seesharper/LightInject

Installing

LightInject provides two distribution models via NuGet

Binary

This adds a reference to the LightInject.dll in the target project.

Source

This will install a single file (LightInject.cs) into the current project.

Creating a container

var container = new LightInject.ServiceContainer();

The container implements IDisposable and should be disposed after usage has completed. It can also be used inside of a using statement for a constrained scope.

Default services

public interface IFoo {}
public class Foo : IFoo {}

container.Register<IFoo, Foo>();
var instance = container.GetInstance<IFoo>();
Assert.IsInstanceOfType(instance, typeof(Foo));

Named services

public class Foo : IFoo {}
public class AnotherFoo : IFoo {}

container.Register<IFoo, Foo>();
container.Register<IFoo, AnotherFoo>("AnotherFoo");
var instance = container.GetInstance<IFoo>("AnotherFoo");
Assert.IsInstanceOfType(instance, typeof(AnotherFoo));

If only one named registration exists, LightInject is capable of resolving this as the default service.

container.Register<IFoo, AnotherFoo>("AnotherFoo");
var instance = container.GetInstance<IFoo>();
Assert.IsInstanceOfType(instance, typeof(AnotherFoo));

Unresolved services

LightInject can resolve services that are not registered with the container using the RegisterFallback method.

var container = new ServiceContainer();
container.RegisterFallback((type, s) => true, request => new Foo());
var foo = container.GetInstance<IFoo>();

The first argument to the RegisterFallback method makes it possible to decide if the service can be "late-resolved". The second argument is a ServiceRequest instance that provides the requested service type and service name.

IEnumerable<T>

When we register multiple services with the same service type, LightInject is capable of resolving these services as an IEnumerable<T>.

public class Foo : IFoo {}
public class AnotherFoo : IFoo {}

container.Register<IFoo, Foo>();
container.Register<IFoo, AnotherFoo>("AnotherFoo");
var instances = container.GetInstance<IEnumerable<IFoo>>();
Assert.AreEqual(2, instances.Count());

Alternatively using the GetAllInstances method.

var instances = container.GetAllInstances<IFoo>();
Assert.AreEqual(2, instances.Count());

In addition, LightInject supports the following IEnumerable<T> sub-types.

• Array
• ICollection<T>
• IList<T>
• IReadOnlyCollection<T> (Net 4.5 and Windows Runtime);
• IReadOnlyList<T> (Net 4.5 and Windows Runtime)

By default, LightInject will resolve all services that are compatible with the requested element type.

container.Register<Foo>();
container.Register<DerivedFoo>();
var instances = container.GetAllInstances<Foo>();
Assert.AreEqual(2, instances.Count());

This behavior can be overridden using the EnableVariance container option.

var container = new ServiceContainer(new ContainerOptions { EnableVariance = false });
container.Register<Foo>();
container.Register<DerivedFoo>();
var instances = container.GetAllInstances<Foo>();
Assert.AreEqual(1, instances.Count());

We can also selectively decide to apply variance only for certain IEnumerable<T> services.

options.VarianceFilter = (enumerableType) => enumerableType.GetGenericArguments()[0] == typeof(IFoo);

Ordering

Sometimes the ordering of the resolved services are important and LightInject solves this by ordering services by their service name.

container.Register<IFoo, Foo1>("A");
container.Register<IFoo, Foo2>("B");
container.Register<IFoo, Foo3>("C");

var instances = container.GetAllInstances<IFoo>().ToArray();
Assert.IsType<Foo1>(instances[0]);
Assert.IsType<Foo2>(instances[1]);
Assert.IsType<Foo3>(instances[2]);

We can also register multiple implementations for a given service type using the RegisterOrdered method.

var container = CreateContainer();
container.RegisterOrdered(
    typeof(IFoo),
    new[] {typeof(Foo1), typeof(Foo2), typeof(Foo3)},
    type => new PerContainerLifetime());

var instances = container.GetAllInstances<IFoo>().ToArray();

Assert.IsType<Foo1>(instances[0]);
Assert.IsType<Foo2>(instances[1]);
Assert.IsType<Foo3>(instances[2]);

