Determine if screensaver is running or machine is locked.

IsScreensaverRunning.cs

using System;
using System.Runtime.InteropServices;

namespace BrutalDev.Helpers
{
  public static class NativeMethods
  {
    // Used to check if the screen saver is running
    [DllImport("user32.dll", CharSet = CharSet.Auto)]
    [return: MarshalAs(UnmanagedType.Bool)]
    public static extern bool SystemParametersInfo(uint uAction, 
                                                   uint uParam, 
                                                   ref bool lpvParam,
                                                   int fWinIni);

    // Used to check if the workstation is locked
    [DllImport("user32", SetLastError = true)]
    private static extern IntPtr OpenDesktop(string lpszDesktop,
                                             uint dwFlags,
                                             bool fInherit,
                                             uint dwDesiredAccess);

    [DllImport("user32", SetLastError = true)]
    private static extern IntPtr OpenInputDesktop(uint dwFlags,
                                                  bool fInherit,
                                                  uint dwDesiredAccess);

    [DllImport("user32", SetLastError = true)]
    private static extern IntPtr CloseDesktop(IntPtr hDesktop);

    [DllImport("user32", SetLastError = true)]
    [return: MarshalAs(UnmanagedType.Bool)]
    private static extern bool SwitchDesktop(IntPtr hDesktop);

    // Check if the workstation has been locked.
    public static bool IsWorkstationLocked()
    {
      const int DESKTOP_SWITCHDESKTOP = 256;
      IntPtr hwnd = OpenInputDesktop(0, false, DESKTOP_SWITCHDESKTOP);

      if (hwnd == IntPtr.Zero)
      {
        // Could not get the input desktop, might be locked already?
        hwnd = OpenDesktop("Default", 0, false, DESKTOP_SWITCHDESKTOP);
      }

      // Can we switch the desktop?
      if (hwnd != IntPtr.Zero)
      {
        if (SwitchDesktop(hwnd))
        {
          // Workstation is NOT LOCKED.
          CloseDesktop(hwnd);
        }
        else
        {
          CloseDesktop(hwnd);
          // Workstation is LOCKED.
          return true;
        }
      }

      return false;
    }

    // Check if the screensaver is busy running.
    public static bool IsScreensaverRunning()
    {
      const int SPI_GETSCREENSAVERRUNNING = 114;
      bool isRunning = false;

      if (!SystemParametersInfo(SPI_GETSCREENSAVERRUNNING, 0, ref isRunning, 0))
      {
        // Could not detect screen saver status...
        return false;
      }

      if (isRunning)
      {
        // Screen saver is ON.
        return true;
      }

      // Screen saver is OFF.
      return false;
    }
  }
}