Screenshot method generates black images

What were the problems you had with DrawToBitmap? It works fine here, (W8.1, VS2013) even with a Webcontrol and also after switching users. (But see the edit at the end for the conditions!)

Bitmap bmp = new Bitmap(this.ClientRectangle.Width, this.ClientRectangle.Height);
this.DrawToBitmap(bmp, this.ClientRectangle);
// Clipboard.SetImage(bmp); for testing only
bmp.Dispose();

Here is code to take a screenshot of your window:

Rectangle formBounds = this.Bounds;
Bitmap bmp = new Bitmap(formBounds.Width, formBounds.Height );
using (Graphics g = Graphics.FromImage(bmp))
       g.CopyFromScreen(formBounds.Location, Point.Empty, formBounds.Size);
//Clipboard.SetImage(bmp); for testing only
bmp.Dispose();

I can switch users like I want, the program keeps working.

BTW, the link you posted is really old, many things may have improved.

Edit:

With the updated question things are a lot clearer.

So you want to continuously get a screenshot of your program even when the user has changed, right? and you want to display a WebControl, right?

A user can have three types of desktop: the logon/logoff screen, the screensaver screen and one or more normal desktop screen(s). But while the user is logged off he has no desktop screen at all.

Therefore the screenshot method will not work if the user has no active desktop, neither as g.CopyFromScreen, which will cause a GDI-error nor using a window handle like in the various solutions on the web, including the ones your link leads to. All these will, at best, show a blank or black screen.

So the DrawToBitmap method is the only one that works.

You wrote that it has random errors. That's not what I see.

The problems come in predictably when the user interacts with the WebBrowser in any way. This includes scrolling or clicking with or without navigation. After these interactions the WebBrowser will draw itself as an empty box until its URL is reloaded - not only refreshed - but really reloaded by webBrowser1.Uri = new Uri(uriPath). This can be done, see my other post

The WebBrowser also has another issue when doing a DrawToBitmap: It will fail (with the said empty box) for any pages that include an <input type="text" element. I'm not sure what's the best way to workaround this, let alone why it happends in the first place.. A screenshot method doesn't have that specific problem.

Edit 2:

The code the OP has dug up code which, using a call to PrintWindow, seems to solve all problems we had: It works while being logged off, works with Refeshing even after clicking in the WebBrowser and scrapes all pages, including those with textual input fields. Hoorah!

After cutting down the slack here is a version that can create a copy of the Form or just the WebBroser (or any other Control) with or without borders:

[DllImport("user32.dll")]
public static extern bool PrintWindow(IntPtr hwnd, IntPtr hdcBlt, uint nFlags);

public Bitmap CaptureWindow(Control ctl)
{
    //Bitmap bmp = new Bitmap(ctl.Width, ctl.Height);  // includes borders
    Bitmap bmp = new Bitmap(ctl.ClientRectangle.Width, ctl.ClientRectangle.Height);  // content only
    using (Graphics graphics = Graphics.FromImage(bmp))
    {
        IntPtr hDC = graphics.GetHdc();
        try      { PrintWindow(ctl.Handle, hDC, (uint)0);   }
        finally  { graphics.ReleaseHdc(hDC);                }
    }
    return bmp;
}