The RegisterOrdered method gives each implementation a service name that can be used for ordering when resolving these services. By default the service name is formatted like 001, 002 and so on. If we need so change this convention, we can do this by passing a format function to the RegisterOrdered method.

container.RegisterOrdered(
    typeof(IFoo<>),
    new[] { typeof(Foo1<>), typeof(Foo2<>), typeof(Foo3<>) },
    type => new PerContainerLifetime(), i => $"A{i.ToString().PadLeft(3,'0')}");

var services = container.AvailableServices.Where(sr => sr.ServiceType == typeof(IFoo<>))
    .OrderBy(sr => sr.ServiceName).ToArray();
Assert.Equal("A001", services[0].ServiceName);
Assert.Equal("A002", services[1].ServiceName);
Assert.Equal("A003", services[2].ServiceName);

Values

Registers the value as a constant.

container.RegisterInstance<string>("SomeValue");
var value = container.GetInstance<string>();
Assert.AreEqual("SomeValue, value");

Compilation

LightInject uses dynamic code compilation either in the form of System.Reflection.Emit or compiled expression trees. When a service is requested from the container, the code needed for creating the service instance is generated and compiled and a delegate for that code is stored for lookup later on so that we only compile it once. These delegates are stored in an AVL tree that ensures maximal performance when looking up a delegate for a given service type. If fact, looking up these delegates is what sets the top performing containers apart. Most high performance container emits approximately the same code, but the approach to storing these delegates may differ.

LightInject provides lock-free service lookup meaning that no locks are involved for getting a service instance after its initial generation and compilation. The only time LightInject actually creates a lock is when generating the code for a given service. That does however mean a potential lock contention problem when many concurrent requests asks for services for the first time.

LightInject deals with this potential problem by providing an API for compilation typically used when an application starts.

The following example shows how to compile all registered services.

container.Compile();

One thing to be aware of is that not all services are backed by its own delegate.

Consider the following service:

public class Foo
{
    public Foo(Bar bar)
    {
        Bar = bar;
    }
} 

Registered and resolved like this:

container.Register<Foo>();
container.Register<Bar>();
var foo = container.GetInstance<Foo>();

In this case we only create a delegate for resolving Foo since that is the only service that is directly requested from the container. The code for creating the Bar instance is embedded inside the code for creating the Foo instance and hence there is only one delegate created.

We call Foo a root service since it is directly requested from the container.

In fact lets just have a look at the IL generated for creating the Foo instance.

newobj Void .ctor() // Bar
newobj Void .ctor(LightInject.SampleLibrary.IBar) //Foo

What happens here is that a new instance of Bar is created and pushed onto the stack and then we create the Foo instance. This is the code that the delegate for Foo points to.

The reason for such a relatively detailed explanation is to illustrate that we don't always create a delegate for a given service and by simply doing a container.Compile() we might create a lot of delegates that is never actually executed. Probably no big deal as long as we don't have tens of thousands of services, but just something to be aware of.

LightInject does not attempt to identify root services as that would be very difficult for various reasons.

We can instead use a predicate when compiling services up front.

container.Compile(sr => sr.ServiceType == typeof(Foo));

Open Generics

LightInject cannot compile open generic services since the actual generic arguments are not known at "compile" time.

We can however specify the generic arguments like this:

container.Compile<Foo<int>>()

LightInject will create a log entry every time a new delegate is created so that information can be used to identify root services that could be compiled up front. In addition to this, a log entry (warning) is also created when trying to compile an open generic service up front.

Lifetime

The default behavior in LightInject is to treat all objects as transients unless otherwise specified.

container.Register<IFoo,Foo>();
var firstInstance = container.GetInstance<IFoo>();
var secondInstance = container.GetInstance<IFoo>();
Assert.AreNotSame(firstInstance, secondInstance);

PerScopeLifetime

Ensures that only one instance of a given service can exists within a scope. The container will call the Dispose method on all disposable objects created within the scope.

container.Register<IFoo,Foo>(new PerScopeLifetime());
using(container.BeginScope())
{    
    var firstInstance = container.GetInstance<IFoo>();
    var secondInstance = container.GetInstance<IFoo>();
    Assert.AreSame(firstInstance, secondInstance);
}

Note: An InvalidOperationException is thrown if a service registered with the PerScopeLifetime is requested outside the scope.

