Unity Dependency Injection error - No parameterless constructor defined for this object

Question

I have an application with a GenericHandler and would like to inject dependencies using Unity. No matter what I try I get the error:

[MissingMethodException: No parameterless constructor defined for this object.]
System.RuntimeTypeHandle.CreateInstance(RuntimeType type, Boolean publicOnly, Boolean

I have tried to follow the example at http://geekswithblogs.net/Rhames/archive/2012/09/11/loosely-coupled-.net-cache-provider-using-dependency-injection.aspx.

My constructor for the handler is as follows:

    public class GetPerson : IHttpHandler
    {
        private IPersonRepository repo;
        public GetPerson(IPersonRepository repo)
        {
            this.repo = repo;
        }

IPersonRepository is implemented by CachedPersonRepository. CachedPersonRepository wraps the PersonRepository (which is used for DataAccess if an item cannot be found in the cache). Both CachedPersonRepository and PersonRepository are IPersonRepository:

public class CachedPersonRepository : IPersonRepository
{
    private ICacheProvider<Person> cacheProvider;
    private IPersonRepository personRepository;

    public CachedPersonRepository(IPersonRepository personRepository, ICacheProvider<Person> cacheProvider)
    {

This IPersonRepository personRepository is parameterless.

ICacheProvider<Person> is implemented by MemcachedCacheProvider<T>:
public class MemcachedCacheProvider<T> : ICacheProvider<T>
{
    public T Get(string key, Func<T> retrieveData, DateTime? absoluteExpiry, TimeSpan relativeExpiry)
    {

I have tried unsuccessfully to initialise the Unity Container in my Global.asax file Application_Start. DI is new to me and I would very much appreciate any advice on where I'm going wrong.

Could you post the Unity configuration method (which you call in Application_Start)? This may just be a configuration error. — Damon
– Damon, Commented Mar 9, 2014 at 10:03
Thanks Damon, I've managed to solve the problem and will post the solution below. — Terra
– Terra, Commented Mar 9, 2014 at 22:39

Terra · Accepted Answer · 2014-03-09 22:58:39Z

There were actually two issues here.

Firstly, CachedPersonRepository uses the Decorator pattern which I didn't properly understand before. Once I understood this I was able to register and resolve the PersonRepository appropriately using this configuration:

public static void Configure(IUnityContainer container)
    {
        container.RegisterType<ICacheProvider<Person>, MemcachedCacheProvider<Person>>();

        container.RegisterType<IPersonRepository, PersonRepository>("PersonRepository", new ContainerControlledLifetimeManager());

        container.RegisterType<IPersonRepository, CachedPersonRepository>(
            new InjectionConstructor(
                    new ResolvedParameter<IPersonRepository>("PersonRepository"),
                    new ResolvedParameter<ICacheProvider<Person>>()));

        container.Resolve<IPersonRepository>();
    }

Having fixed this I still saw the same "No parameterless constructor defined for this object" error.

The reason for this, is that I was working with an IHttpHandler and it is not possible to inject dependencies in the constructor.

I got around this by using Property injection:

A Repository property with the Dependency Attribute has been added to the GetPerson handler:

public class GetPerson : HandlerBase
    {
        [Dependency]
        public IPersonRepository Repository { get; set; }

A new http module was needed to check for requests from handlers which implemented my HandlerBase:

public class UnityHttpModule : IHttpModule
{
    public void Init(HttpApplication context)
    {
        context.PreRequestHandlerExecute += OnPreRequestHandlerExecute;
    }

    public void Dispose() { }

    private void OnPreRequestHandlerExecute(object sender, EventArgs e)
    {
        IHttpHandler currentHandler = HttpContext.Current.Handler as HandlerBase;

        if (currentHandler != null)
        {
            HttpContext.Current.Application.GetContainer().BuildUp(
                currentHandler.GetType(), currentHandler);
        }
    }
}

Resources:

http://download.microsoft.com/download/4/D/B/4DBC771D-9E24-4211-ADC5-65812115E52D/DependencyInjectionWithUnity.pdf (Chapter 4, pages 60-63)

http://msdn.microsoft.com/en-us/library/ff664534(v=pandp.50).aspx

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