Contextual Binding

One of the more powerful (and complex) features of Ninject is its contextual binding system. We mentioned earlier that you can register more than one binding for a type. There are two primary reasons one might want to register multiple type bindings:-

Multiple bindings – for contextual binding taking the context into consideration

In this case, the appropriate implementation that should be used depends on the target context for which the service type is being resolved and the binding metadata on each binding. Up until this point, we’ve only been talking about default bindings — bindings that are used unconditionally, in any context. However, unless you make one or more of the bindings conditional, you’ll get an exception at runtime indicating that the appropriate binding to use in a given context is ambiguous (you’ll run into the same case if you do a kernel.Get<T>() and there are multiple possible results for the given service type T).

Multiple bindings – for Multi-Injection

This mechanism allows you to get a set of related service implementations at the same time – this is not contextual binding. You use this mechanism by requesting a IEnumerable<>, T[] array or List<T> in the target – see Multi-injection

class Warrior {
    readonly IEnumerable<IWeapon> _weapons;
    public Warrior( IEnumerable<IWeapon> weapons) {
        _weapons=weapons;
    }
    public void Attack(string victim) {
        foreach(var weapon in _weapons) 
            Console.WriteLine(weapon.Hit(victim));
    }
}

Simple constrained resolution: Named bindings

Named bindings are the simplest (and most common) form of conditional binding, and are used as follows:

1. In your type bindings, you register multiple type bindings for the same service type, but add binding metadata so you’ll be able to indicate which one is appropriate in your condition later:-

Bind<IWeapon>().To<Shuriken>().Named("Strong");
Bind<IWeapon>().To<Dagger>().Named("Weak");

2. At the target location, you indicate the name you want to use in the given context:-

class WeakAttack {
    readonly IWeapon _weapon;
    public([Named("Weak")] IWeapon weakWeapon)
        _weapon = weakWeapon;
    }
    public void Attack(string victim){
        Console.WriteLine(_weapon.Hit(victim));
    }
}

3. alternately, you can programmatically achieve the same effect directly (aka Service Location antipattern) by doing:

kernel.Get<IWeapon>("Weak");

Other Constrained Resolution mechanisms

The Named Bindings in the previous section are built-in and have associated overloads and attribute types for a very simple reason – they’re a very common use case that can often satisfy the requirements of most straightforward applications. However they’re just a specific application of a more general concept: Constrained Resolution.

Constrained resolution involves the following elements:

• binding metadata: extra information we include in a type binding
• the target context: the receiver which the service is being resolved for
• One or more of the following
  1. constraint predicates on the target using the binding metadata to take into consideration when matching a resolution target against a set of bindings
  2. constraints on the type binding which take into account the target context

Constraining the binding selection via ConstraintAttribute-derived attributes on the injection target

ConstraintAttribute is an abstract attribute class from which one can derive your own constraints which filter the set of eligible bindings for injection into a target. Example usage is as follows:

// will work just as well without this line, but it's more correct and important for IntelliSense etc.
[AttributeUsage(AttributeTargets.Property | AttributeTargets.Field | AttributeTargets.Parameter,
    AllowMultiple = true, Inherited = true)]
public class Swimmer : ConstraintAttribute {
    public bool Matches(IBindingMetadata metadata) {
        return metadata.Has("CanSwim") && metadata.Get<bool>("CanSwim");
    }
}
class WarriorsModule : Ninject.Modules.NinjectModule {
    public override void Load() {
        Bind<IWarrior>().To<Ninja>();
        Bind<IWarrior>().To<Samurai>().WithMetadata("CanSwim", false);
        Bind<IWarrior>().To<SpecialNinja>().WithMetadata("CanSwim", true);
    }
}
class AmphibiousAttack {
    public AmphibiousAttack([Swimmer]IWarrior warrior) {
        Assert.IsType<SpecialNinja>(warrior);
    }
}

The above is a basic example of using the metadata to guide the selection of the appropriate binding. Here are some other examples:

[AttributeUsage(AttributeTargets.Property | AttributeTargets.Field | AttributeTargets.Parameter, 
    AllowMultiple = true, Inherited = true)]
public class NonSwimmer : ConstraintAttribute {
    public bool Matches(IBindingMetadata metadata) {
        return metadata.Has("CanSwim") && !metadata.Get<bool>("CanSwim");
    }
}
class OnLandAttack {
    public OnLandAttack([NonSwimmer]IWarrior warrior) {
        Assert.IsType<Samurai>(warrior);
    }
}

Since the conditionals are attached to the target, if no conditional is specified on the target then all bindings will be used. The following code will throw an ActivationException because the results are ambiguous.

class JustAttack {
    // Note: This will fail because we have three matching bindings.
    public JustAttack(IWarrior warrior) {
    }
}

If this is not what you want then see the next section on binding-based conditionals.

