Question

I’m trying to bind a service and specifying a constructor argument using Ninject in our application. The constructor argument is a value that can be pulled from the query string or a cookie. The code we currently have is something like this

kernel.Bind<SomeService>()
      .ToSelf()
      .InRequestScope()
      .WithConstructorArgument("someID", ctx =>
           // Try to get it from the posted form values
           System.Web.HttpContext.Current.Request.Form["someID"] != null ? 
           long.Parse(System.Web.HttpContext.Current.Request.Form["someID"]) :                                          

           // Try to get it from the query string
           System.Web.HttpContext.Current.Request.QueryString["someID"] != null ?
           long.Parse(System.Web.HttpContext.Current.Request.QueryString["someID"]) 

           : 0);

This works but is pretty ugly. I realize there are other ways of accomplishing this such as passing in the Form value or the QueryString value as a parameter, but we like having it defined in the Binding. What we would ideally like to do is something like this:

   kernel.Bind<SomeService>()
          .ToSelf()
          .InRequestScope()
          .WithConstructorArgument("someID", ctx => GetSomeID());

From what I can tell, this is not possible. Is there another way to break out the constructor argument injection logic into another method so we don’t have to nested one line if statements?

Answer 1

I’d suggest binding the dependency on the Query String / HTTP form via an interface. This approach seems more in line with the dependency injection pattern (de-coupling code from specific implementations and classes).

public interface IParameters
{
    string SomeID { get; }
}

public class ParametersFromHttpContext
{
    IQueryString _queryString;
    IRequestForm _requestForm;

    public ParametersFromHttpContext(IQueryString queryString, IRequestForm requestForm)
    {
        _queryString = queryString;
        _requestForm = requestForm;
    }

    public string SomeID
    {
        get
        {
            return
                // Try to get it from the posted form values
               _requestForm["someID"] != null ? 
               long.Parse(_requestForm["someID"]) :                                          

               // Try to get it from the query string
               _queryString["someID"] != null ?
               long.Parse(_queryString["someID"]) 

               : 0;
        }
    }
}

Now logic you want can be contained in the binding, without the need to reference HttpContext in the kernel.

kernel.Bind<IParameters>().To<ParametersFromHttpContext>();