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In a search for flawless Visitor (design pattern) Part IV

Updated on February 8, 2013

Part I | Part II | Part III | Part IV

Imagine situation where in some part of our big big project there is an abstraction named a "Big Vehicle" and to drive this big vehicle a driver has to implement IBigVehicleDriver interface.

To model this situation we will add BigVehicle(IBigVehicleDriver driver) method to our "Vehicles" static class.

public static void BigVehicle(IBigVehicleDriver driver)
    Console.WriteLine("Has got into Big Vehicle");

And what we want now is to plugin this method to Type Router and to be able to route to this method those subtypes of polymorphic structure that implement "IBigVehicleDriver" interface.

SubType to Interface routing

To set up this behavior we have to make some trick. Apparently we can not provide generic type parameter of type "IBigVehicleDriver" to SetActionFor<T> method since interface is not subtype of a Driver type.

Therefore we will instantiate second Type Router ("irouter") with generic parameter of "IBigVehicleDriver" as a base type and use it as an adapter.

We will then plugin "Vehicles.BigVehicle" method to "irouter" and plugin "irouter.Execute" to router for Drivers.

static void Main(string[] args)



static void RoutingToMethodsWithInterfaceParameter()
    var driver = new FireTruckDriver();

    var router = new TypeRouter<Driver>();
    var irouter = new TypeRouter<IBigVehicleDriver>();



And Oooops! We've got an empty screen. Nothing happens... What went wrong?

The reason why nothing happens is because we set the key to store "Vehicles.BigVehicle" method in "irouter" as a "RuntimeTypeHandle" of "IBigVehicleDriver" type whereas object coming into "Execute" method of "irouter" is of type "FireTruckDriver".

Therefore when we give to storage the key to retrieve "Vehicles.BigVehicle" processing Method we do not get the Method due to this mismatch.

Bad luck.

Well, it makes us think about adapter again.

We have to have a key to be set as for subtype of Driver, but then we apparently will have to cast subtype of Driver to Interface it implements. It sounds familiar.

(See "Overcoming contravariant nature of Action(T) delegate" section from Part II).

It seems like we need something like this:

Actions[typeof(T).TypeHandle] = 
	baseType => action((U)(T)baseType);

Where T generic parameter is subtype of Driver and U is an Interface this subtype implements.

Therefore here is how second overloaded "SetActionFor" method for Type Router looks:

public void SetActionFor<T, U>(Action<U> action)
    where T : TBase, U
    Actions[typeof(T).TypeHandle] = 
	baseType => action((U)(T)baseType);

The cool thing is that we now do not have to create second Type Router anymore as we did in first attempt.

And (BTW) we do not have to provide second generic type argument to "SetActionFor" since compiler figures it out for us as soon as it sees that argument we provide to it, is a method with Interface (different type) parameter.

Here is a revised version of "RoutingToMethodsWithInterfaceParameter()" test method:

static void Main(string[] args)



static void RoutingToMethodsWithInterfaceParameter()
    var driver = new FireTruckDriver();
    var router = new TypeRouter<Driver>();


And voilà!

We have successfully routed Driver's subtype that implements Interface to processing Method with a parameter of that Interface type.

Here is the result:

Interface to SubTypes routing

And finally, to demonstrate the opposite to previous routing we model situation where we have an array (or IEnumerable<Interface>) of some Interface type filled with subtypes implementing that Interface and we would like to process items in array differently; as subtypes.

static void Main(string[] args)



static void RoutingInterfaceToMethodsWithSubtypeParameter()
    var drivers = new IBigVehicleDriver[] {
            new FireTruckDriver(),
            new SchoolBusDriver()

    var router = new TypeRouter<IBigVehicleDriver>();


    foreach (var dr in drivers)

We set generic parameter for Type Router to be of "IBigVehicleDriver" type and

that gives us a following result:


As we can see now we have more then fulfilled all requirements stated in section "The Goal" from Part II.

Being more robust than Visitor the Type Router allows dispatching types without being bound by dependencies the Visitor design pattern has.

The whole implementation can be packaged now in shared library and be reused in many projects without being forced to change it every time when a new subtype is derived from any base type.

We have got an ability to plug in processing methods incrementally anywhere in our code.

As a bonus Type Router offers different variations of type dispatching that Visitor has been missing, which allows for even more code flexibility.

And the most important thing is that by utilizing Type Router, code can be maintained (changed) without fear of breaking another part of it anywhere else.

Alex Movshovich.

Software Developer.


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

© 2012 softomiser


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