Question

I am working on a parser for a serial data protocol. I have an overarching Packet class, a couple sub-classes such as CommandPacket and StatusPacket, and then a few sub-classes of each of those:

• Packet
• • CommandPacket
• • • CommandTypeA
• • • CommandTypeB
• • Status Packet
• • • StatusTypeA
• • • StatusTypeB

The Packet class has a static method for taking data off of a buffer, and if it represents a valid packet, returning a new instance of the Packet class, instantiated with the necessary data.

Now, this is where my problem comes up. I would like to be able to return the most specific type of packet. To facilitate this, I have created a static .isValid() method, which is overridden on the sub classes. The idea is that I could loop through each type of specific packet (CommandTypeA, CommandTypeB, StatusTypeA, StatusTypeB, etc.) calling .isValid() until one of them returned TRUE. At that point, I would return a new instance of that specific packet type.

While of course it is possible for me to create this method directly, how do I account for types of packets that are not added to my project yet? I want someone to be able to extend my class in the future without having to modify the original Packet class.

I have considered utilizing reflection for this, but am avoiding it, since these methods would be called for each and every packet received, and must be efficient.

Any thoughts on how I should re-work my design pattern?

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While I don’t think it is entirely relelvant to the discussion, I am doing this in VB.NET. There is also a similar (but not quite the same) question posted here: Java – subclass validation design pattern

Answer 1

Based on the comments and MarkJ’s lead, here’s a suggestion (which is eventually based on an interface instead of an attribute for stronger type checking):

public interface IPacketValidator
{
    bool IsPacketValid( Packet packet );
    Type PacketType { get; }
}

public class ValidatorsRegistry
{
    private List<IPacketValidator> m_validatorsList;

    private static ValidatorsRegistry m_Instance = new ValidatorsRegistry();
    public static ValidatorsRegistry Instance { get { return m_Instance; } }

    private ValidatorsRegistry()
    {
        InitValidatorsRegistry();
    }

    private void InitValidatorsRegistry()
    {
        m_validatorsList = new List<IPacketValidator>();

        Type iPacketValidatorType = typeof( IPacketValidator );

        foreach ( Assembly asm in AppDomain.CurrentDomain.GetAssemblies() )
        {
            foreach ( Type type in asm.GetTypes() )
            {
                if ( iPacketValidatorType.IsAssignableFrom( type ) && type != iPacketValidatorType )
                {
                    IPacketValidator validator = (IPacketValidator)Activator.CreateInstance(type);
                    m_validatorsList.Add( validator );
                }
            }
        }
    }

    public Type GetSpecificPacketType(Packet packet)
    {
        Type packetType = typeof( Packet );
        foreach ( IPacketValidator validator in m_validatorsList )
        {
            if ( validator.IsPacketValid( packet ) )
            {
                packetType = validator.PacketType;
                break;
            }
        }

        return packetType;
    }
}