Ninject contextual binding for 2+ dependent classe

Question

Having trouble figuring out how to manage contextual binding in the scenario where two classes have the same underlying interface dependency, but each class ctor's parameter is named differently. Pseudo code below to demonstrate my situation:

    interface IThing { }
    public class Thing1 : IThing { public Thing1(string fileCode) { } }
    public class Thing2 : IThing { public Thing2(string fileCode) { } }
    interface IThingFactory { IThing CreateThing(string fileCode); }

    interface IDependentThing { }
    public class A : IDependentThing { public A(string fileCode, IThingFactory thingFactory) { } }
    public class B : IDependentThing { public B(string fileCd, IThingFactory thingFactory) { } } //How to handle binding for this dependent?
    interface IDependentThingFactory { IDependentThing CreateDependentThing(string fileCode); }

    //...

    public override void Load()
    {
        Bind<IThing>().ToMethod(ctx =>
        {
            var fileCode = ctx.Parameters.First(p => p.Name == "fileCode").GetValue(ctx, null) as string;
            IThing thing = null;

            if (fileCode == "FileType1")
            {
                Bind<Thing1>().ToSelf().WithConstructorArgument("fileCode", fileCode);
                thing = Kernel.Get<Thing1>();
            }
            else if (fileCode == "FileType2")
            {
                Bind<Thing2>().ToSelf().WithConstructorArgument("fileCode", fileCode);
                thing = Kernel.Get<Thing2>();
            }
            return thing;
        });

        Bind<IThingFactory>().ToFactory();
        Bind<IDependentThingFactory>().ToFactory();
    }

//Later...
using (TextReader tr = new StreamReader(path))
{
    string firstLine = tr.ReadLine();

    if (firstLine.Substring(838, 1) == ".")
    {
        fileCode = "FileType1";
    }
    else if (firstLine.Substring(883, 1) == ".")
    {
        fileCode = "FileType2";
    }

    //won't work for creating B
    Kernel.Get<IDependentThing>(new ConstructorArgument("fileCode", fileCode));

    //or maybe...

    //seems to eliminate my problem by allowing me to handle variations
    //in parameter names  from within A and B's ctors, but looks like it
    //requires injecting factories along the chain (see A & B ctor arguments).
    dependentThingFactory.CreateDependentThing(fileCode) 
};

fileCode is computed based off of some analysis of local files. Once the type of file is determined, I want Ninject to hand back the appropriate object for processing that file

How would I handle the binding for B since the existing binding I defined requires a constructor parameter with a different name? Is there a better way to do this in general?

I guess I could just use p.Name == "fileCode" || p.Name == "fileCd", but I can't shake the feeling that I'm doing something wrong (feels messy). Also, I'm not thrilled about pulling parameters by name, and I've thought about maybe creating a custom type that would give Ninject something more concrete to match against versus a string parameter. From where I'm standing it looks like I either just manage the multiple parameter names situation, or switch to custom types as my parameters instead of strings.

Answer 1

Making parameter injection more refactor safe and making them available for the whole resolution context

Instead of "named parameters" you can use a "type matching" or "typed" parameter. The factories IInstanceProvider can be exchanged for another one which does so:

kernel.Bind<IThingFactory>()
      .ToFactory(() => new TypeMatchingArgumentInheritanceInstanceProvider());

Note:

• this IInstanceProvider will also make the argument available further "downstream" (it "inherits" the parameter)
• a string is very verbose so you may want to choose to wrap it in another type, like class ConnectionInfo.

Contextual Binding combined with parameter injection

So let's say we create our own FileType type to be more verbose than just using string:

public class FileCode
{
    public FileCode(string value)
    {
        Value = value;
    }

    public string Value { get; private set; }
}

(maybe you want to replace that with an enum?)

Since your requirements are more complex we're going to have to change things up a little bit. We're going to create our own IConstructorArgument to easily be able to match it for When-contextual bindings and also inject it's value based on type-matching (as above):

internal class FileCodeParameter : IConstructorArgument
{
    private readonly FileCode fileCode;

    public FileCodeParameter(FileCode fileCode)
    {
        this.fileCode = fileCode;
    }

    public string Name { get { return "File Code Parameter"; } }

    public bool ShouldInherit { get { return true; } }

    public FileCode FileCode  { get { return this.fileCode; } }

    public bool Equals(IParameter other)
    {
        var otherFileCodeParameter = other as FileCodeParameter;
        if (otherFileCodeParameter == null)
        {
            return false;
        }

        return otherFileCodeParameter.fileCode == this.fileCode;
    }

    public object GetValue(IContext context, ITarget target)
    {
        return this.fileCode;
    }

    public bool AppliesToTarget(IContext context, ITarget target)
    {
        return target.Type == typeof(FileCode);
    }
}