Note that one is allowed to have multiple attributes on a target – e.g., you can combine the built-in [Named("X")] with a custom constraint such as [CanSwim] above and it’ll add the two conditions together.

Specifying constraints on the type binding using the built-in attribute-based helpers

A key issue with explicitly managing binding metadata as string->object dictionary mappings is that things can easily go wrong with magic strings – as a result it’s generally recommended to prefer tagging things with attributes in preference whereever possible.

An important point about previous section regarding Attribute driven constraints at the target site is that a sensible default approach is to use Constructor Injection and not have any container-dependencies in your business logic unless you have a really good reason. This is why constraints are more typically specified alongside the type bindings, although Ninject doesn’t mind either way – it’ll even let you constrain from the target side and the type binding side at the same time. Examples of constraints on a type binding are (specifying the same constraints as the previous section but on the bindings instead):

Bind<IWarrior>().To<Samurai>().WhenInjectedInto(typeof(OnLandAttack));
Bind<IWarrior>().To<SpecialNinja>().WhenInjectedInto(typeof(AmphibiousAttack));

There are a multiple constraint options, all specified via a When... option at the end of the type binding. The simplest form are ones driven by straightforward marker attributes, without any need to derive from base classes etc.

class SwimmerNeeded : Attribute{}
class ClimberNeeded : Attribute{}

This allows one to define constraints based on any of the following:

1. the Target: the parameter being injected into
2. the Member: the property, method or constructor itself
3. the Class: the class Type within which the Member or Target is defined within

class WarriorsModule : Ninject.Modules.NinjectModule {
    public override void Load() {
        Bind<IWarrior>().To<Ninja>();
        Bind<IWarrior>().To<Samurai>().WhenClassHas<ClimberNeeded>();
        Bind<IWarrior>().To<Samurai>().WhenTargetHas<ClimberNeeded>();
        Bind<IWarrior>().To<SpecialNinja>().WhenMemberHas<SwimmerNeeded>();
    }
}

Then the right things will happen when one resolves services such as:

class MultiAttack {
    public MultiAttack([ClimberNeeded] IWarrior MountainWarrior) {
    }
    [Inject, SwimmerNeeded]
    IWarrior OffShoreWarrior { get; set; }
    [Inject]
    IWarrior AnyOldWarrior {get;set;}
}

[ClimberNeeded]
class MountainousAttack {
    [Inject, SwimmerNeeded]
    IWarrior HighlandLakeSwimmer { get; set; }
    [Inject]
    IWarrior StandardMountainWarrior { get; set; }
}

Specifying constraints on the type binding using arbitrary elements of the resolution request context

All of the .WhenXXX() helpers in the preceding section are just built-in specific applications of the generalized .When(Func<IRequest,bool>) method, which one can use as follows:

Bind<IWarrior>().To<Samurai>().When(request => request.Target.Member.Name.StartsWith("Climbing"));
Bind<IWarrior>().To<Samurai>().When(request => request.Target.Type.Namespace.StartsWith("Samurais.Climbing"));

Other examples of using the request context

As with all of Ninject, it’s highly recommended to look at the tests

They go through all the possibilities in detail, and are extremley short and readable – try it; you won’t regret looking and you may learn something along the way!

Using the request context in a Factory or Factory Method to do contextual binding

Another way of doing contextual resolution is to have the factory take the context into consideration. For example, people often use a pattern as follows to do logging:

class ClassThatLogs {
    ILog _log = LogFactory.CreateLog(typeof(ClassThatLogs));
    // Sometimes one sees reflection based approaches too, which have efficiency issues
}

In the above, each logger

1. needs to be customized for the context into which it is cut and pasted
2. is being located rather than injected (aka Service Location antipattern)

An alternate approach is to make the Factory Method’s generated result contextual is to bind as follows:

Bind<LogSink>().ToMethod( context => LogFactory.CreateLog( context.Request.Target.Type ) );

Which enables having the loggers injected as follows:

class ClassThatLogs {
    readonly ILog _log;
    public ClassThatLogs(ILog log){
        _log = log;
    }
}

Specifying constraints in terms of Nested Contexts

There are .Depth, .ParentContext and .ParentRequest members on the IRequest passed to the When predicate which are used to define rules based on the hierarchy of requests in a service resolution where nesting takes place. See The Activation Process for more details on this.

Some articles on topics related to nested resolution:

• Remo Gloor’s article on Context Preservation

Continue reading: Conventions-Based Binding