I clean the solution and start debuging again,the output information is below:
1>------ Build started: Project: Modeless, Configuration: Debug Win32 ------
1>Compiling...
1>stdafx.cpp
1>Compiling...
1>ChildFrm.cpp
1>MainFrm.cpp
1>Modeless.cpp
1>ModelessDialog.cpp
1>ModelessDoc.cpp
1>ModelessView.cpp
1>Generating Code...
1>Compiling resources...
1>Microsoft (R) Windows (R) Resource Compiler Version 6.1.6723.1
1>Copyright (C) Microsoft Corporation. All rights reserved.
1>Compiling manifest to resources...
1>Microsoft (R) Windows (R) Resource Compiler Version 6.1.6723.1
1>Copyright (C) Microsoft Corporation. All rights reserved.
1>Linking...
1>ModelessDialog.obj : error LNK2001: unresolved external symbol "public: void __thiscall ModelessDialog::OnLoginLogin(void)" (?OnLoginLogin@ModelessDialog@@QAEXXZ)
1>D:\WorkDocuments\Visual C++ Programs\MFC\Modeless\Debug\Modeless.exe : fatal error LNK1120: 1 unresolved externals
1>Build log was saved at "file://d:\WorkDocuments\Visual C++ Programs\MFC\Modeless\Debug\BuildLog.htm"
1>Modeless - 2 error(s), 0 warning(s)
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========