Multiple Interface injection with castle windsor

Question

How can you get castle Windsor to choose the right implantation of a interface at run time when you have multiple implementations in the container.

For example lets say i have a simple interface called IExamCalc that does calculations to work out how someone did in that exam.

No we have several implementation of this like bellow for example,

public interface IExamCalc
{
    int CalculateMark(ExamAnswers examAnswers)
}

public class WritenExam : IExamCalc
{
    public int CalculateMark(ExamAnswers examAnswers)
    {
         return 4;
    }
}

public class OralExam : IExamCalc
{
    public int CalculateMark(ExamAnswers examAnswers)
    {
         return 8;
    }
}

public class ExamMarkService
{
    private IExamCalc _examCalc;
    public ExamMarkService(IExamCalc examCalc)
    {
        _examCalc = examCalc;
    }

    public int[] CalculateExamMarks(ExamAnswers[] examAnswers)
    {
        IList<int> marks = new List<int>;
        foreach(ExamAnswers examanswer in examaAnswers)
        {
            marks.Add(_examCalc.CalculateMark);
        }
    }
}

Say the ExamMarkService is being resloved through Windor how can i make sure that the correct implementation is injected in the constructor and is this an example of a multi-tenancy problem?

Hope that all makes sence

Colin G

Answer 1

The short answer is, you can't. This kind of choice is dependent on application code, so if you just did container.Resolve<IExamCalc>, then Windsor couldn't know which one you wanted.

The question to ask is how do you know which type to use?

Answer 2

As David said, you can't, but IHandlerSelector will let you take control. Check out the tests to get an idea of how to use them: https://svn.castleproject.org/svn/castle/trunk/InversionOfControl/Castle.Windsor.Tests/HandlerSelectorsTestCase.cs

Basically, you would do something like:

public class WritenExamHandler : IHandlerSelector
    {
        public bool HasOpinionAbout(string key, Type service)
        {
            // Decision logic here
            return somethingThatWouldBeTrueToSelectWritenExam && service == typeof(IExamCalc);
        }

        public IHandler SelectHandler(string key, Type service, IHandler[] handlers)
        {
            return handlers.Where(handler => handler.ComponentModel.Implementation == typeof (WritenExam)).First();
        }
    }

and then you register it with:

container.Kernel.AddHandlerSelector(new WritenExamHandler());

This will allow you to easily deal with multi-tenency issues :)

Answer 3

Multi-tenancy is defined as being able to run your software on one instance, serving multiple tenants/customers/clients. I guess you could run into problems like yours more often in a multi-tenancy setup.

All your components have keys which are unique strings, so you may always so a container.Resolve("someKey") to get a specific implementation.

If you want to have a specific implementation automatically injected, you may configure your component like this (off my memory, may not be 100% precise):

<component id="someService.customer1" service="ISomeService" type="Customer1SomeService" />

<component id="anotherId" service="IAnotherService" type="AnotherService">
    <parameters>
        <parameterName> <!-- as written in the ctor's signature -->
           ${someService.customer1}
        </parameterName>
    </parameters> 
</component>