.NET 中选择合适的文件打开模式（CreateNew, Create, Open, OpenOrCreate, Truncate, Append）

public static FileStream Open(string path, FileMode mode);

public enum FileMode
{
    CreateNew = 1,
    Create = 2,
    Open = 3,
    OpenOrCreate = 4,
    Truncate = 5,
    Append = 6,
}

private unsafe SafeFileHandle CreateFileOpenHandle(FileMode mode, FileShare share, FileOptions options)
{
    Interop.Kernel32.SECURITY_ATTRIBUTES secAttrs = GetSecAttrs(share);

    int fAccess =
        ((_access & FileAccess.Read) == FileAccess.Read ? Interop.Kernel32.GenericOperations.GENERIC_READ : 0) |
        ((_access & FileAccess.Write) == FileAccess.Write ? Interop.Kernel32.GenericOperations.GENERIC_WRITE : 0);

    // Our Inheritable bit was stolen from Windows, but should be set in
    // the security attributes class.  Don't leave this bit set.
    share &= ~FileShare.Inheritable;

    // Must use a valid Win32 constant here...
    if (mode == FileMode.Append)
        mode = FileMode.OpenOrCreate;

    int flagsAndAttributes = (int)options;

    // For mitigating local elevation of privilege attack through named pipes
    // make sure we always call CreateFile with SECURITY_ANONYMOUS so that the
    // named pipe server can't impersonate a high privileged client security context
    // (note that this is the effective default on CreateFile2)
    flagsAndAttributes |= (Interop.Kernel32.SecurityOptions.SECURITY_SQOS_PRESENT | Interop.Kernel32.SecurityOptions.SECURITY_ANONYMOUS);

    using (DisableMediaInsertionPrompt.Create())
    {
        Debug.Assert(_path != null);
        return ValidateFileHandle(
            Interop.Kernel32.CreateFile(_path, fAccess, share, ref secAttrs, mode, flagsAndAttributes, IntPtr.Zero));
    }
}

// Contains constants for specifying how the OS should open a file.
// These will control whether you overwrite a file, open an existing
// file, or some combination thereof.
//
// To append to a file, use Append (which maps to OpenOrCreate then we seek
// to the end of the file).  To truncate a file or create it if it doesn't
// exist, use Create.
//
public enum FileMode
{
    // Creates a new file. An exception is raised if the file already exists.
    CreateNew = 1,

    // Creates a new file. If the file already exists, it is overwritten.
    Create = 2,

    // Opens an existing file. An exception is raised if the file does not exist.
    Open = 3,

    // Opens the file if it exists. Otherwise, creates a new file.
    OpenOrCreate = 4,

    // Opens an existing file. Once opened, the file is truncated so that its
    // size is zero bytes. The calling process must open the file with at least
    // WRITE access. An exception is raised if the file does not exist.
    Truncate = 5,

    // Opens the file if it exists and seeks to the end.  Otherwise,
    // creates a new file.
    Append = 6,
}