(How)
C^# You Are - 13 April 2008

This week's questions can be answered by using some unfamiliar, but available, types in .NET.

1. How can you periodically check for the existing of another machine on the network?  Answer

   For such a task you generally ping the remote server periodically. Ping doesn't work in all cases since it can be blocked but in general it is a good alternative.

   To ping a remote machine you could use P/Invoke to call into the ICMP API of Windows but there is an easier way.  Visual Basic exposes a My.Computer.Network class with some common functionality.  One of these functions is pinging a remote machine.  Contrary to popular belief you can invoke this method outside VB.

   To use this class you need to do the following:

   1. Add a reference to the Microsoft.VisualBasic assembly.  It is available in the GAC.
   2. Add a using statement for the namespace Microsoft.VisualBasic.Devices.
   3. Create an instance of the Network class.  It has no parameters.
   4. Call the Ping() method with the URL of the server to ping.  For VB users you can call the method directly by using My.Computer.Network.Ping.
2. You need to copy a large byte array from one location to another.  How can you do it efficiently?  Answer

   The most efficient mechanism is to use the Buffer.BlockCopy method.  This static method allows you to copy an array of value types to another array in an efficient manner.  The only downside to this method is that the offset you must use are in bytes rather than elements.

3. Inside a class you maintain a collection of child objects.  While the parent class can manipulate the collection you do not want anyone else to be able to.  However you want to expose the children as a collection for simplicity.  How can you do this?  Answer

   The ReadOnlyCollection<T> class was created to represent a read-only collection of objects.   Both Array and List<T> expose methods to convert the object to a read-only collection.  To implement this in the parent class you would create an instance of the class directly or call the method to get the read-only collection and then return it.

   The collection is only read-only in the sense that others cannot modify the list.  Technically ReadOnlyCollection<T> wraps the original collection.  If the original list is modified then the read-only version will reflect that change.  Therefore enumeration of a ReadOnlyCollection<T> is still not thread-safe.

4. You want to get the list of tables from a database.  How can you do this?  Answer

   The GetSchema method exposed by DbConnection-derived classes can give you access to a database's schema information including tables, columns, sprocs and type system.  The downside to this method is that it is very generalized to the point where getting even something as simple as sprocs can be difficult unless you know the provider in advance. 

   This is a complex method to get right.  You should refer to online documentation and trial and error to verify your information.  I hope to write an article on this topic, and add some support in Kraken, in the future.

5. You are building the About box for your application and you'd like to retrieve this information from the assembly data.  How can you do it?  Answer

   The core assembly information such as product name, version, copyright and trademark are merely attributes on the assembly.  You can use the various attribute classes (such as AssemblyProductAttribute or AssemblyCopyrightAttribute).

   An easier way though is to use the AssemblyInfo class.  This class effectively wraps these attributes into properties.  It is part of the Microsoft.VisualBasic assembly under the namespace Microsoft.VisualBasic.ApplicationServices.  Creating an instance of this class just requires an assembly reference.  You can get the EXE assembly by calling Assembly.GetEntryAssembly().