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static extern bool GetProfilesDirectory(StringBuilder path, ref int size);

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   if (GetProfilesDirectory(path, ref size) )

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    if (GetProfilesDirectory(path, &size) ) {

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        MessageBox(NULL, path, "GetProfilesDirectory()", MB_OK);

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WINBOOL WINAPI GetProfilesDirectoryA(LPSTR lpProfileDir, LPDWORD lpcchSize);

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    if (GetProfilesDirectory(path, &size) ) {

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        MessageBox(NULL, path, "GetProfilesDirectory()", MB_OK);

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Documentation

[GetProfilesDirectory] on MSDN