PerContainerLifetime

Ensures that only one instance of a given service can exist within the container. The container will call the Dispose method on all disposable objects when the container itself is disposed.

using(container = new ServiceContainer())
{
    container.Register<IFoo,Foo>(new PerContainerLifetime());    
    var firstInstance = container.GetInstance<IFoo>();
    var secondInstance = container.GetInstance<IFoo>();
    Assert.AreSame(firstInstance, secondInstance);
}

PerRequestLifeTime

A new instance is created for each request and the container calls Dispose when the scope ends. This lifetime is used when the conrete class implements IDisposable.

container.Register<IFoo,Foo>(new PerRequestLifeTime());
using(container.BeginScope())
{        
    var firstInstance = container.GetInstance<IFoo>();
    var secondInstance = container.GetInstance<IFoo>();
    Assert.AreNotSame(firstInstance, secondInstance);
}    

Note: An InvalidOperationException is thrown if a service registered with the PerRequestLifeTime is requested outside the scope.

Custom lifetime

A custom lifetime is created by implementing the ILifetime interface

internal interface ILifetime
{
    object GetInstance(Func<object> instanceFactory, Scope currentScope);        
}

The following example shows to create a custom lifetime that ensures only one instance per thread.

public class PerThreadLifetime : ILifetime
{
    ThreadLocal<object> instances = new ThreadLocal<object>();     

    public object GetInstance(Func<object> instanceFactory, Scope currentScope)
    {
        if (instances.value == null)
        {
            instances.value = instanceFactory();
        }
        return instances.value;
    }
}

That is all it takes to create a custom lifetime, but what about disposable services?

public class PerThreadLifetime : ILifetime
{
    ThreadLocal<object> instances = new ThreadLocal<object>();     

    public object GetInstance(Func<object> instanceFactory, Scope currentScope)
    {            
        if (instances.value == null)
        {                
            object instance = instanceFactory();                
            IDisposable disposable = instance as IDisposable;                
            if (disposable != null)
            {
                if (currentScope == null)
                {
                    throw new InvalidOperationException(
                        "Attempt to create an disposable object without a current scope.");
                }
                currentScope.TrackInstance(disposable);
            }

            instances.value = instance;
        }
        return instance.value;
    }
}

Important

A lifetime object controls the lifetime of a single service and can never be shared for multiple service registrations.

Wrong

ILifetime lifetime = new PerContainerLifeTime();
container.Register<IFoo,Foo>(lifetime);
container.Register<IBar,Bar>(lifetime);

Right

container.Register<IFoo,Foo>(new PerContainerLifeTime());
container.Register<IBar,Bar>(new PerContainerLifeTime());

A lifetime object is also shared across threads and that is something we must take into consideration when developing new lifetime implementations.

Async and Await

By default scopes are managed per thread which means that when the container looks for the current scope, it will look for a scope that is associated with the current thread.

With the introduction of the async/await pattern chances are that the code that is requesting a service instance is running on another thread.

To illustrate this lets consider an example that is going to cause an instance to be resolved on another thread.

We start of by creating an interface that returns a Task<IBar>

public interface IAsyncFoo
{
    Task<IBar> GetBar();
}

Next we implement this interface in such a way that the IBar instance is requested on another thread.

public class AsyncFoo : IAsyncFoo
{
    private readonly Lazy<IBar> lazyBar;

    public AsyncFoo(Lazy<IBar> lazyBar)
    {
        this.lazyBar = lazyBar;
    }

    public async Task<IBar> GetBar()
    {
        await Task.Delay(10);
        return lazyBar.Value; <--This code is executed on another thread (continuation).
    }
}

The we register the dependency (IBar) with the PerScopeLifetime that is going to cause the container to ask for the current scope so that the instance can be registered with that scope.

var container = new ServiceContainer();
container.Register<IBar, Bar>(new PerScopeLifetime());
container.Register<IAsyncFoo, AsyncFoo>();

using (container.BeginScope())
{
    var instance = container.GetInstance<IAsyncFoo>();
    ExceptionAssert.Throws<AggregateException>(() => instance.GetBar().Wait());                
}

This will throw an exception that states the following:

Attempt to create a scoped instance without a current scope.  

