I tend to avoid breaking CQS principe in OOP. But there are sometimes cases that it would be possible to do it for a simpler code.

cqs samples

//1. breaking CQS

var sor = new SimpleOrderRouting();
var orderConfirmation = sor.Route(new OrderRequest(..));

bool isOrderExecuted = orderConfirmation.ExecutionCompleted;

//2. adhering to CQS not blocking

var sor = new SimpleOrderRouting();
bool isOrderExecuted = false;
sor.ExecutionCompleted += response => {
  isOrderExecuted = response.ExecutionCompleted;
}

sor.Route(new OrderRequest(..));

//3. adhering to CQS way blocking

var sor = new SimpleOrderRouting();

var orderRequest = sor.PrepareRequest(...);
sor.Route(orderRequest);

bool isOrderExecuted = orderRequest.ExecutionCompleted;

//4. adhering to CQS with observer pattern

public interface IOrderExecutionHandler
{
     void Notify(ExecutionResponse response);
}



//...

var sor = new SimpleOrderRouting();
var orderRequest = new OrderRequest(..);
IOrderExecutionHandler oeHandler = new OrderExecutionHandler();
sor.Subscribe(oeHandler);
sor.Route(orderRequest); // invokes Notify method on observers


// other way ?

What's wrong with number 2?

To be fair, it's difficult to see if you're violating CQS in any of these examples, because we can't see the signature of the containing method.

For simplicity. The containing method call "sor.Route" is running an algorithm on OrderRequest object, finds to which financial market the order should be routed, creates a Market object which is a proxy to the financial market and calls a method on it "market.Send(marketOrder)". Then the market proxy calls back when order is executed so "sor" can notify the client with this bool isOrderExecuted = orderConfirmation.ExecutionCompleted;

I like the most the number 2 because I'm being called back when something of interest happens to me. But it seems that without using events it's hard to adhere to CQS and have readable code.

Is it more clrear or more context should be provided ? How would you write a CQS compliant code differently that in a two last steps ?

Ah, use Rx instead of .NET events :)

Or just use the Observer pattern.

or return a promise/future/task (could be custom code)?

@ploeh : With the observer pattern that I've added it seems to clunky. Or maybe you would do it in another way.

@yreynhout could you elaborate ?