Now let me create some sample codes so we can later verify that it works:

public interface IThing
{
    FileCode FileCode { get; }
}

public abstract class Thing : IThing
{
    protected Thing(FileCode fileCode)
    {
        FileCode = fileCode;
    }

    public FileCode FileCode { get; private set; }
}

public class ThingFoo : Thing
{
    public ThingFoo(FileCode fileCode) : base(fileCode) { }
}

public class ThingBar : Thing
{
    public ThingBar(FileCode fileCode) : base(fileCode) { }
}

public interface IOtherThing
{
    FileCode FileCode { get; }
}

public abstract class OtherThing : IOtherThing
{
    protected OtherThing(FileCode fileCode)
    {
        FileCode = fileCode;
    }

    public FileCode FileCode { get; private set; }
}

public class OtherThingFoo : OtherThing
{
    public OtherThingFoo(FileCode fileCode) : base(fileCode) { }
}

public class OtherThingBar : OtherThing
{
    public OtherThingBar(FileCode fileCode) : base(fileCode) { }
}

public class OtherThingWrapper
{
    public OtherThingWrapper(IOtherThing otherThing)
    {
        OtherThing = otherThing;
    }

    public IOtherThing OtherThing { get; private set; }
}

public class FileProcessor
{
    public FileProcessor(IThing thing, OtherThingWrapper otherThingWrapper)
    {
        Thing = thing;
        OtherThingWrapper = otherThingWrapper;
    }

    public IThing Thing { get; private set; }
    public OtherThingWrapper OtherThingWrapper { get; private set; }
}

What's missing? The factory. We could use the ToFactory binding with custom IInstanceProvider but unless we're going to create lots of factories with FileCodeParameters i don't think it makes sense, so let's keep it simple:

public interface IFileProcessorFactory
{
    FileProcessor Create(FileCode fileCode);
}

internal class FileProcessorFactory : IFileProcessorFactory
{
    private readonly IResolutionRoot resolutionRoot;

    public FileProcessorFactory(IResolutionRoot resolutionRoot)
    {
        this.resolutionRoot = resolutionRoot;
    }

    public FileProcessor Create(FileCode fileCode)
    {
        return this.resolutionRoot.Get<FileProcessor>(new FileCodeParameter(fileCode));
    }
}

Now let's have it all come together:

public class Test
{
    [Fact]
    public void FactMethodName()
    {
        var fooFileCode = new FileCode("foo");
        var barFileCode = new FileCode("bar");

        var kernel = new StandardKernel();
        kernel
            .Bind<IFileProcessorFactory>()
            .To<FileProcessorFactory>();

        kernel
            .Bind<IThing>()
            .To<ThingFoo>()
            .WhenFileCode(fooFileCode);
        kernel
            .Bind<IThing>()
            .To<ThingBar>()
            .WhenFileCode(barFileCode);

        kernel
            .Bind<IOtherThing>()
            .To<OtherThingFoo>()
            .WhenFileCode(fooFileCode);
        kernel
            .Bind<IOtherThing>()
            .To<OtherThingBar>()
            .WhenFileCode(barFileCode);


        var fileProcessor = kernel.Get<IFileProcessorFactory>().Create(barFileCode);
        fileProcessor.Thing.Should().BeOfType<ThingBar>();
        fileProcessor.Thing.FileCode.Should().Be(barFileCode);
        fileProcessor.OtherThingWrapper.OtherThing.Should().BeOfType<OtherThingBar>();
        fileProcessor.OtherThingWrapper.OtherThing.FileCode.Should().Be(barFileCode);
    }
}

public static class BindingExtensionsForFileCodes
{
    public static IBindingInNamedWithOrOnSyntax<T> WhenFileCode<T>(
        this IBindingWhenSyntax<T> syntax,
        FileCode fileCode)
    {
        return syntax.When(req => req
            .Parameters
            .OfType<FileCodeParameter>()
            .Single()
            .FileCode.Value == fileCode.Value);
    }
}

That's it! - the FileCode is both being injected and being used for selection of implementation - as the parameter is "inherited", it also works deeper down the object tree.

