Asp.Net Web Api - attribute for not binding/formatting a parameter?

Question

I have a method on an ApiController that looks like this:

public IEnumerable<Items> GetSlideSets() {
        IServiceClass serviceClass = new ServiceClass();
        //...

Yes, I am aware that this is not good design but I'm addressing this issue in a different iteration.

At a certain point in my application I need to call this functionality from within the project itself so I thought I could simply reuse the controller (and why not, I can pluck it out of my IoC container). The only problem is that in this case, I need to inject my own implementation of IServiceClass, easy enough:

public IEnumerable<Items> GetSlideSets(IServiceClass serviceClass = null) {
        serviceClass = serviceClass ?? new ServiceClass();
        //...

Except now I am getting errors when calling this via a regular Api call Optionalparameter 'serviceClass' is not supported by FormatterParameterBinding.

I know that there are various attributes that control bindings. Is there one that I can put on the parameter to say it shouldn't bind.

Can you move the serviceClass to the scope of the controller instead of the action method scope where you have it now? — David Tansey
– David Tansey, Commented May 6, 2013 at 2:06
IoC happens at object construction usually, thus the IServiceClass parameter would need to be a constructor parameter (that you copy to a private instance field and use from there) and not a method parameter. I.e., what he said. — ChrisF
– ChrisF, Commented May 6, 2013 at 3:08
That would mean that ServiceClass initializes for any controller method call. In a better architecture this wouldn't be a problem, but this is a prototype gone live and things like routing and controller refactoring are for another iteration. @ChrisF - you're right, this part of it is just regular DI, not IoC, I'm just injecting a dependency via parameter. I could also inject it via a property which would achieve a similar result but I'm looking specifically for the attribute. — George Mauer
– George Mauer, Commented May 6, 2013 at 14:19

Youssef Moussaoui · Accepted Answer · 2013-05-06 19:13:53Z

2

Like others have mentioned, it's probably a better idea to inject the dependency in the constructor.

But if you really must avoid binding an action parameter, there isn't a built-in attribute but you can create one pretty easily. Here's what it could look like:

public class DontBindAttribute : ParameterBindingAttribute
{
    public override HttpParameterBinding GetBinding(HttpParameterDescriptor parameter)
    {
        return new DontBindParameterBinding(parameter);
    }

    private class DontBindParameterBinding : HttpParameterBinding
    {
        public DontBindParameterBinding(HttpParameterDescriptor parameter) : base(parameter)
        {
        }

        public override Task ExecuteBindingAsync(ModelMetadataProvider metadataProvider, HttpActionContext actionContext, CancellationToken cancellationToken)
        {
            actionContext.ActionArguments.Add(Descriptor.ParameterName, Descriptor.DefaultValue);
            var completedTaskSource = new TaskCompletionSource<object>();
            completedTaskSource.SetResult(null);
            return completedTaskSource.Task;
        }
    }
}

You just need to apply the attribute to the parameter afterwards:

public IEnumerable<Items> GetSlideSets([DontBind] IServiceClass serviceClass = null)

answered May 6, 2013 at 19:13

Youssef Moussaoui

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1 Comment

George Mauer Over a year ago

Thanks, I ended up just going with making it Lazy<> and using property injection in this case, but thanks for showing me how to do it.

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