将派生的 class object 传递给 function

Question

I am going through the following code:

public interface IBus
{
  Task Publish(BaseMessage message,string topicName);
}

The BaseMessage is:

public class BaseMessage
{
  public Guid Id {get;set;}
  public DateTime CreationDateTime{get;set;}
}

There is a class which implements IBus

public class AzureServiceBusMessageBus : IBus
{
  public async Task Publish(BaseMessage message,string topicName)
  {
    ISenderClient topic = new TopicClient(message,topicName);
    
    var jsonMess = JsonConvert.SerializeObject(message);
    var serviceBusMessage = new Message(Encoding.UTF8.GetBytes(jsonMessage))
    {
      CorrelationId = Guid.NewGuid().ToString()
    }
   await topicClient.Sendasync(serviceBusMessage);
  }

The following code is from Controller of client:

public BasketCheckOutMessage : BaseMessage
{
 public string FirstName {get;set;}
}

public Task CheckController(IBus messageBus)
{
  ...
   BasketCheckOutMessage basket = new BasketCheckOutMessage ();
   try
   {
    await messageBus.Publish(basket ,"checkout");
   }
   catch(Exception e)
   {
   }
}

Here, I dont understand one thing:

What is the purpose of passing " basket " object in clientcontroller whereas it is received as " BaseMessage " base object in Publish(). My understanding is this is a redundant code because if we set FirstName = "A" in BasketCheckOutMessage from client code in ClientController and pass to "Publish" method, it wont be received since the argument has baseclass reference(BaseMessage).

Is my understand wrong in this context?

Answer 1

Yes, your understanding needs a bit of tweaking.

When you pass a derived object, you are indeed passing it, with all its fields and other data.

The receiving end however can expect the base class, but if the receiver "looks into it", for instance, casts message to BasketCheckOutMessage , it would be able to use all its members. The message.GetType() call would also produce the typeof(BasketCheckOutMessage) output.

Another matter is C#'s Reflection. The JsonConvert.SerializeObject method will investigate which properties on the input members are available to serializing, and it will find FirstName member, because it is there.