The reason that this is happening is that the current scope is associated with the thread that created it and when the continuation executes, we are essentially requesting an instance on another thread.

To deal with this issue, LightInject now supports scopes across the logical CallContext.

var container = new ServiceContainer();
container.ScopeManagerProvider = new PerLogicalCallContextScopeManagerProvider();
container.Register<IBar, Bar>(new PerScopeLifetime());
container.Register<IAsyncFoo, AsyncFoo>();

using (container.BeginScope())
{
    var instance = container.GetInstance<IAsyncFoo>();
    var bar = instance.GetBar().Result;
    Assert.IsInstanceOfType(bar, typeof(IBar));
}

Note that the PerLogicalCallContextScopeManagerProvider is only available when running under .Net 4.5. For more information, please refer to the following article by Stephen Cleary.

Scope

The purpose of the scope is to track the services created within the scope. For instance, the PerScopeLifetime uses the scope to ensure that we only create a single service instance even if it requested multiple times.

One of the most canonical examples would be in a web application where we need to inject IDbConnection into different services. Let say that we have an OrderController and we need two services to process the order.

public class CustomerService : ICustomerService
{
    public CustomerService(IDbConnection dbConnection)
    {
    }
}

public class OrderService : IOrderService
{
    public OrderService(IDbConnection dbConnection)
    {
    }
}  

public class OrderController
{
    public OrderController(ICustomerService customerService, IOrderService orderService)
    {
    }
}

As we can see the OrderController depends on both the CustomerService and the OrderService which are both dependant upon an IDbConnection.

By registering the IDbConnection as a scoped service we ensure two things.

• Only a single instance of IDbConnection will ever be created inside a scope.
• The IDbConnection instance is disposed when the scope ends.

container.RegisterScoped<IDbConnection>(factory => new ProviderSpecificConnection());

So when and how do we start these scopes?

She short answer is that most of the time, we don't. For instance, in AspNetCore, the scopes are started and ended by the AspNetCore infrastructure so we don't have to think about that when developing web application. A scope is started when the web request starts and it ended when the web request ends. It is really that simple, one request equals one scope.

So in LightInject, we register a scoped service using RegisterScoped without really thinking about when and how the scopes are started and ended. In a web application this usually means a web request, but for other applications it can mean something else. Maybe for a UI application it means a page/window/form or something similar.

To start a scope manually we can create scope using the BeginScope method

using (container.BeginScope())
{
    var dbConnection = container.GetInstance<IDbConnection>();  
}        

Note: The Scope implement IDisposable and should always be wrapped in a using block to ensure its disposal

In this example we start a new scope and retrieve the service from the container which means that LightInject uses the "current" scope to resolve the service. This is only supported for backwards compatibility and should be avoided if possible. The recommended approach is to retrieve services directly from the scope.

using (var scope = container.BeginScope())
{
    var dbConnection = scope.GetInstance<IDbConnection>();  
}    

Since we are retrieving the service directly from the scope, the current scope is ignored and we simply use the scope from which the service was requested. This is not only much faster, but it is also a much safer way to deal with scopes.

This also allows for multiple active scopes.

using (var outerScope = container.BeginScope())
{    
    using (var innerScope = container.BeginScope())
    {
        var outerDbConnection = outerScope.GetInstance<IDbConnection>();
        var innerDbConnection = innerScope.GetInstance<IDbConnection>();
    }  
}    

In addition to the PerScopeLifetime which ensures disposal and a single instance within a scope, we also have the PerRequestLifetime. This lifetime behaves just a transient meaning that we get a new instance for every time it is requested with the only difference to transients being that instances are disposed when the scope ends.

Note: The PerRequestLifetime has NO relation to the notion of a web request.

If we don't need access to an ambient scope, we can disable this in the ContainerOptions

var container = new ServiceContainer(o => o.EnableCurrentScope = false);

This also improves performance ever so slightly as we don't need to maintain a current scope when scopes are started and ended.

