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Lazy Resolution

Lamar has some built in functionality for "lazy" resolved dependencies, so that instead of your application service taking a direct dependency on IExpensiveToBuildService that might not be necessary, you could instead have Lamar fulfil a dependency on Lazy<IExpensiveToBuildService> or Func<IExpensiveToBuildService> that could be used to retrieve that expensive service only when it is needed from whatever Container originally created the parent object.

Do note that the Lazy<T> and Func<T> approaches respect the lifecycle of the underlying registration rather than automatically building a unique object instance.

Also note that Lazy<T> or Func<T> is your best (only) viable approach if you wish to have Lamar inject bi-directional relationships.

Lazy<T>

Assuming that Lamar either has an existing configuration for T or can derive a way to build T, you can just declare a dependency on Lazy<T> like this sample:

cs
public class WidgetLazyUser
{
    private readonly Lazy<IWidget> _widget;

    public WidgetLazyUser(Lazy<IWidget> widget)
    {
        _widget = widget;
    }

    public IWidget Widget => _widget.Value;
}

[Fact]
public void lazy_resolution_in_action()
{
    var container = new Container(_ => { _.For<IWidget>().Use<AWidget>(); });

    container.GetInstance<WidgetLazyUser>()
        .Widget.ShouldBeOfType<AWidget>();
}

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cs
public class WidgetLazyUser
{
    private readonly Lazy<IWidget> _widget;

    public WidgetLazyUser(Lazy<IWidget> widget)
    {
        _widget = widget;
    }

    public IWidget Widget
    {
        get { return _widget.Value; }
    }
}

[Fact]
public void lazy_resolution_in_action()
{
    var container = new Container(_ =>
    {
        _.For<IWidget>().Use<AWidget>();
    });

    container.GetInstance<WidgetLazyUser>()
        .Widget.ShouldBeOfType<AWidget>();
}

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Func<T>

Likewise, you can also declare a dependency on Func<T> with very similar mechanics:

cs
[Fact]
public void build_a_func_that_returns_a_singleton()
{
    var container = new Container(x => { x.ForSingletonOf<IWidget>().Use<AWidget>(); });

    var func = container.GetInstance<Func<IWidget>>();
    var w1 = func();
    var w2 = func();
    var w3 = func();

    w1.ShouldBeOfType<AWidget>();

    w1.ShouldBeSameAs(w2);
    w1.ShouldBeSameAs(w3);
    w2.ShouldBeSameAs(w3);
}

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cs
[Fact]
public void build_a_func_that_returns_a_singleton()
{
    var container = new Container(x =>
    {
        x.ForSingletonOf<IWidget>().Use<ColorWidget>().Ctor<string>("color").Is("green");
    });

    var func = container.GetInstance<Func<IWidget>>();
    var w1 = func();
    var w2 = func();
    var w3 = func();

    w1.ShouldBeOfType<ColorWidget>().Color.ShouldBe("green");

    w1.ShouldBeTheSameAs(w2);
    w1.ShouldBeTheSameAs(w3);
    w2.ShouldBeTheSameAs(w3);
}

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This functionality predates the introduction of the Lazy type to .Net

Func<string, T>

Finally, you can also declare a dependency on Func<string, T> that will allow you to lazily resolve a dependency of T by name:

cs
[Fact]
public void build_a_func_by_string()
{
    var container = new Container(x =>
    {
        x.For<IWidget>().Add<GreenWidget>().Named("green");
        x.For<IWidget>().Add<BlueWidget>().Named("blue");
        x.For<IWidget>().Add<RedWidget>().Named("red");
    });

    var func = container.GetInstance<Func<string, IWidget>>();
    func("green").ShouldBeOfType<GreenWidget>();
    func("blue").ShouldBeOfType<BlueWidget>();
    func("red").ShouldBeOfType<RedWidget>();
}

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cs
[Fact]
public void build_a_func_by_string()
{
    var container = new Container(x =>
    {
        x.For<IWidget>().Add<ColorWidget>().Ctor<string>("color").Is("green").Named("green");
        x.For<IWidget>().Add<ColorWidget>().Ctor<string>("color").Is("blue").Named("blue");
        x.For<IWidget>().Add<ColorWidget>().Ctor<string>("color").Is("red").Named("red");
    });

    var func = container.GetInstance<Func<string, IWidget>>();
    func("green").ShouldBeOfType<ColorWidget>().Color.ShouldBe("green");
}

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Bi-relational Dependency Workaround

Lamar does not directly support bi-directional dependency relationships -- but will happily tell you in an exception when you accidentally manage to create one without cratering your AppDomain with a StackOverflowException.

Either Func<T> or Lazy<T> can be used as a workaround for purposeful bi-directional dependencies between types. The following is an example of using this strategy:

cs
public class Thing1
{
    private readonly Lazy<Thing2> _thing2;

    public Thing1(Lazy<Thing2> thing2)
    {
        _thing2 = thing2;
    }

    public Thing2 Thing2 => _thing2.Value;
}

public class Thing2
{
    public Thing2(Thing1 thing1)
    {
        Thing1 = thing1;
    }

    public Thing1 Thing1 { get; set; }
}

[Fact]
public void use_lazy_as_workaround_for_bi_directional_dependency()
{
    var container = Container.For(_ =>
    {
        _.AddSingleton<Thing1>();
        _.AddSingleton<Thing2>();
    });

    var thing1 = container.GetInstance<Thing1>();
    var thing2 = container.GetInstance<Thing2>();

    thing1.Thing2.ShouldBeSameAs(thing2);
    thing2.Thing1.ShouldBeSameAs(thing1);
}

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cs
[Singleton]
public class Thing1
{
    private readonly Lazy<Thing2> _thing2;

    public Thing1(Lazy<Thing2> thing2)
    {
        _thing2 = thing2;
    }

    public Thing2 Thing2
    {
        get { return _thing2.Value; }
    }
}

[Singleton]
public class Thing2
{
    public Thing1 Thing1 { get; set; }

    public Thing2(Thing1 thing1)
    {
        Thing1 = thing1;
    }
}

[Fact]
public void use_lazy_as_workaround_for_bi_directional_dependency()
{
    var container = new Container();
    var thing1 = container.GetInstance<Thing1>();
    var thing2 = container.GetInstance<Thing2>();

    thing1.Thing2.ShouldBeSameAs(thing2);
    thing2.Thing1.ShouldBeSameAs(thing1);
}

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