ASP.NET MVC: What if your model is just a Dictionary?

Question

(I'm new to MVC).

In my application I don't have a model in the sense of a class with properties. Oh no it's much simpler: my users basically fill in a bunch of string values for various keys setup elsewhere in the system (the keys are arbitrary and not known ahead of time, thus no pre-coded class)¹.

My "model" thus is just:

Dictionary<string, string>

Pretty simple.

As I understand it model binding, html helpers, model state, validation summaries all rely on reflection of an arbitrary class' properties. But can they just use the key/values in my dictionary instead?

For example, can I have:

<label for="Name">Name:</label>
<%= Html.TextBox("Name") %>
<%= Html.ValidationMessage("Name", "*") %>

and:

[AcceptVerbs(HttpVerbs.Post)]
public ActionResult Create(Dictionary<string, string> valuesEntered)
{            
    // ...
}

and have MVC use the "Name" key/value as found in my Dictionary<string, string> "model" to do all its behind-the-scenes magic? (MVC 1.0 preferrably, but please shed some light if this is better addressed in 2.0 as I'd still like to know)?

---

_{1: sounds silly I'm sure. It's for a reporting app, where the "keys" are the report parameter names and the "values" are the values the report will run with.}

Answer 1

True, a model can be as simple as a collection of name/value pairs. You can go down that route but i would suggest at least wrapping your Dictionary in a class and using properties or getters at the very least to allow yourself some flexibility down the road.

And yes, you can bind collections but only in list/grid/combobox-type scenarios. E.g. UI controls that are designed to hold collections. You can't really use reflection to arbitrarily bind to a given value within a collection.

EDIT: I don't think a label/text box is the right UI for your scenario. Try a grid or a list.

Answer 2

I would suggest creating a model for a report name-value pair and then use a method such as that outlined in this (see the BeginCollectionItem html helper discussion) to achieve binding a collection of these.

Validation could then be inserted into the name-value pair model as required depending on the rules for the particular instance.

Answer 3

According to ASP.NET MVC architecture you will need to do the following steps:

1. Create your own binder (a suitable example I found here)

   public class DefaultDictionaryBinder : DefaultModelBinder  
   {  
       //...  
   }
   

2. Substitute default binder in Application_Start

   ModelBinders.Binders.DefaultBinder = new DefaultDictionaryBinder();
   

3. Create action filter

   public class DictionaryFilter : ActionFilterAttribute  
   {  
       public override void OnActionExecuting(ActionExecutingContext filterContext)  
       {  
           // Process filterContext.HttpContext.Request.InputStream here  
       }    
       //...  
   }
   

4. Now apply the filter to your action

   [DictionaryFilter]  
   [AcceptVerbs(HttpVerbs.Post)]  
   public ActionResult Create(Dictionary<string, string> valuesEntered)  
   {  
       //...  
   }  
   

5. Enjoy your work :-)

Answer 4

Creating the custom model binder is a good suggestion, however, I see one big problem with using a Dictionary to supply the values: How does the Dictionary-based model get the keys? According to your comment, you plan to give your controls the same names as your keys. So, if you already know the names of your keys, why go through the trouble of creating a Dictionary?

You can accomplish the same result by creating a single ViewModel object that holds all of the parameters you will use for your reports as properties. Populate the values you need, ignore the ones you don't.

For example:

public class ReportParameterViewModel
{
    public DateTime OrderRangeBegin { get; set; }
    public DateTime OrderRangeEnd { get; set; }
    public string Department { get; set; }
    public int Customer_Number { get; set; }
    public double Commission_Rate { get; set; }
    public int Page_Start { get; set; }
    public int Page_End { get; set; }
    public bool IsExecutiveVersion { get; set; }
    //
    // ...and so on...
    //
}

Make sure all of your controller action methods responsible for report generation can accept an instance of ReportParameterViewModel and make sure all of your report views inherit from ReportParameterViewModel. The dictionary idea just sounds like more work than is necessary, given the fact that the views for your reports are likely to be static.

Answer 5

Another alternative might be looping over the dictionary to generate the fields, using the key as the field name. In this case, however the label might have to be pulled from another location.

Prior to posting the form use jquery.form.js to combine the data being posted back into a json string representation for a dictionary and then use the JavaScriptSerializer to convert the json strin into an dictionary. Maybe, there is even support for doing this automatically.

For the validation you could also switch to client side validation, for example using jquery.validate.js and build the java scipt automatically.

Answer 6

The default model binder in MVC 1.0 can bind to a dictionary as long as it uses the magic form field identifiers 'dictionaryName[index].key' and 'dictionaryName[index].value', where dictionaryName is the name of your dictionary parameter, and index is a 0-based sequential number. Typically the 'key' value will be a hidden field, and the 'value' value is your text field. In your example:

<%= Html.Hidden("valuesEntered[0].key", "Name") %>
<%= Html.TextBox("valuesEntered[0].value") %>
<%= Html.ValidationMessage("valuesEntered[0].value", "*") %>

As I understand it, binding to dictionaries is different in MVC 2.

Answer 7

Goood Luck! I had the same issue, and i'm working on it now

Answer 8

Had the same problem