IAsyncDisposable

LightInject also supports IAsyncDisposable meaning that IAsyncDisposable.DisposeAsync will be called if the scope is started with a using-block adding the await await keyword.

await using (var scope = container.BeginScope())
{
    asyncDisposable = container.GetInstance<AsyncDisposable>();
}

The Scope returned from BeginScope also implements IAsyncDisposable and will call DisposeAsync on all scoped services resolved within the Scope. Services only implementing IDisposable will also be disposed the the async scope ends.

If on the other hand, a service ONLY implements IAsyncDisposable and is resolved within a synchronous scope, an exception will be thrown

using (var scope = container.BeginScope())
{
    asyncDisposable = container.GetInstance<AsyncDisposable>();
}

Dependencies

Constructor Injection

public interface IFoo {}        
public interface IBar {}

public class Foo : IFoo
{
    public Foo(IBar bar) 
    {
        Bar = bar;
    }

    public IBar Bar { get; private set; } 
}

public class Bar : IBar {}

Implicit service registration

Registers a service without specifying any information about how to resolve the constructor dependencies of the implementing type.

container.Register<IFoo, Foo>();
container.Register<IBar, Bar>();
var foo = (Foo)container.GetInstance<IFoo>();
Assert.IsInstanceOfType(foo.Bar, typeof(Bar)); 

Note: In the case where the implementing type(Foo) has more than one constructor, LightInject will choose the constructor with the most parameters.

For fine grained control of the injected constructor dependencies, we can provide a factory that makes it possible to create an instance of a given constructor dependency.

container.RegisterConstructorDependency<IBar>((factory, parameterInfo) => new Bar());

This tells the container to inject a new Bar instance whenever it sees an IBar constructor dependency.

Explicit service registration

Registers a service by providing explicit information about how to create the service instance and how to resolve the constructor dependencies.

container.Register<IBar, Bar>();
container.Register<IFoo>(factory => new Foo(factory.GetInstance<IBar>()));
var foo = (Foo)container.GetInstance<IFoo>();
Assert.IsNotNull(foo.Bar);

Parameters

Parameters are used when we want to supply one or more values when the service is resolved.

public class Foo : IFoo
{
    public Foo(int value)
    {
        Value = value;
    }

    public int Value { get; private set; }
}   

container.Register<int, IFoo>((factory, arg) => new Foo(arg));
var foo = (Foo)container.GetInstance<int, IFoo>(42);
Assert.AreEqual(42,foo.Value);

We can also do a combination of supplied values and dependencies.

public class Foo : IFoo
{
    public Foo(int value, IBar bar)
    {
        Value = value;
    }

    public int Value { get; private set; }
    public IBar Bar { get; private set; }
}    

container.Register<IBar, Bar>();
container.Register<int, IFoo>((factory, value) => new Foo(value, factory.GetInstance<IBar>()));
var foo = (Foo)container.GetInstance<int, IFoo>(42);
Assert.AreEqual(42, foo.Value);
Assert.IsNotNull(foo.Bar);

Optional arguments

LightInject will allow for default values to be used when a constructor dependency cannot be resolved.

public class Foo
{
    public Foo(string value = "42")
    {
        Value = value;
    }

    public string Value { get; }
}

We can still resolve Foo even though we have not registered a string service.

var container = new ServiceContainer(options => options.EnableOptionalArguments = true);
container.Register<Foo>();
var instance = container.GetInstance<Foo>();
Assert.AreEqual("42", instance.Value)

Note that the use cases for optional dependencies should be rare and are to be used with caution.

Property Injection

public interface IFoo {}

public interface IBar {}

public class Foo : IFoo
{
    public IBar Bar { get; set; }
}

public class Bar : IBar {}

Implicit service registration

Registers the service without specifying any information about how to resolve the property dependencies.

container.Register<IFoo, Foo>();
container.Register<IBar, Bar>();
var foo = (Foo)container.GetInstance<IFoo>();
Assert.IsNotNull(foo.bar);

Note: LightInject considers all read/write properties a dependency, but implements a loose strategy around property dependencies, meaning that it will NOT throw an exception in the case of an unresolved property dependency.

For fine grained control of the injected property dependencies, we can provide a factory that makes it possible to create an instance of a given property dependency.

container.RegisterPropertyDependency<IBar>((factory, propertyInfo) => new Bar());

This tells the container to inject a new Bar instance whenever it sees an IBar property dependency.

