error C2664: 'CWnd::MessageBoxW' : cannot convert parameter 1 from 'const char [12]' to 'LPCTSTR'解决方案

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You will have to do one of two things:

1. Change your project configuration to use multibyte strings. Press ALT+F7 to open the properties, and navigate to Configuration Properties > General. Switch Character Set to "Use Multi-Byte Character Set".
2. Indicate that the string literal, in this case "Hello world!" is of a specific encoding. This can be done through either prefixing it with L, such as L"Hello world!", or surrounding it with the generic _T("Hello world!") macro. The latter will expand to the L prefix if you are compiling for unicode (see #1), and nothing (indicating multi-byte) otherwise.