C#

public abstract class Message
{
    // Properties & methods representing fields common to all messages
    protected byte[] CompileCommonPacketPart()
    {
         // Compiles a part of the packet from the common fields
    }
}

public abstract class Command : Message
{
    // Additional fields and methods available for all commands
    protected byte[] CompileCommandPacketPart()
    { 
         // Compiles a part of the packet from command-specific data
    }
}

public abstract class Response : Message
{
    // Additional fields and methods available for all responses
    protected byte[] CompileResponsePacketPart()
    { 
         // Compiles a part of the packet from response-specific data
    }
}

public class MyCommand : Command
{
    // Concrete command class, adds data 
    // specific to this particular command
    public byte[] CompilePacket()
    { 
         // Compiles the generic parts of the command
         CompileCommonPacketPart();
         CompileCommandPacketPart();
         
         // Adds parts specific to this command
         ...
    }
}