Explicit service registration

Registers a service by providing explicit information about how to create the service instance and how to resolve the property dependencies.

container.Register<IBar, Bar>();
container.Register<IFoo>(factory => new Foo() {Bar = factory.GetInstance<IBar>()}) 
var foo = (Foo)container.GetInstance<IFoo>();
Assert.IsNotNull(foo.bar);

Property injection on existing instances.

In the cases where we don't control the creation of the service instance, LightInject can inject property dependencies into an existing instance.

container.Register<IBar, Bar>();
var foo = new Foo();
container.InjectProperties(foo);
Assert.IsNotNull(foo);

Disabling ProperyInjection

Property injection is enabled by default in LightInject, but it can be disabled like this.

var container = new ServiceContainer(new ContainerOptions { EnablePropertyInjection = false });

It is actually recommended to turn off property injection unless it is really needed. Backward compatibility is the only reason that this is not the default.

Initializers

Use the Initialize method to perform service instance initialization/post-processing.

container.Register<IFoo, FooWithPropertyDependency>();
container.Initialize(
    registration => registration.ServiceType == typeof(IFoo), 
    (factory, instance) => ((FooWithPropertyDependency)instance).Bar = new Bar());
var foo = (FooWithProperyDependency)container.GetInstance<IFoo>();
Assert.IsInstanceOfType(foo.Bar, typeof(Bar));

Assembly Scanning

LightInject is capable of registering services by looking at the types of a given assembly.

container.RegisterAssembly(typeof(IFoo).Assembly)

To filter out the services to be registered with the container, we can provide a predicate that makes it possible to inspect the service type and the implementing type.

container.RegisterAssembly(typeof(IFoo).Assembly, (serviceType, implementingType) => serviceType.NameSpace == "SomeNamespace");

It is also possible to scan a set assembly files based on a search pattern.

container.RegisterAssembly("SomeAssemblyName*.dll");  

When scanning assemblies, LightInject will register services using a service name that by default is the implementing type name. This behavior can be changed by specifying a function delegate to provide the name based on the service type and the implementing type.

container.RegisterAssembly(typeof(IFoo).Assembly, () => new PerContainerLifetime(), (serviceType, implementingType) => serviceType.NameSpace == "SomeNamespace", (serviceType, implementingType) => "Provide custom service name here");

We can also change this behavior globally for all registrations by implementing the IServiceNameProvider interface.

public class CustomServiceNameProvider : IServiceNameProvider
{
    public string GetServiceName(Type serviceType, Type implementingType)
    {
        return "Provide custom service name here";  
    }
}

To change the default behavior for all registrations we simply change this dependency on the container before we start scanning assemblies.

container.ServiceNameProvider = new CustomServiceNameProvider();

Composition Root

When LightInject scans an assembly it will look for an implementation of the ICompositionRoot interface.

public class SampleCompositionRoot : ICompositionRoot
{               
    public void Compose(IServiceRegistry serviceRegistry)
    {     
        serviceRegistry.Register(typeof(IFoo),typeof(Foo));
    }
}

If one or more implementations of the ICompositionRoot interface is found, they will be created and executed.

Note: Any other services contained within the target assembly that is not registered in the composition root, will NOT be registered.

Rather that having a single composition root that basically needs to reference all other assemblies, having multiple composition roots makes it possible to group services naturally together. Another advantage of registering services in a ICompositionRoot, is that they can easily be reused in automated tests.

Lazy Composition Roots

LightInject is capable of registering services on a need to have basis. For a large application that has a lot of services, it might not be the best solution to register all these services up front as this could seriously hurt the startup time of our application due to extensive assembly loading.

If an unregistered service is requested, LightInject will scan the assembly where this service is contained.

CompositionRootAttribute

When an assembly is being scanned, LightInject will look for implementations of the ICompositionRoot interface. For large assemblies that contains many type, this might be an expensive operation. The CompositionRootAttribute is an assembly level attribute that simply helps LightInject to locate the compostion root.

