I have been writing and recording on design patterns for past some days. You can see some of the design pattern videos we have made on http://www.questpond.com/FreeDesign1.htm.
You can download by architecture interview question book from here.
In the previous article we have discussed about the fundamentals of IOC and DI design patterns. In case if you have missed it you can read more about it here
In the same article we also discussed about how Windsor can be used to solve this problem. In this article we will take up a simple example and try to implement DI using unity application blocks thus resulting in loosely coupled architecture.
Any good architecture always stands on the base that components are loosely coupled. let me stress the point truly loosely coupled. Let's try to understand this with a scenario and how we normally approach this problem using pure object oriented way.
Consider a scenario where we have a customer class which needs to perform insert operation on database. The customer class should be customizable to save in either SQL server or oracle database. In future we should also be able to add new databases with out disturbing the customer class.
Below is what we will do. Define two classes one for SQL server and other for Oracle. Both these classes inherit from some kind of same interface 'Idatabase'. The customer class points to this interface. So the customer class is theoretically shielded from the concrete implementation of SQL Server or oracle. But because the object creational activity needs to be done by the customer class it still needs to be aware of the concrete classes.
Figure: - General loosely coupled thinking
This is what will happen in actual implementation as shown in figure 'Concrete classes'. The customer class will be creating objects of either oracle or SQL server depending on condition. That means the customer class is exposed to the concrete classes which defeats the purpose of interface. Any change in the database classes will lead to compiling of the customer class.
Figure :- Concrete classes
The next thing comes in mind is creation patterns. If we can introduce a factory who takes the creational aspect of the concrete classes thus isolating the concrete classes from the customer class.
Here are the issues with factory which makes us force to think about some other solutions:-
. Everything is hardcoded: - The biggest issues with factory are it can not be reused across applications. All the options are hardcoded in the factory itself which makes the factory stringent to particular implementation.
. Interface dependent: - The base on which factories stands are common interfaces. Interfaces decouple the implementation and the object creation procedure. But then all the classes should implement a common interface. This is a limitation by itself again.
. Factories are custom: - They are very much custom to a particular implementation.
. Everything is compile time: - All dependent objects for an object in factory have to be known at compile time.
I will not be explaining factory pattern in case you are not aware of you can read the same in my previous article SoftArchInter1.aspx
Ok, I will show you a magic.Rather than writing those huge line of code for factory.lets go the DI way (Dependency injection).
Rather than writing factory pattern, how about injecting the object directly in to the customer class. So let the customer class references the interface and we should be able to inject the concrete type in to the customer class. With this the customer class does not need to use the new keyword and is complete decoupled from the concrete classes.
Figure: - Dependency injection
Injection can be done using containers. So basically you need to put both the classes in to the container and the container will create object and inject the same in to the other class.
Figure: - Containers
With this your customer class does not need to worry about creating objects and knowing the concrete objects.
Now that we know the benefit of containers we will explore unity application block which helps us to achieve dependency injection.
The first step is to download the unity application block from http://msdn.microsoft.com/en-us/library/cc468366.aspx and install the same. Depending on whether you have 2008 or 2005 the documentation will vary. For this tutorial I will be using VS 2005 because majority professionals are still using 2005.
Figure: - unity block installed
Once you install the block you should be able to see the same in Microsoft patterns and practices.
The first thing you need to do is to get the references of at least two components Unity and Unity configuration.
Figure: - Add referenced to Unity components
Once you have got reference to the components import the "Microsoft.Practices.Unity" namespace in to your code as shown in figure 'Import unity namespace'.
Figure: - Import unity namespace
As said previously we should be able to inject SQL Server database object or oracle database object in the customer class. So as a good practice we will inherit the SQL server implementation and oracle implementation from a common interface 'Idatabase'.
Figure: - Database interfaces
In the customer class we will reference the interface and expose the public property using [Dependency] attribute. This exposure is essential for the unity container. Unity container needs some kind of gateway by which it can inject the object. So what we have done is we have exposed the interface 'iDatabase' publicly. There are lot of other ways by which we can achieve the same you can read my previous article on the different ways of providing injection gateways for containers here.
Figure: - Providing injection gateway
To achieve high customization unity application allows us to specify how the injection will work in the config files ( web.config and app.config ). As this example is a windows command application we will be defining an app.config file.
You can see below a simple template of App.config file from unity perspective. The Configsection is compulsory and keep it as it is. The important section to be noted is the types section which comes under container section. In this we specify which interfaces map to which concrete class. The unity container creates object from this section and inserts in to the customer object through the dependency attribute.
Figure: - App.config file
The client code is pretty simple. The first step us we create the unity container object. In the second step we read the unity section. In step 3 we configure the container using the unity section data.
In step 4 we tell the container to create the customer object. This is the most important step of all. In this the container using the config data and the dependency attribute injects either the SQL server database object or oracle database object in to the customer class. This object creation depends on what is defined in the app.config file.
Finally in step 5 we call the save method
Figure: - Client code
So what happens internally in the container is that the unity container creates the object of either oracle or SQL server data objects and injects in to the customer class through the dependency defined attribute.
So if you define the 'clsSqlServer' class it will create the object of 'clsSqlServer' and inject it in to the customer class. If you define the 'ClsOracle' class it will inject the 'clsOracle' object. You can see in figure 'The real fun' how easy it to change implementation by just modifying the config file type sections
Figure: - The real fun