Question

I have a situation where I need to write an HTML Helper to extend another html helper. Normally, the helper would look like this.

@Html.TextAreaFor(model => model.Content, new { @class = "some css", @data_bind = "some other stuff..." })

This works fine, but it has to be wrapped in some other HTML that is always the same. I wanted to encapsulate it for convenience, like this.

public static MvcHtmlString CondensedHelperFor<TModel, TProperty>(this HtmlHelper<TModel> htmlHelper, Expression<Func<TModel, TProperty>> expression, object htmlAttributes) {
            var stringBuilder = new System.Text.StringBuilder();
            var tag = new TagBuilder("div"); tag.AddCssClass("some_css");
            stringBuilder.Append(toolbar.ToString(TagRenderMode.SelfClosing));

            stringBuilder.Append(htmlHelper.TextAreaFor(expression, htmlAttributes));
            // more tags and such...

            return new MvcHtmlString(stringBuilder.ToString());
        }

The line stringBuilder.Append(htmlHelper.TextAreaFor... is what I want to change. The CSS class that has to go there is always going to be present. So I would rather include it here. However I would like to be able to specify additional CSS classes in the top-level helper. So …

@Html.CondensedHelperFor(model => model.Content, new { @class = "some_other_css" })

And the static css that will always be there get blanketed in through the Helper.

Any ideas?

Answer 1

First, create a method (the best would be to create an extension method) that converts an object to IDictionary via type reflection:

public static IDictionary<string, object> ToDictionary(this object data) 
{
        if(data == null) return null; // Or throw an ArgumentNullException if you want

        BindingFlags publicAttributes = BindingFlags.Public | BindingFlags.Instance;
        Dictionary<string, object> dictionary = new Dictionary<string, object>();

        foreach (PropertyInfo property in 
                 data.GetType().GetProperties(publicAttributes)) { 
            if (property.CanRead) {
                dictionary.Add(property.Name, property.GetValue(data, null));
            }
        }
        return dictionary;
}

Now, make use of C# 4.0 ExpandoObject, which allows adding properties at runtime.
You would end up with something like this:

public static MvcHtmlString CondensedHelperFor<TModel, TProperty>(this HtmlHelper<TModel> htmlHelper, Expression<Func<TModel, TProperty>> expression, object htmlAttributes) {
{
    var dictAttributes = htmlAttributes.ToDictionary();

    var result = new ExpandoObject();
    var d = result as IDictionary<string, object>; //work with the Expando as a Dictionary

    if(dictAttributes != null)
    {
        foreach (var pair in dictAttributes)
        {
            d[pair.Key] = pair.Value;
        }
    }

    // Add other properties to the dictionary d here
    // ...

    var stringBuilder = new System.Text.StringBuilder();
    var tag = new TagBuilder("div"); tag.AddCssClass("some_css");
    stringBuilder.Append(toolbar.ToString(TagRenderMode.SelfClosing));
    stringBuilder.Append(htmlHelper.TextAreaFor(expression, result));

    return new MvcHtmlString(stringBuilder.ToString());
}