[assembly: CompositionRootType(typeof(SampleCompositionRoot))]

RegisterFrom

Allows explicit execution of a composition root.

container.RegisterFrom<SampleCompositionRoot>();

Alternatively we can also pass an existing composition root instance.

container.RegisterFrom(new SampleCompositionRoot());

Generics

public interface IFoo<T> {};
public class Foo<T> : IFoo<T> {};

The container creates the closed generic type based on the service request.

container.Register(typeof(IFoo<>), typeof(Foo<>));
var instance = container.GetInstance(typeof(IFoo<int>));
Assert.IsInstanceOfType(instance, typeof(Foo<int>));

Constraints

LightInject enforces generic constrains

Lazy<T>

LightInject can resolve a service as an instance of Lazy<T> when we want to postpone resolving the underlying service until it is needed.

public interface IFoo {}
public class Foo : IFoo {}

container.Register<IFoo, Foo>();
var lazyFoo = container.GetInstance<Lazy<IFoo>>();
Assert.IsNotNull(lazyFoo.Value);

Function Factories

Function factories allows services to resolved as a function delegate that in turn is capable of returning the underlying service instance. We can think of this as an alternative to the Service Locator (anti)pattern.

public interface IFoo {}
public class Foo : IFoo {}

container.Register<IFoo,Foo>();
var func = container.GetInstance<Func<IFoo>>();
var foo = func();
Assert.IsNotNull(foo); 

Note: A function factory is effectively a delegate that redirects back to the corresponding GetInstance method on the service container.

Named Factories

The container returns a function delegate that represents calling the GetInstance method with "SomeFoo" as the service name argument.

container.Register<IFoo, Foo>("SomeFoo");
var func = container.GetInstance<Func<IFoo>>("SomeFoo");   
var foo = func();
Assert.IsNotNull(foo);

Parameters

Function factories can also take parameters that will be used create the service instance.

public class Foo : IFoo
{
    public Foo(int value)
    {
        Value = value;
    }

    public int Value { get; private set; }
}

container.Register<int, IFoo>((factory, value) => new Foo(value));
var fooFactory = container.GetInstance<Func<int, IFoo>>();
var foo = (Foo)fooFactory(42); 
Assert.AreEqual(foo.Value, 42);

Note : The service must be explicitly registered in order for the container to resolve it as a parameterized function factory.

IDisposable

The only way to deal with disposable objects when using function factories, is to let the service type inherit from IDisposable.

public interface IFoo : IDisposable {}
public class Foo : IFoo {}

container.Register<IFoo, Foo>();
var fooFactory = container.GetInstance<Func<IFoo>>();

using(IFoo foo = fooFactory())
{
    
} <--Instance is disposed here          

Note: Although this is common practice even in the BCL, this kind of interfaces are often referred to as leaky abstractions.

Typed Factories

A typed factory is a class that wraps the function factory that is used to create the underlying service instance. As opposed to just function factories, typed factories provides better expressiveness to the consumer of the factory.

public interface IFooFactory
{
    IFoo GetFoo();
}

public class FooFactory : IFooFactory
{
    private Func<IFoo> createFoo;

    public FooFactory(Func<IFoo> createFoo)
    {
        this.createFoo = createFoo;
    }

    public IFoo GetFoo()
    {
        return createFoo();
    }
} 

container.Register<IFoo, Foo>();
container.Register<IFooFactory, FooFactory>(new PerContainerLifetime());
var fooFactory = container.GetInstance<IFooFactory>();
var foo = fooFactory.GetFoo();
Assert.IsNotNull(foo);

Note: Register typed factories with the PerContainerLifetime unless a compelling reason exists to choose a different lifetime.

Parameters

Types factories can also wrap a parameterized function factory and allows us to pass arguments.

public class Foo : IFoo
{
    public Foo(int value)
    {
        Value = value;
    }

    public int Value { get; private set; }
}

public interface IFooFactory
{
    IFoo GetFoo(int value);
} 

public class FooFactory : IFooFactory
{
    private Func<int, IFoo> createFoo;

    public FooFactory(Func<int, IFoo> createFoo)
    {
        this.createFoo = createFoo;
    }

    public IFoo GetFoo(int value)
    {
        return createFoo(value);
    }
} 

container.Register<int, IFoo>((factory, value) => new Foo(value));
container.Register<IFooFactory, FooFactory>(new PerContainerLifetime());
var typedFooFactory = container.GetInstance<IFooFactory>();
var foo = typedFooFactory.GetFoo(42);
Assert.AreEqual(foo.Value, 42);

