error LNK2019: unresolved external symbol "__declspec(dllimport) public: bool __thiscall CMarkup::AddElem(char const *,char const *,int)" (__imp_?AddElem@CMarkup@@QAE_NPBD0H@Z) referenced in function "public: void __thiscall CUpDlgView::OnButtonadd(void)" (?OnButtonadd@CUpDlgView@@QAEXXZ)

UpDlgView.obj : error LNK2019: unresolved external symbol "__declspec(dllimport) public: __thiscall CMarkup::CMarkup(void)" (__imp_??0CMarkup@@QAE@XZ) referenced in function "public: void __thiscall CUpDlgView::OnButtonadd(void)" (?OnButtonadd@CUpDlgView@@QAEXXZ)

分析：
錯誤的類別是:unresolved external symbol ,是一個沒有解決的外部變量，因為一直認為第三方類不可能有錯誤所以一直在自己編寫的代碼中尋找錯誤，其實錯誤的關鍵在這里："__declspec(dllimport)" 這說明這是一個外部導出類，所以需要?將class? AFX_EXT_CLASS CMarkup?改為 class CMarkUp .