How do I pass a Dictionary as a parameter to an ActionResult method from jQuery/Ajax?

At last I figured it out!! Thanks for the suggestions everyone! I finally figured out the best solution is to pass JSON via the Http Post and use a custom ModelBinder to convert the JSON to a Dictionary. One thing I did in my solution is created a JsonDictionary object that inherits from Dictionary so that I can attach the custom ModelBinder to the JsonDictionary type, and it wont cause any conflicts in the future if I use Dictionary as a ActionResult parameter later on for a different purpose than JSON.

Here's the final ActionResult method:

public ActionResult AddItems([Bind(Include="values")] JsonDictionary values)
{
    // do something
}

And the jQuery "$.post" call:

$.post("/Controller/AddItems",
{
    values: Sys.Serialization.JavaScriptSerializer.serialize(
            {
                id: 200,
                "name": "Chris"
            }
        )
},
function(data) { },
"json");

Then the JsonDictionaryModelBinder needs to be registered, I added this to the Application_Start method within the Global.asax.cs:

protected void Application_Start()
{
    ModelBinders.Binders.Add(typeof(JsonDictionary), new JsonDictionaryModelBinder());
}

And, finally here's the JsonDictionaryModelBinder object and JsonDictionary object I created:

public class JsonDictionary : Dictionary<string, object>
{
    public JsonDictionary() { }

    public void Add(JsonDictionary jsonDictionary)
    {
        if (jsonDictionary != null)
        {
            foreach (var k in jsonDictionary.Keys)
            {
                this.Add(k, jsonDictionary[k]);
            }
        }
    }
}

public class JsonDictionaryModelBinder : IModelBinder
{
    #region IModelBinder Members

    public object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
    {
        if (bindingContext.Model == null) { bindingContext.Model = new JsonDictionary(); }
        var model = bindingContext.Model as JsonDictionary;

        if (bindingContext.ModelType == typeof(JsonDictionary))
        {
            // Deserialize each form/querystring item specified in the "includeProperties"
            // parameter that was passed to the "UpdateModel" method call

            // Check/Add Form Collection
            this.addRequestValues(
                model,
                controllerContext.RequestContext.HttpContext.Request.Form,
                controllerContext, bindingContext);

            // Check/Add QueryString Collection
            this.addRequestValues(
                model,
                controllerContext.RequestContext.HttpContext.Request.QueryString,
                controllerContext, bindingContext);
        }

        return model;
    }

    #endregion

    private void addRequestValues(JsonDictionary model, NameValueCollection nameValueCollection, ControllerContext controllerContext, ModelBindingContext bindingContext)
    {
        foreach (string key in nameValueCollection.Keys)
        {
            if (bindingContext.PropertyFilter(key))
            {
                var jsonText = nameValueCollection[key];
                var newModel = deserializeJson(jsonText);
                // Add the new JSON key/value pairs to the Model
                model.Add(newModel);
            }
        }
    }

    private JsonDictionary deserializeJson(string json)
    {
        // Must Reference "System.Web.Extensions" in order to use the JavaScriptSerializer
        var serializer = new System.Web.Script.Serialization.JavaScriptSerializer();
        return serializer.Deserialize<JsonDictionary>(json);
    }
}

This is what I tried. Saves a lot of work. Javascript:

  var dict = {};       
        dict["id"] = "200";
        dict["FirstName"] = "Chris";
        dict["DynamicItem1"] = "Some Value";
        dict["DynamicItem2"] = "Some Other Value";

        var theObject = {};
        theObject.dict = dict;
        $.post(URL, theObject, function (data, textStatus, XMLHttpRequest) {
            console.log("success");
        }, "json");

Action Method:

public ActionResult MethodName(DictionaryModel obj)
    {
       //Action method logic
    }

public class DictionaryModel
{
    public Dictionary<string, string> dict { get; set; }

}

It's possible with custom model binders or filters. Behind the scenes - you will have to do it manually anyway (Request.Form, parse strings, create dictionary tralala), but at least - your controller will be clean and code will be reusable for another actions.

I don't think it's possible to pass in a Dictionary from jQuery/Ajax to an ActionResult method via an Http Post. One thing I figured out that seems to be the easiest to work with is to pass in a JSON object and then parse that out into a Dictionary.

Here's the modified version of of the above calling "$.post" from jQuery that sends JSON as a pseudo-Dictionary:

$.post("/Controller/AddItems",
    {
        values: Sys.Serialization.JavaScriptSerializer.serialize(
                {
                    id: 200,
                    "name": "Chris"
                }
            )
    },
    function(data) { },
    "json");

The "Sys.Serialization.JavaScriptSerializer.serialize" function is a method of the ASP.NET AJAX JavaScript library.

Here's the modified version of the above ActionResult method:

public ActionResult AddItems(Dictionary<string, object> values)
{
    // Must Reference "System.Web.Extensions" in order to use the JavaScriptSerializer
    var json = new System.Web.Script.Serialization.JavaScriptSerializer();
    var data = json.Deserialize<Dictionary<string, string>>(routeValues);

    // do something
}

I think this makes it much easier to Unit Test by passing JSON, instead of using the Form Collection to send/retrieve the collection of key/value pairs. Also, it's easier to get working than figuring out how to build a custom IModelBinder, and a custom IModelBinder might cause issues with other ActionResult methods when this is the only one I need to do this.