IDisposable

Working with typed factories gives us the possibility to release disposable services registered as transients without exposing a leaky abstraction.

public interface IFooFactory
{
    IFoo GetFoo(int value);
    void Release(IFoo foo);
} 

public class FooFactory : IFooFactory
{
    private Func<IFoo> createFoo;

    public FooFactory(Func<IFoo> createFoo)
    {
        this.createFoo = createFoo;
    }

    public IFoo GetFoo(int value)
    {
        return createFoo(value);
    }

    public void Release(IFoo foo)
    {
        var disposable = foo as IDisposable;
        if (disposable != null)
        {
            disposable.Dispose();
        }
    }
}    

Recursive dependency detection

A recursive dependency graph is when a service depends directly or indirectly on itself.

public class FooWithRecursiveDependency : IFoo
{
    public FooWithRecursiveDependency(IFoo foo)
    {
    }
}

The following code will throw an InvalidOperationException stating that there are existing recursive dependencies.

container.Register(typeof(IFoo), typeof(FooWithRecursiveDependency));
container.GetInstance<IFoo>()

Internals

When running under the .Net platform, LightInject is capable of creating instances of classes that has the internal modifier.

The only requirement is that the internal class exposes a public constructor.

internal class InternalFooWithPublicConstructor : IFoo
{
    public InternalFooWithPublicConstructor () {}
}

Logging

Sometimes it might be useful to obtain information about what is going on inside the container and LightInject provides a very simple log abstraction that is used to log information and warnings from within the container.

var containerOptions = new ContainerOptions();
containerOptions.LogFactory = (type) => logEntry => Console.WriteLine(logEntry.Message);

Unit Testing

Sometimes it might be useful to use the service container within our unit tests. LightInject also provides the LightInject.xUnit extension that enables dependencies to be injected into test methods. One side effect of using that extension is that it is tightly coupled to xUnit and it we have less control with regards to container instances.

Instead consider this simple base class

public class ContainerFixture : IDisposable
{
    public ContainerFixture()
    {
        var container = CreateContainer();
        Configure(container);
        container.RegisterFrom<CompositionRoot>();
        ServiceFactory = container.BeginScope();
        InjectPrivateFields();
    }

    private void InjectPrivateFields()
    {
        var privateInstanceFields = this.GetType().GetFields(BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance);
        foreach (var privateInstanceField in privateInstanceFields)
        {
            privateInstanceField.SetValue(this, GetInstance(ServiceFactory, privateInstanceField));
        }
    }

    internal Scope ServiceFactory { get; }

    public void Dispose() => ServiceFactory.Dispose();

    public TService GetInstance<TService>(string name = "")
        => ServiceFactory.GetInstance<TService>(name);

    private object GetInstance(IServiceFactory factory, FieldInfo field)
        => ServiceFactory.TryGetInstance(field.FieldType) ?? ServiceFactory.GetInstance(field.FieldType, field.Name);

    internal virtual IServiceContainer CreateContainer() => new ServiceContainer();

    internal virtual void Configure(IServiceRegistry serviceRegistry) {}
}

This can be use with any test framework as long as it creates a new instance of the test class for each test method and that it calls Dispose after the test completes. For xUnit this is the default behaviour.

Injecting services now becomes incredible easy. Just declare the service to test as a private field like this.

public class SampleTests : ContainerFixture
{
    private ICalculator calculator;
    
    [Fact]
    public void ShouldAddNumbers()
    {
        calculator.Add(2,2).ShouldBe(2);
    }
}

If we need to configure the container before executing the test, we can do that by simply overriding the Configure method. This could for instance be used to register mock services into the container.

public class SampleTests : ContainerFixture
{
    private ICalculator calculator;
    
    [Fact]
    public void ShouldAddNumbers()
    {
        calculator.Add(2,2).ShouldBe(2);
    }
    
    internal override Configure(IServiceRegistry serviceRegistry)
    {
        // Add registrations related to testing here
    }
}