Immutable Messages

Usually messages are designed as DTOs, i.e. a plain class with public properties. This model is simple and will always work. Immutable messages are sometimes considered from a coding philosophy that messages should not be able to be changed after they are created.

Message objects can be made immutable at runtime by:

1. Creating properties with only public getters and initializing these properties via constructor initialization.
2. Having regular message classes with public getters/setters at the sender, where these classes implement an interface with only public getters. Receivers reference only the interface.

Properties with only public getters

public class CancelOrder : ICommand
{
    public CancelOrder(int orderId)
    {
        OrderId = orderId;
    }

    public int OrderId { get; private set; }
}

Classes with public setters, interfaces with only getters

Using private setters is not supported by all serializers. An alternative is to make use of NServiceBus's support for multiple inheritance and polymorphic dispatch. In this case, a message contract is defined through an interface containing getters only, which is used by the message handler. An implementation of the message contract, which exposes public setters, is used to create the message and pass it to Send or Publish.

public class CancelOrder : ICancelOrder
{
    public CancelOrder(int orderId)
    {
        OrderId = orderId;
    }

    public int OrderId { get; set; } // Public setter
}


public interface ICancelOrder : IMessage
{
    int OrderId { get; } // Only getter
}