cannot convert 'const char*' to 'LPCWSTR {aka const wchar_t*}'

Try this:

RS232Handle=OpenRS232(L"COM1", 9600);

HANDLE OpenRS232(const wchar_t* ComName, DWORD BaudRate)
{
    ComHandle=CreateFileW(ComName, GENERIC_READ|GENERIC_WRITE, 0, NULL, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL);
}

On Windows the wchar_t type is used to represent characters in UTF-16 encoding. This is what the Windows kernel uses internally and therefore modern versions of Visual C++ default to Unicode functions. If you insist on using the ANSI functions instead (thus going back to your original code), remove the L-prefix from the string "COM1" and change the call from CreateFileW to CreateFileA.

Most Windows API functions that deal with strings have both a W and an A version; the only exception that I am aware of is the function GetProcAddress which always takes an ANSI string regardless of whether you're working with ANSI or Unicode in your project.

The Windows CreateFile function is actually a macro that expands to one of:

• CreateFileA, which takes a file path of type const char*
• CreateFileW, which takes a file path of type const wchar_t*.

(The same is true for most of the functions in the Windows API that take a string.)

You're declaring the parameter const char* ComName, but apparently compiling with UNICODE defined, so it's calling the W version of the function. There's no automatic conversion from const wchar_t* to const char*, hence the error.

Your options are to:

1. Change the function parameter to a UTF-16 (const wchar_t*) string.
2. Keep the char* parameter, but have your function explicitly convert it to a UTF-16 string with a function like MultiByteToWideChar.
3. Explicitly call CreateFileA instead of CreateFile.
4. Compile your program without UNICODE, so that the macros expand to the A versions by default.
5. Kidnap a prominent Microsoft developer and force him to read UTF-8 Everywhere until he agrees to have Windows fully support UTF-8 as an “ANSI” code page, thus freeing Windows developers everywhere from this wide-character stuff.

Edit: I don't know if a kidnapping was involved, but Windows 10 1903 finally added support for UTF-8 as an ANSI code page.

const char* and const wchar_t* are two different types with no implicit conversion between them. Therefore, you need to perform the conversion to before passing the value on to the CreateFile function. Take a look at this answer for a possible conversion approach: https://stackoverflow.com/a/3074785/6710751

Alternatively you might just use the CreateFileA function instead of CreateFile, as was suggested by Ben Voigt.

There are many ways of fixing this

1. Open the project properties, General/Character Set. This will be set to either Unicode or Multi byte character set. If you wish to use char* change from Unicode to MBCS. This will convert CreateFile to CreateFileW if Unicode is specified and CreateFileA if MBCS is specified.
2. Enclose all strings in _T() eg _T("COM1"). What this does is to compile the string as a char* if MBCS is specified, wchar_t if unicode is specified
3. Force all strings to be wide strings by prefixing with L eg L"COM1"

Note that in some error handling routine the strings are specifically MBCS