Closing OpenFileDialog/SaveFileDialog

This is going to require pinvoke, the dialogs are not Forms but native Windows dialogs. The basic approach is to enumerate all toplevel windows and check if their class name is "#32770", the class name for all dialogs owned by Windows. And force the dialog to close by sending the WM_CLOSE message.

Add a new class to your project and paste the code shown below. Call DialogCloser.Execute() when the logout timer expires. Then close the forms. The code will work for MessageBox, OpenFormDialog, FolderBrowserDialog, PrintDialog, ColorDialog, FontDialog, PageSetupDialog and SaveFileDialog.

using System;
using System.Text;
using System.Windows.Forms;
using System.Runtime.InteropServices;

static class DialogCloser {
    public static void Execute() {
        // Enumerate windows to find dialogs
        EnumThreadWndProc callback = new EnumThreadWndProc(checkWindow);
        EnumThreadWindows(GetCurrentThreadId(), callback, IntPtr.Zero);
        GC.KeepAlive(callback);
    }

    private static bool checkWindow(IntPtr hWnd, IntPtr lp) {
        // Checks if <hWnd> is a Windows dialog
        StringBuilder sb = new StringBuilder(260);
        GetClassName(hWnd, sb, sb.Capacity);
        if (sb.ToString() == "#32770") {
            // Close it by sending WM_CLOSE to the window
            SendMessage(hWnd, 0x0010, IntPtr.Zero, IntPtr.Zero);
        }
        return true;
    }

    // P/Invoke declarations
    private delegate bool EnumThreadWndProc(IntPtr hWnd, IntPtr lp);
    [DllImport("user32.dll")]
    private static extern bool EnumThreadWindows(int tid, EnumThreadWndProc callback, IntPtr lp);
    [DllImport("kernel32.dll")]
    private static extern int GetCurrentThreadId();
    [DllImport("user32.dll")]
    private static extern int GetClassName(IntPtr hWnd, StringBuilder buffer, int buflen);
    [DllImport("user32.dll")]
    private static extern IntPtr SendMessage(IntPtr hWnd, int msg, IntPtr wp, IntPtr lp);
}