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A watchdog application

This article is taken from content omitted during technical review of the a book entitled Network programming for .NET (Buy at Amazon UK) (Buy at Amazon US)

                When you have a server application that runs unattended for long periods of time, you may wish to restart the computer on a daily basis, to free up lost memory, and improve performance.

                This program will reboot the computer at a time specified by the user. It does so in unattended mode, ensuring that any requests for user intervention are ignored where possible. This includes messages such as ‘Do you wish to save changes?’  There is no provision for saving settings, or restarting this application once the computer has rebooted. This program does not provide any means of automatically logging in.

                To include a program for automatic start-up, it may be included in the start up menu manually. Alternatively, an entry can be made in the registry at HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run. Information on accessing the registry can be read in chapter 4

In order to reboot a computer you need to use a windows API (application programming interface) function named ExitWindowsEx contained in user32.dll. Functions in the Windows API have a calling convention, which can be determined by reading up on the particular function. A good reference point for the windows API in C# and VB.NET is a available at http://custom.programming-in.net

The technology behind this interoperability between the Windows API and .NET is known as P/Invoke (platform invoke). This technology allows you to declare Windows API functions in your code in almost the same way as you would declare native .NET functions. Each Win32 function call is declared with a static extern method. The DllImport attribute is then applied to that method. Parameters of the DllImport attribute indicate the specific DLL and DLL entry point that is called.

The significant parameters of the DllImport attribute are as follows:

·         CallingConvention indicates the method by which arguments are passed to the unmanaged implementation.

·         CharSet manages name mangling and indicates how to marshal String arguments to the method.

·         EntryPoint indicates the name or ordinal of the DLL entry point to be called. To discover entry points in DLLs, a utility named dependency walker is indispensable ( http://www.dependencywalker.com/ ).

·         ExactSpelling indicates whether the name of the entry point in the unmanaged DLL should be modified to correspond to the CharSet value, if not set then a case-insensitive search is made for the entry point.

·         PreserveSig indicates whether or not the name of the entry point in the unmanaged DLL should be modified to correspond to the CharSet value specified in the CharSet field.

·         SetLastError indicates the caller may call the Win32 API GetLastError function to retrieve more detailed information for errors that may have occurred during the call to the API.

The trickiest part of P/Invoke is determining how to map types between the Win32 call and the static extern method that you declare. When migrating C++ style API declarations to .NET, there are some general rules for mapping types. When a Win32 method needs a handle or pointer (int *), you should use an IntPtr. For pointers to strings (char * or LPCTSTR), you can simply use the .NET String type.

It is sufficient to simply call ExitWindowsEx on Windows 98 to reboot a computer. However from Windows NT upwards, you need to acquire privileges to reboot a computer from code.  In this case we request a privilege named SeShutdownPrivilege from the AdjustTokenPrivileges API call. Unfortunately the privilege architecture of the Windows NT kernel is beyond the scope of this book.

Start a new project in Visual Studio .NET. Add a label named lblCurrentTime to the form, along with a DateTimePicker of the same name. The latter control should be set with format to time. A check box named chkReboot is required to enable & disable automatic rebooting. Finally, add a timer named tmrReboot. Ensure that the timer is set with enabled to true.

The timer will poll on a check that will compare the current time with the time specified by the user. A check box is used to prevent the computer from rebooting as soon as the program is started.