mono/netcore/System.Private.CoreLib/shared/System/IO/FileMode.cs

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.

namespace System.IO
{
    // 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,
    }
}