How to use Dependency Injection with ASP.NET

Question

I am trying to work out a way to use Dependency Injection with ASP.NET controls.

I have got lots of controls that create repositories directly, and use those to access and bind to data etc.

I am looking for a pattern where I can pass repositories to the controls externally (IoC), so my controls remain unaware of how repositories are constructed and where they come from etc.

I would prefer not to have a dependency on the IoC container from my controls, therefore I just want to be able to construct the controls with constructor or property injection.

(And just to complicate things, these controls are being constructed and placed on the page by a CMS at runtime!)

Any thoughts?

Answer 1

The best way is to have a base class for the controls like:

public class PartialView : UserControl
{
    protected override void OnInit(System.EventArgs e)
    {
        ObjectFactory.BuildUp(this);
        base.OnInit(e);
    }
}

That will inject any control that inherits from that base class (uses structuremap). Combining that with a property based config, you will be able to have controls like:

public partial class AdminHeader : PartialView
{
   IMyRepository Repository{get;set;}
}

Update 1: If you can't have the controls inherit, perhaps the CMS has a hook right after creating the controls, in there you can call the BuildUp. Also if the CMS allows you to hook something to fetch the instance you could use constructor based injection, but I prefer BuildUp on this specific scenario as asp.net doesn't have a hook for this.

Answer 2

You could also create some singleton instances in the Application_Start global.asax event and have them available as public static readonly properties.

Answer 3

Autofac supports fairly unobtrusive dependency injection in ASP.NET WebForms. My understanding is it just hooks into the ASP.NET page lifecycle using an http module and does property injection. The only catch is that for controls I don't think this happens until after the Init event.

Answer 4

You can use automatic constructor injection by replacing the default PageHandlerFactory with a custom one. This way you can use an overloaded constructor to load the dependencies. Your page might look like this:

public partial class HomePage : System.Web.UI.Page
{
    private readonly IDependency dependency;

    public HomePage(IDependency dependency)
    {
        this.dependency = dependency;
    }

    protected HomePage () { }
}

Configuring that custom PageHandlerFactory can be done in the web.config as follows:

<?xml version="1.0"?>
<configuration>
  <system.web>
    <httpHandlers>
      <add verb="*" path="*.aspx"
        type="YourApp.CustomPageHandlerFactory, YourApp"/>
    </httpHandlers>
  </system.web>
</configuration>

Your CustomPageHandlerFactory can look like this:

public class CustomPageHandlerFactory : PageHandlerFactory
{
    private static object GetInstance(Type type)
    {
        // TODO: Get instance using your favorite DI library.
        // for instance using the Common Service Locator:
        return Microsoft.Practices.ServiceLocation
            .ServiceLocator.Current.GetInstance(type);
    }

    public override IHttpHandler GetHandler(HttpContext cxt, 
        string type, string vPath, string path)
    {
        var page = base.GetHandler(cxt, type, vPath, path);

        if (page != null)
        {
            // Magic happens here ;-)
            InjectDependencies(page);
        }

        return page;
    }

    private static void InjectDependencies(object page)
    {
        Type pageType = page.GetType().BaseType;

        var ctor = GetInjectableCtor(pageType);

        if (ctor != null)
        {
            object[] arguments = (
                from parameter in ctor.GetParameters()
                select GetInstance(parameter.ParameterType)
                .ToArray();

            ctor.Invoke(page, arguments);
        }
    }

    private static ConstructorInfo GetInjectableCtor(
        Type type)
    {
        var overloadedPublicConstructors = (
            from constructor in type.GetConstructors()
            where constructor.GetParameters().Length > 0
            select constructor).ToArray();

        if (overloadedPublicConstructors.Length == 0)
        {
            return null;
        }

        if (overloadedPublicConstructors.Length == 1)
        {
            return overloadedPublicConstructors[0];
        }

        throw new Exception(string.Format(
            "The type {0} has multiple public " +
            "ctors and can't be initialized.", type));
    }
}

You can read more about it here.