How does DllImport decide between CreateFileA and CreateFileW?

When I use P/Invoke for calling CreateFile like:

[DllImport("kernel32.dll")]
public static extern SafeFileHandle CreateFile(
    string lpFileName,
    uint dwDesiredAccess,
    uint dwShareMode,
    IntPtr SecurityAttributes,
    uint dwCreationDisposition,
    uint dwFlagsAndAttributes,
    IntPtr hTemplateFile
);

how does Windows decide to call CreateFileA or CreateFileW?