Specify run time parameter dependency in Unity

Question

I have a class which needs a string as a parameter in its constructor but this parameter will be decided by the calling code. At the same point of time, the life time of this class has to be tied to per HTTP request. So, I created a custom PerWebRequestTimelineManager and used that for my target type in the config file. But since the string in the constructor has to be dynamically determined, I cannot use the ConstructorInjection via the config file. I can use an abstract factory to solve the problem of dynamic dependency, but I am not sure about the implementation: Can you check the code below and validate the approach. Specifically the RegisterType and Resolve calls seem a bit out of place though the successive Resolve calls across the application will be able to retrieve the same instance.:

public class PerformanceTracerFactory : IPerformanceTracerFactory
{
    private readonly IPerformanceTracer tracer;

    public IPerformanceTracer CreateInstance(string operationTitle)
    {
        _container.RegisterType<IPerformanceTracer, PerformanceTracer>(new InjectionConstructor(operationTitle));

        return _container.Resolve<IPerformanceTracer>();
    }
}

Relevant portion of config file:

<register type="IPerformanceTracer" mapTo="PerformanceTracer">
  <lifetime type="PerWebRequest"/>
</register>
<register type="IPerformanceTracerFactory" mapTo="PerformanceTracerFactory"/>

I have another question. In case if the above way of configuring and injecting the dependency using code is correct, then I think I do not need the config entries. I can always use the suitable overload to push the custom lifetime manager. In case, I would want to achieve the same thing using only config file, then how do I code the solution?

Answer 1

If you use a container-based factory you don't have to register/resolve your IPerformanceTracer in each call.

Register the mapping IPerformanceTracer --> PerformanceTracer once in your config file and use a ParameterOverride when you resolve your interface.

public IPerformanceTracer CreateInstance(string operationTitle)
{
    return _container.Resolve<IPerformanceTracer>(new ParameterOverride("nameOfTheParameterInTheConstructorOfPerformanceTracer", operationTitle);
}