Differentiate between TCHAR and _TCHAR
In addition to what @RussC said, TCHAR is used by the Win32 API and is based on the UNICODE define, whereas _TCHAR is used by the C runtime and is based on the _UNICODE define instead. UNICODE and _UNICODE are usually defined/omitted together, making TCHAR and _TCHAR interchangable, but that is not a requirement. They are semantically separated for use by different frameworks.
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TCHAR and _TCHAR are identical, although since TCHAR doesn't have a leading underscore, Microsoft aren't allowed to reserved it as a keyword (imagine if you had a variable called TCHAR. Think what would happen). Hence TCHAR will not be #defined when Language Extensions are disabled (/Za).
TCHAR is defined in winnt.h (which you'll get when you #include ), and also tchar.h under /Ze. _TCHAR is available only in tchar.h (which also #defines _TSCHAR and _TUCHAR). Those are unsigned/signed variants of the normal TCHAR data type.