Question

I have just changed from Ninject to TinyIoC for dependency injection and I’m having trouble with constructor injection.

I have managed to simplify it down to this snippet:

public interface IBar { } 

public class Foo
{
    public Foo(IBar bar) { }
}

public class Bar : IBar
{
    public Bar(string value) { }
}

class Program
{
    static void Main(string[] args)
    {
        var container = TinyIoCContainer.Current;

        string value = "test";
        container.Register<IBar, Bar>().UsingConstructor(() => new Bar(value));

        var foo = container.Resolve<Foo>();
        Console.WriteLine(foo.GetType());
    }
}

which causes a TinyIoCResolutionException to be thrown with:

"Unable to resolve type: TinyIoCTestApp.Foo"

and inside that exception is a chain of inner exceptions:

"Unable to resolve type: TinyIoCTestApp.Bar"
"Unable to resolve type: System.String"
"Unable to resolve type: System.Char[]"
"Value cannot be null.\r\nParameter name: key"

Is there something wrong with the way I’m using the constructor injection? I realize I could call

container.Register<IBar, Bar>(new Bar(value));

and that does indeed work, however the result is a global instance of Bar which is not what I’m after.

Any ideas?

Answer 1

I’m not familiar with TinyIOC, but I think I can answer your question.

The UsingConstructor registers a lambda that points at a constructor (the ctor(string)) that TinyIOC will use to do automatic constructor injection into. TinyIOC will analyse the constructor arguments, finds an argument of type System.String and tries to resolve that type. Since you haven’t registered System.String explicitly (which you shouldn’t btw), resolving IBar (and thus Foo) fails.

The incorrect assumption you made is that TinyIOC will execute your () => new Bar(value)) lambda, which it will not. If you look at the UsingConstructor method you will propable see that it takes an Expression<Func<T>> instead of a Func<T>.

The thing you want, is to register a factory delegate that does the creation. I expect TinyIOC to contain a method for this. It might look something like this:

container.Register<IBar>(() => new Bar(value));