ASP.NET MVC3 - Dynamic forms (ModelBind to a Dictionary<string, string> or NameValueCollection) - How?

Question

Lets say I have a a form that can have new text inputs created in JavaScript at runtime. And I want to bind the values to a NameValueCollection (or a Dictionary). Does ASP.NET MVC3 natively allow this?

In other words, how do I get this to work?

Assuming this is the HTML form...

<!-- if someone posted this form -->
<form action="MyExample">
    <input type="hidden" name="id" value="123" />
    <input type="text" name="things.abc" value="blah" />
    <input type="text" name="things.def" value="happy" />
    <input type="text" name="things.ghi" value="pelicans" />
    <input type="submit" />
</form>

... and this is is the "Action" in the Controller ...

public ActionResult MyExample(int id, NameValueCollection things)
{
    // At this point, `things["abc"]` should equal `"blah"`
    return Content(string.Format("Things has {0} values.", things.Count));
}

Do I need to make my own custom model binder? Or am I just naming the input boxes incorrectly?

Answer 1

I don't think the default ASP.NET MVC3 model binder does this, so I've made the following helper class. It works, I just didn't want to make this if the DefaultModelBinder handles this already.

I won't mark this as the answer for a while in hopes that someone will tell me how to get this to work correctly without a custom class. But for those of you who have the same need, here's the code.

Global.asax.cs

// Add this line in the Application_Start() method in the Global.asax.cs
ModelBinders.Binders.DefaultBinder = new NameValueAwareModelBinder();

Custom model binder

using System.Collections.Specialized;
using System.Web.Mvc;

namespace Lil_Timmys_Example.Helpers
{
    public class NameValueAwareModelBinder : DefaultModelBinder
    {
        public override object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
        {
            if (bindingContext.ModelMetadata.ModelType == typeof(NameValueCollection))
            {
                var result = new NameValueCollection();

                string prefix = bindingContext.ModelName + ".";

                var queryString = controllerContext.HttpContext.Request.QueryString;
                foreach (var key in queryString.AllKeys)
                {
                    if (key.StartsWith(prefix))
                    {
                        result[key.Substring(prefix.Length)] = queryString.Get(key);
                    }
                }

                var form = controllerContext.HttpContext.Request.Form;
                foreach (var key in form.AllKeys)
                {
                    if (key.StartsWith(prefix))
                    {
                        result[key.Substring(prefix.Length)] = form.Get(key);
                    }
                }

                return result;
            }
            else
            {
                return base.BindModel(controllerContext, bindingContext);
            }
        }
    }
}