Question

EditorFor() can take an object additionalViewData parameter which the typical method to populate is something like:

EditorFor(model => model.PropertyName, new { myKey = "myValue" })

How can I inspect the contents of additionalViewData, add or append to existing value for a key, etc in a custom HTML Helper?

I’ve tried these approaches:

• convert to Dictionary<string, object>() and add/append values: doesn’t work as it looks like the implementation of EditorFor in MVC uses new RouteValueDictionary(additionalViewData) which embeds the dictionary within a dictionary
• convert to RouteValueDictionary using new RouteValueDictionary(additionalViewData) but that has same (or very similar) issue as above

I’m also open to “you’re doing it wrong” — maybe I’m missing a simpler approach. Keep in mind what I’m trying to do is write an HTML helper that is reusable and adds some values to the additionalViewData to be used by custom views. Some of the values depend on metadata from the property so it is not quite so easy as just use a bunch of different templates.

Update with example of what I’m doing:

    public static MvcHtmlString myNullableBooleanFor<TModel, TValue>(this HtmlHelper<TModel> htmlHelper, Expression<Func<TModel, TValue>> choice, string templateName, object additionalViewData)
    {            
        ModelMetadata metadata = ModelMetadata.FromLambdaExpression(choice, htmlHelper.ViewData);

        /*
    here need to add to additionalViewData some key values among them:
    { propertyName, metadata.PropertyName }

     */

        StringBuilder sb = new StringBuilder();
        sb.AppendLine(htmlHelper.EditorFor(choice, templateName, additionalViewData).ToString());
        MvcHtmlString validation = htmlHelper.ValidationMessageFor(choice);
        if (validation != null)
            sb.AppendLine(validation.ToString());
        return new MvcHtmlString(sb.ToString());
    }

Update with what happens when I convert the anonymous object to a Dictionary<string, object>() and pass that dictionary to EditorFor():

I put a break point in the Razor view and examined ViewData. It appears that the dictionary passed into EditorFor() is put inside another Dictionary<string, object>(). In the “Immediate Window”, ViewData looks like this:

ViewData.Keys
Count = 4
    [0]: "Comparer"
    [1]: "Count"
    [2]: "Keys"
    [3]: "Values"

See how the dictionary has the contents of a dictionary within it? Yes, the actual data is in that inner dictionary however unsurprisingly, this doesn’t work.

Added bounty.

Answer 1

If I understand correctly, you are trying to iterate over properties of an anonymous type. If so: How do I iterate over the properties of an anonymous object in C#?

[Edit]
Ok, it’s more than that. This is really bothering me now because I love C# and it won’t let me do what Python does, which I also love. So here’s a solution, if you are using C# 4 but it’s messy and will work for a similar problem I have but maybe not exactly for you:

    // Assume you've created a class to hold commonly used additional view data
    // called MyAdditionalViewData. I would create an inheritance hierarchy to
    // contain situation-specific (area-specific, in my case) additionalViewData.
class MyAdditionalViewData
{
    public string Name { get; set; }

    public string Sound { get; set; }
}     

/* In your views */
// Pass an instance of this class in the call to your HTML Helper
EditorFor(model => model.PropertyName, 
    new { adv = new MyAdditionalViewData { Name = "Cow", Sound = "Moo" } }) ;               

    /* In your HTML helper */
    // Here's the C# 4 part that makes this work.
dynamic bovine = new ExpandoObject();

// Copy values
var adv = x.adv;
if (adv.Name != null) bovine.Name = adv.Name;
if (adv.Sound != null) bovine.Sound = adv.Sound;

// Additional stuff
bovine.Food = "Grass";
bovine.Contents = "Burgers";

    // When the MVC framework converts this to a route value dictionary
    // you'll get a proper object, not a dictionary within a dictionary
var dict = new RouteValueDictionary(bovine);

There has got to be a better way.