ObjectBuilder

ObjectBuilder is a C# dependency injection framework, more precisely it is a framework for building a dependency injectors. There were two versions, the Object Builder 2 was later integrated into Unity Application Block.

The earlier versions is used by Web Client Software Factory (WCSF), the library for building web applications in WebForms. It is tool we are using for our internal system.

The first thing I notices about Object Builder is that it is woefully undocumented (official MSDN documentation), the source code is available, but no quick start or anything. I have googled a little and found very helpful post about how to actually use it to create objects. – sort of quick start tutorial.

Good, but not enough, I am trying to transfer out C# app from Web Site Project in Web Forms and WCSF to to MVC, but since WCSF auto-magically inserts all dependencies, I had to dive into source code of OB in order to understand it and later integrate it for MVC controller creation (I want to insert already existing services and other stuff into MVC controllers).

First, it is good to have an idea what functionality of ObjectBuilder do I have to replicate:

  • Services – Singletons that are alive during the whole life of web app. When constructor of object or property of object has a attribute [ServiceDependency], the OB will put there a singleton instance of some object.
  • New objects – whenever asked (through attribute [CreateNew]), the OB creates a new object.
  • There is also some registration of services and type mappings, but I am only interested in how to create a new object / get service just like the WCSF would.

Since the OB is not a dependency injection framework, but rather framework for building DI, it has only very simple DI framework – the one that can either create a new object or use singleton singleton instance.

The WCSF can use DI in two ways, either through constructor or by filling proprties (see official documentation):

public class MyClass {
  public MyClass(
    [CreateNew] IStoreInventory storeInventory,
    [ServiceDependency] ITimeService timeService)
  {
    // ... object initialization
  }
  
  [ServiceDependency]
  public IMyService MyService {get;set;}
}

Core concepts of the ObjectBuilder:

  • IBuilder – The builder that builds up or tears own the objects. The BuilderBase class is easy to understand. You can create a object like this
builder.BuildUp<MyClass>(locator, idToBuild, existing)
// In reality all parameters can be null when not used

The builder has a chain of building strategies (the various ways to create an object).

  • IBuilderStrategy – Strategy how will the objects be build.

The strategies can be varied, e.g. singleton strategy can look through locator and if there already is an object, return it; if the object is not in locator, create it, add to locator and return it. Or the the object can be build by some factory method.

  • IBuilderPolicy – Policy tailoring concrete implementations of IBuilderStartegy, e.g. IMethodPolicy can tailor which method will be called by MethodExecutionStrategy.
  • IBuilderContext – context used for one request to build up an object. It consists from locator, chain of building strategies and list of policies. It basically only holds tailored data passed to the chain of strategies of IBuilder.
  • ILocator – Basically a dictionary of id-object, it is used mostly for singletons, so when someone asks for an object with specified id, the OB will use locator to locate it.

When we request an object from IBuilder, it

  1. takes passed paramaters,
  2. creates a new IBuilderContext from passed parameters and other internal data (strategies, policies)
  3. IBuilder asks the head of builder strategy chain to build up an object.
  4. The IBuilderStartegy will look at the IBuilderContext and other data and determines if it can build up an object. If it can, it returns an object. If it can’t, it asks the next strategy in builder chain to try to build up the object. Note that each link of chain must call previous link.
  5. The base implementation BuilderStartegy of IBuilderStartegy will return the existing object (the one passed as parameter into IBuilder.BuildUp), if all strategies in chain fail.

This is rather high level description of OB, concrete example how to set it up, look at the David Hayden blog.

Leave a Reply

Your email address will not be published. Required fields are marked *