What does the value 1027 actually mean for the flags parameter in GetDCEx(IntPtr hWnd, IntPtr hRgn, int flags) method call?

Most of what I have found just has people using a similar block of code as what I have done below (which I use to custom draw my border with a specific color on a user control):

// PInvoke declaration
[DllImport("User32.dll", EntryPoint = "GetDCEx")]
internal static extern IntPtr GetDCEx(IntPtr hWnd, IntPtr hRgn, int flags);

// Event Override
protected override void OnPaint(PaintEventArgs e)
{
    base.OnPaint(e);

    IntPtr hWnd = this.Handle;
    IntPtr hRgn = IntPtr.Zero;
    IntPtr hdc = GetDCEx(hWnd, hRgn, 1027);

    using (Graphics grfx = Graphics.FromHdc(hdc))
    {
        Rectangle rect = new Rectangle(0, 0, this.Width - 1, this.Height -
            1);
        Pen pen = new Pen(Color.FromArgb(192, 192, 192), 1);
        grfx.DrawRectangle(pen, rect);
    }
}

But I don't actually know why I'm passing 1027 into the flags param. Looking at Microsoft's documentation for this method, I see that there is a list of DeviceContext values (copied below). So what does 1027 actually mean in this context?

/// <summary>Values to pass to the GetDCEx method.</summary>
[Flags()]
private enum DeviceContextValues : uint
{
    /// <summary>DCX_WINDOW: Returns a DC that corresponds to the window rectangle rather
    /// than the client rectangle.</summary>
    Window       = 0x00000001,
    /// <summary>DCX_CACHE: Returns a DC from the cache, rather than the OWNDC or CLASSDC
    /// window. Essentially overrides CS_OWNDC and CS_CLASSDC.</summary>
    Cache        = 0x00000002,
    /// <summary>DCX_NORESETATTRS: Does not reset the attributes of this DC to the
    /// default attributes when this DC is released.</summary>
    NoResetAttrs     = 0x00000004,
    /// <summary>DCX_CLIPCHILDREN: Excludes the visible regions of all child windows
    /// below the window identified by hWnd.</summary>
    ClipChildren     = 0x00000008,
    /// <summary>DCX_CLIPSIBLINGS: Excludes the visible regions of all sibling windows
    /// above the window identified by hWnd.</summary>
    ClipSiblings     = 0x00000010,
    /// <summary>DCX_PARENTCLIP: Uses the visible region of the parent window. The
    /// parent's WS_CLIPCHILDREN and CS_PARENTDC style bits are ignored. The origin is
    /// set to the upper-left corner of the window identified by hWnd.</summary>
    ParentClip       = 0x00000020,
    /// <summary>DCX_EXCLUDERGN: The clipping region identified by hrgnClip is excluded
    /// from the visible region of the returned DC.</summary>
    ExcludeRgn       = 0x00000040,
    /// <summary>DCX_INTERSECTRGN: The clipping region identified by hrgnClip is
    /// intersected with the visible region of the returned DC.</summary>
    IntersectRgn     = 0x00000080,
    /// <summary>DCX_EXCLUDEUPDATE: Unknown...Undocumented</summary>
    ExcludeUpdate    = 0x00000100,
    /// <summary>DCX_INTERSECTUPDATE: Unknown...Undocumented</summary>
    IntersectUpdate  = 0x00000200,
    /// <summary>DCX_LOCKWINDOWUPDATE: Allows drawing even if there is a LockWindowUpdate
    /// call in effect that would otherwise exclude this window. Used for drawing during
    /// tracking.</summary>
    LockWindowUpdate = 0x00000400,
    /// <summary>DCX_USESTYLE: Undocumented, something related to WM_NCPAINT message.</summary>
    UseStyle = 0x00010000,
    /// <summary>DCX_VALIDATE When specified with DCX_INTERSECTUPDATE, causes the DC to
    /// be completely validated. Using this function with both DCX_INTERSECTUPDATE and
    /// DCX_VALIDATE is identical to using the BeginPaint function.</summary>
    Validate     = 0x00200000,
}

1027 is 0100 0000 0011 in binary, so it’s a combination of the following flags:

• Window (0x1)
• Cache (0x2)
• LockWindowUpdate (0x400)

The value comes from using a binary OR on the flags:

  Window | Cache | LockWindowUpdate
= 0x1 | 0x2 | 0x400
= 0x403
= 1027

For clarity, you should specify the flags in code that way instead of just using a magic number:

DeviceContextValues.Window | DeviceContextValues.Cache | DeviceContextValues.LockWindowUpdate