Below, just for reference, all of the code for easier copy & paste:

using FluentAssertions;
using Ninject;
using Ninject.Activation;
using Ninject.Parameters;
using Ninject.Planning.Targets;
using Ninject.Syntax;
using System.Linq;
using Xunit;

namespace NinjectTest.ParameterContextual
{
    public class FileCode
    {
        public FileCode(string value)
        {
            Value = value;
        }

        public string Value { get; private set; }
    }

    public interface IThing
    {
        FileCode FileCode { get; }
    }

    public abstract class Thing : IThing
    {
        protected Thing(FileCode fileCode)
        {
            FileCode = fileCode;
        }

        public FileCode FileCode { get; private set; }
    }

    public class ThingFoo : Thing
    {
        public ThingFoo(FileCode fileCode) : base(fileCode) { }
    }

    public class ThingBar : Thing
    {
        public ThingBar(FileCode fileCode) : base(fileCode) { }
    }

    public interface IOtherThing
    {
        FileCode FileCode { get; }
    }

    public abstract class OtherThing : IOtherThing
    {
        protected OtherThing(FileCode fileCode)
        {
            FileCode = fileCode;
        }

        public FileCode FileCode { get; private set; }
    }

    public class OtherThingFoo : OtherThing
    {
        public OtherThingFoo(FileCode fileCode) : base(fileCode) { }
    }

    public class OtherThingBar : OtherThing
    {
        public OtherThingBar(FileCode fileCode) : base(fileCode) { }
    }

    public class OtherThingWrapper
    {
        public OtherThingWrapper(IOtherThing otherThing)
        {
            OtherThing = otherThing;
        }

        public IOtherThing OtherThing { get; private set; }
    }

    public class FileProcessor
    {
        public FileProcessor(IThing thing, OtherThingWrapper otherThingWrapper)
        {
            Thing = thing;
            OtherThingWrapper = otherThingWrapper;
        }

        public IThing Thing { get; private set; }
        public OtherThingWrapper OtherThingWrapper { get; private set; }
    }

    public interface IFileProcessorFactory
    {
        FileProcessor Create(FileCode fileCode);
    }

    internal class FileProcessorFactory : IFileProcessorFactory
    {
        private readonly IResolutionRoot resolutionRoot;

        public FileProcessorFactory(IResolutionRoot resolutionRoot)
        {
            this.resolutionRoot = resolutionRoot;
        }

        public FileProcessor Create(FileCode fileCode)
        {
            return this.resolutionRoot.Get<FileProcessor>(new FileCodeParameter(fileCode));
        }
    }

    public class Test
    {
        [Fact]
        public void FactMethodName()
        {
            var fooFileCode = new FileCode("foo");
            var barFileCode = new FileCode("bar");

            var kernel = new StandardKernel();
            kernel
                .Bind<IFileProcessorFactory>()
                .To<FileProcessorFactory>();

            kernel
                .Bind<IThing>()
                .To<ThingFoo>()
                .WhenFileCode(fooFileCode);
            kernel
                .Bind<IThing>()
                .To<ThingBar>()
                .WhenFileCode(barFileCode);

            kernel
                .Bind<IOtherThing>()
                .To<OtherThingFoo>()
                .WhenFileCode(fooFileCode);
            kernel
                .Bind<IOtherThing>()
                .To<OtherThingBar>()
                .WhenFileCode(barFileCode);


            var fileProcessor = kernel.Get<IFileProcessorFactory>().Create(barFileCode);
            fileProcessor.Thing.Should().BeOfType<ThingBar>();
            fileProcessor.Thing.FileCode.Should().Be(barFileCode);
            fileProcessor.OtherThingWrapper.OtherThing.Should().BeOfType<OtherThingBar>();
            fileProcessor.OtherThingWrapper.OtherThing.FileCode.Should().Be(barFileCode);
        }
    }

    internal class FileCodeParameter : IConstructorArgument
    {
        private readonly FileCode fileCode;

        public FileCodeParameter(FileCode fileCode)
        {
            this.fileCode = fileCode;
        }

        public string Name { get { return "File Code Parameter"; } }

        public bool ShouldInherit { get { return true; } }

        public FileCode FileCode  { get { return this.fileCode; } }

        public bool Equals(IParameter other)
        {
            var otherFileCodeParameter = other as FileCodeParameter;
            if (otherFileCodeParameter == null)
            {
                return false;
            }

            return otherFileCodeParameter.fileCode == this.fileCode;
        }

        public object GetValue(IContext context, ITarget target)
        {
            return this.fileCode;
        }

        public bool AppliesToTarget(IContext context, ITarget target)
        {
            return target.Type == typeof(FileCode);
        }
    }

    public static class BindingExtensionsForFileCodes
    {
        public static IBindingInNamedWithOrOnSyntax<T> WhenFileCode<T>(
            this IBindingWhenSyntax<T> syntax,
            FileCode fileCode)
        {
            return syntax.When(req => req
                .Parameters
                .OfType<FileCodeParameter>()
                .Single()
                .FileCode.Value == fileCode.Value);
        }
    }
}