Payloads as dynamic Objects in ASP.NET MVC

Even though the dynamic type seems to be everywhere these days, ASP.NET MVC still doesn’t support having dynamic parameters in controller action methods out of the box; which is to say, this doesn’t work as expected:

[HttpPost]

public ActionResult Post(dynamic payload)

{

    //payload will be an object without any properties

 

    return this.View();

}

However, because MVC is so extensible, it is very easy to achieve it. For that, we need to build a custom model binder and apply it to our action method parameter. We’ll assume that the content will come as JSON from the HTTP POST payload. Note that this does not happen with Web API, but still happens with MVC Core!

There are a couple of ways by which we can bind a model binder to a parameter:

First, let’s focus on the actual model binder, the core for any of the above solutions; we need to implement the IModelBinder interface, which isn’t really that hard to do:

public sealed class DynamicModelBinder : IModelBinder

{

    private const string ContentType = "application/json";

 

    public DynamicModelBinder(bool useModelName = false)

    {

        this.UseModelName = useModelName;

    }

 

    public bool UseModelName { get; private set; }

 

    public object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)

    {

        dynamic data = null;

 

        if ((controllerContext.HttpContext.Request.AcceptTypes.Any(x => x.StartsWith(ContentType, StringComparison.OrdinalIgnoreCase) == true) &&

            (controllerContext.HttpContext.Request.ContentType.StartsWith(ContentType, StringComparison.OrdinalIgnoreCase) == true)))

        {

            controllerContext.HttpContext.Request.InputStream.Position = 0;

 

            using (var reader = new StreamReader(controllerContext.HttpContext.Request.InputStream))

            {

                var payload = reader.ReadToEnd();

 

                if (string.IsNullOrWhiteSpace(payload) == false)

                {

                    data = JsonConvert.DeserializeObject(payload);

 

                    if (this.UseModelName == true)

                    {

                        data = data[bindingContext.ModelName];

                    }

                }

            }

        }

 

        return data;

    }

}

Nothing fancy here; it will check to see if both the Accept and the Content-Type HTTP headers are present and set to application/json, the official MIME type for JSON, before parsing the posted content. If any content is present, JSON.NET will parse it into it’s own object. The UseModelName property is used to bind to a specific property of the payload, for example, say you are binding to a parameter called firstName, and you want it populated with the contents of the firstName field in the payload. In our case, we don’t need it, we want the whole thing, so it is set to false.

Now, the way I recommend for applying this model binder is through a custom attribute:

[Serializable]

[AttributeUsage(AttributeTargets.Parameter | AttributeTargets.Property, AllowMultiple = false, Inherited = false)]

public sealed class DynamicModelBinderAttribute : CustomModelBinderAttribute

{

    public DynamicModelBinderAttribute(bool useModelName = false)

    {

        this.UseModelName = useModelName;

    }

 

    public bool UseModelName { get; private set; }

 

    public override IModelBinder GetBinder()

    {

        return new DynamicModelBinder(this.UseModelName);

    }

}

It goes like this:

[HttpPost]

public ActionResult Post([DynamicModelBinder] dynamic payload)

{

    //payload will be populated with the contents of the HTTP POST payload

 

    return this.View();

}

Or, if you want to do it for several dynamic parameters, just set a global model binder provider for type Object; this is safe, because we would never have a parameter of type Object:

public sealed class DynamicModelBinderProvider : IModelBinderProvider

{

    public static readonly IModelBinderProvider Instance = new DynamicModelBinderProvider();

 

    public IModelBinder GetBinder(Type modelType)

    {

        if (modelType == typeof (object))

        {

            return new DynamicModelBinder();

        }

 

        return null;

    }

}

And we register is as this:

ModelBinderProviders.BinderProviders.Insert(0, DynamicModelBinderProvider.Instance);

And that’s it. This is one of what I consider to be ASP.NET MVC flaws, and they will deserve another post, soon. Web API already solves this problem, but it is still there in the future version of MVC Core, and can be solved in the same way.