Looking for a Ninject scope that behaves like InRequestScope

Question

On my service layer I have injected an UnitOfWork and 2 repositories in the constructor. The Unit of Work and repository have an instance of a DbContext

Answer 1

As discussed in the other answer, InCallScope is not a good approach to solving this problem.

For now I'm dumping some code that works against the latest NuGet Unstable / Include PreRelease / Instal-Package -Pre editions of Ninject.Web.Common without a clear explanation. I will translate this to an article in the Ninject.Extensions.NamedScope wiki at some stagehave started to write a walkthrough of this technique in the Ninject.Extensions.NamedScope wiki's CreateNamedScope/GetScope article.

Possibly some bits will become Pull Request(s) at some stage too (Hat tip to @Remo Gloor who supplied me the outline code). The associated tests and learning tests are in this gist for now), pending packaging in a proper released format TBD.

The exec summary is you Load the Module below into your Kernel and use .InRequestScope() on everything you want created / Disposed per handler invocation and then feed requests through via IHandlerComposer.ComposeCallDispose.

If you use the following Module:

public class Module : NinjectModule
{
    public override void Load()
    {
        Bind<IHandlerComposer>().To<NinjectRequestScopedHandlerComposer>();

        // Wire it up so InRequestScope will work for Handler scopes
        Bind<INinjectRequestHandlerScopeFactory>().To<NinjectRequestHandlerScopeFactory>();
        NinjectRequestHandlerScopeFactory.NinjectHttpApplicationPlugin.RegisterIn( Kernel );
    }
}

Which wires in a Factory[1] and NinjectHttpApplicationPlugin that exposes:

public interface INinjectRequestHandlerScopeFactory
{
    NamedScope CreateRequestHandlerScope();
}

Then you can use this Composer to Run a Request InRequestScope():

public interface IHandlerComposer
{
    void ComposeCallDispose( Type type, Action<object> callback );
}

Implemented as:

class NinjectRequestScopedHandlerComposer : IHandlerComposer
{
    readonly INinjectRequestHandlerScopeFactory _requestHandlerScopeFactory;

    public NinjectRequestScopedHandlerComposer( INinjectRequestHandlerScopeFactory requestHandlerScopeFactory )
    {
        _requestHandlerScopeFactory = requestHandlerScopeFactory;
    }

    void IHandlerComposer.ComposeCallDispose( Type handlerType, Action<object> callback )
    {
        using ( var resolutionRoot = _requestHandlerScopeFactory.CreateRequestHandlerScope() )
            foreach ( object handler in resolutionRoot.GetAll( handlerType ) )
                callback( handler );
    }
}

The Ninject Infrastructure stuff:

class NinjectRequestHandlerScopeFactory : INinjectRequestHandlerScopeFactory
{
    internal const string ScopeName = "Handler";

    readonly IKernel _kernel;

    public NinjectRequestHandlerScopeFactory( IKernel kernel )
    {
        _kernel = kernel;
    }

    NamedScope INinjectRequestHandlerScopeFactory.CreateRequestHandlerScope()
    {
        return _kernel.CreateNamedScope( ScopeName );
    }

    /// <summary>
    /// When plugged in as a Ninject Kernel Component via <c>RegisterIn(IKernel)</c>, makes the Named Scope generated during IHandlerFactory.RunAndDispose available for use via the Ninject.Web.Common's <c>.InRequestScope()</c> Binding extension.
    /// </summary>
    public class NinjectHttpApplicationPlugin : NinjectComponent, INinjectHttpApplicationPlugin
    {
        readonly IKernel kernel;

        public static void RegisterIn( IKernel kernel )
        {
            kernel.Components.Add<INinjectHttpApplicationPlugin, NinjectHttpApplicationPlugin>();
        }

        public NinjectHttpApplicationPlugin( IKernel kernel )
        {
            this.kernel = kernel;
        }

        object INinjectHttpApplicationPlugin.GetRequestScope( IContext context )
        {
            // TODO PR for TrgGetScope
            try
            {
                return NamedScopeExtensionMethods.GetScope( context, ScopeName );
            }
            catch ( UnknownScopeException )
            {
                return null;
            }
        }

        void INinjectHttpApplicationPlugin.Start()
        {
        }

        void INinjectHttpApplicationPlugin.Stop()
        {
        }
    }
}

Answer 2

UPDATE: This approach works against NuGet current, but relies in an anomaly in the InCallscope implementation which has been fixed in the current Unstable NuGet packages. I'll be tweaking this answer in a few days to reflect the best approach after some mulling over. NB the high level way of structuring stuff will stay pretty much identical, just the exact details of the Bind<DbContext>() scoping will work. (Hint: CreateNamedScope in unstable would work or one could set up the Command Handler as DefinesNamedScope. Reason I dont just do that is that I want to have something that composes/plays well with InRequestScope)

---

I highly recommend reading the Ninject.Extensions.NamedScope integration tests (seriously, find them and read and re-read them)

The DbContext is a Unit Of Work so no further wrapping is necessary.

As you want to be able to have multiple 'requests' in flight and want to have a single Unit of Work shared between them, you need to:

Bind<DbContext>()
    .ToMethod( ctx => 
        new DbContext( 
            connectionStringName: ConfigurationUtility.GetConnectionString() ))
    .InCallScope();

The InCallScope() means that:

1. for a given object graph composed for a single kernel.Get() Call (hence In Call Scope), everyone that requires an DbContext will get the same instance.
2. the IDisposable.Dispose() will be called when a Kernel.Release() happens for the root object (or a Kernel.Components.Get<ICache>().Clear() happens for the root if it is not .InCallScope())

There should be no reason to use InNamedScope() and DefinesNamedScope(); You don't have long-lived objects you're trying to exclude from the default pooling / parenting / grouping.

If you do the above, you should be able to:

var command = kernel.Get<ICommand>();
try {
    command.Execute();
} finally {
    kernel.Components.Get<ICache>().Clear( command ); // Dispose of DbContext happens here
}

The Command implementation looks like:

class Command : ICommand {
    readonly IAccountRepository _ar;
    readonly IBlockedIpRepository _br;
    readonly DbContext _ctx;
    public Command(IAccountRepository ar, IBlockedIpRepository br, DbContext ctx){
        _ar = ar;
        _br = br;
        _ctx = ctx;
    }
    void ICommand.Execute(){
        _ar.Insert(a);
        _br.Insert(b);
        _ctx.saveChanges();
    }
}

---

Note that in general, I avoid having an implicit Unit of Work in this way, and instead surface it's creation and Disposal. This makes a Command look like this:

class Command : ICommand {
    readonly IAccountService _as;
    readonly IBlockedIpService _bs;
    readonly Func<DbContext> _createContext;
    public Command(IAccountService @as, IBlockedIpServices bs, Func<DbContext> createContext){
        _as = @as;
        _bs = bs;
        _createContext = createContext;
    }
    void ICommand.Execute(){
        using(var ctx = _createContext()) {
            _ar.InsertA(ctx);
            _br.InsertB(ctx);
            ctx.saveChanges();
        }
   }

This involves no usage of .InCallScope() on the Bind<DbContext>() (but does require the presence of Ninject.Extensions.Factory's FactoryModule to synthesize the Func<DbContext> from a straightforward Bind<DbContext>().