Does Structuremap support Lazy out of the box?

问题:

Does structuremap allow you to do constructor injection in a lazy fashion? Meaning not creating the object which is injected until it is used?

回答1:

UPDATE: StructureMap v3 implements this out of the box, so this trick is no longer necessary.

---

StructureMap version 2 doesn't, but with a few tricks you can get it to do what I believe you are looking for. First of all, you can already wire up Lazy<T> instances manually like this:

container = new Container(x =>
{
    x.Scan(y =>
    {
        y.TheCallingAssembly();
        y.WithDefaultConventions();
    });

    x.For<Lazy<IFoo>>().Use(y => new Lazy<IFoo>(y.GetInstance<Foo>));
    x.For<Lazy<IBar>>().Use(y => new Lazy<IBar>(y.GetInstance<Bar>));
    x.For<Lazy<IBaz>>().Use(y => new Lazy<IBaz>(y.GetInstance<Baz>));
});

This works just fine, but you have to register each and every type individually. It would be nicer if you could take advantage of a more convention-based approach. Ideally, the following syntax would be nice.

x.For(typeof(Lazy<>)).Use(typeof(Lazy<>));

This syntax actually works... somewhat. Unfortunately, at runtime, StructureMap will attempt to find the "greediest" constructor for Lazy<T> and settle on public Lazy(Func<T> valueFactory, bool isThreadSafe). Since we didn't tell it what to do with the boolean isThreadSafe parameter, it will throw an exception when it tries to resolve `Lazy'.

The documentation for Lazy states that the "thread safety mode" of the default Lazy(Func<T> valueFactory) constructor is LazyThreadSafetyMode.ExecutionAndPublication, which just so happens to be what you get by passing true into the isThreadSafe parameter of the constructor above. So, if we could just tell StructureMap to pass true for isThreadSafe, we would get the same behavior as if we called the constructor we actually wanted to use in the first place (e.g. Lazy(Func<T> valueFactory)).

Simply registering x.For(typeof(bool)).Use(y => true) would be very reckless and dangerous since we would be telling StructureMap to go ahead and use the value true for any boolean anywhere. Instead, we need to tell StructureMap what value to use for just this one boolean parameter, which we can do like this.

x.For(typeof(Lazy<>)).Use(typeof(Lazy<>))
 .CtorDependency<bool>("isThreadSafe").Is(true);

StructureMap now knows to use the value of "true" for the isThreadSafe parameter when resolving Lazy<T>. We can now use Lazy<T> in constructor parameters, and get the behavior I believe you were looking for.

You can read about the Lazy class in more detail here.

回答2:

Yes, it does. The latest version of StructureMap (2.6.x) is compiled against .NET Framework 3.5, and so does not have access to the Lazy<T> type introduced in .NET 4. However, it does support the same functionality - "not creating the object which is injected until it is used". Instead of depending on a Lazy<T>, you depend on a Func<T>. No special container registration is required.

I've included a sample program that creates the following output:

Created Consumer
Consuming
Created Helper
Helping

Sample.cs:

class Program
{
    static void Main(string[] args)
    {
        var container = new Container(x =>
        {
            x.For<IConsumer>().Use<Consumer>();
            x.For<IHelper>().Use<Helper>();
        });

        var consumer = container.GetInstance<IConsumer>();
        consumer.Consume();
    }
}

public class Consumer : IConsumer
{
    private readonly Func<IHelper> _helper;

    public Consumer(Func<IHelper> helper)
    {
        _helper = helper;
        Console.WriteLine("Created Consumer");
    }

    public void Consume()
    {
        Console.WriteLine("Consuming");
        _helper().Help();
    }
}

public interface IConsumer
{
    void Consume();
}

public interface IHelper
{
    void Help();
}

public class Helper : IHelper
{
    public Helper()
    {
        Console.WriteLine("Created Helper");
    }

    public void Help()
    {
        Console.WriteLine("Helping");
    }
}