Lazy<> Ninject Injection

Question

I use ninject framework. In my code I have a Lazy object. I can create an instance, but when I call the value property I got an exception.

 private Lazy<IPsoriasisReportUserControl> psoriasisReportUserControl;

[Inject]
    public Lazy<IPsoriasisReportUserControl> PsoriasisReportUserControl
    {
        get { return psoriasisReportUserControl; }
        set { psoriasisReportUserControl = value; }
    }

I got

The lazily-initialized type does not have a public, parameterless constructor

exception because the injection does not inject the method into the constructor. I think I have to write a method to the bind what Creates a new instance.

Freestone

Answer 1

Bind(typeof (Lazy<IPsoriasisReportUserControl>)).ToMethod(
            ctx => new Lazy<IPsoriasisReportUserControl>(() =>
                  Kernel.Get<IPsoriasisReportUserControl>()));

Freestone

This answer works. But you can also do the other answer by simply using the NInject Extension factory. It will work automatically. – Ascanius

Answer 2

Use the factory extension for Ninject https://github.com/ninject/ninject.extensions.factory

Khat

Actually, a link to the wiki might be the more useful reference than the source code repository, as it shows how the injection of Lazy<T> is supposed to happen: github.com/ninject/Ninject.Extensions.Factory/wiki/Lazy – Cesium

Answer 3

You need a default public constructor on Lazy :

public Lazy() {}

Miscellanea

there is an parameterless constructor. The injection does not use that I think. Do not know how can I configure that< – Freestone

Lazy probably refers to the type in the BCL. You'll be hard pressed to change it. – Senter

[TestMethod] public void UC_DiagnosticReport_How_Lazy_Works() { Lazy<IPsoriasisReportUserControl> a = new Lazy<IPsoriasisReportUserControl>( NinjectService.Get<IPsoriasisReportUserControl>); var b = a.Value; Assert.IsNotNull(b); } this works. I need this – Freestone

You have to bind IPsoriasisReportUserControl like this : kernel.Bind(typeof(IPsoriasisReportUserControl)).To(typeof(PsoriasisReportUserControl); BEFORE the bind of Lazy<> – Miscellanea