Html.Labelfor use DisplayName of object not property

Question

Given a view model like this:

public class ParentViewModel
{
    public object ChildViewModel { get; set; }
}

If I use Html.LabelFor like this:

@Html.LabelFor(model => model.ChildViewModel)

I would get an output like this:

<label for="Model_ChildViewModel">ChildViewModel</label>

What I actually want though is for the generated label to use the DisplayName attribute applied to the object E.G.

[DisplayName("My Custom Label")]
public class ChildViewModel
{
}

with an output of:

<label for="Model_ChildViewModel">My Custom Label</label>

I understand that the Html.LabelFor method takes an expression that expects a property and it will look for the DisplayName attribute on that property rather than the object itself.

I have created an Html helper method to achieve what I want that looks like this:

public static IHtmlString CreateLabel<TModel>(this HtmlHelper html, Func<TModel, object> func) 
    where TModel : class
    {
        TagBuilder tb = new TagBuilder("label");

        var model = html.ViewData.Model as TModel;
        if (model != null)
        {
            object obj = func(model);
            if (obj != null)
            {
                var attribute = obj.GetType().GetCustomAttributes(
                    typeof(DisplayNameAttribute), true)
                    .FirstOrDefault() as DisplayNameAttribute;

                if (attribute != null)
                {
                    tb.InnerHtml = attribute.DisplayName;
                    return MvcHtmlString.Create(tb.ToString());
                }
                else
                {
                    tb.InnerHtml = obj.ToString();
                    return MvcHtmlString.Create(tb.ToString());
                }
            }
        }

        tb.InnerHtml = html.ViewData.Model.ToString();
        return MvcHtmlString.Create(tb.ToString());
    }

Instead of taking an expression, my helper takes a Func<TModel, object> which returns the object that I want to check for the DisplayName attribute.

The first problem I had was when I tried to call this method in razor like this:

@Html.CreateLabel(model => model.ChildObject)

I get the following error:

The type arguments for method 'CreateLabel<TModel>(this HtmlHelper,
Func<TModel, object>) cannot be inferred from usage. Try specifying
the arguments explicitly.'

So I call the method like this instead:

 @{ Html.CreateLabel<ChildViewModel>(model => model.ChildObject); }

but nothing gets rendered. If I use the debugger to step through my helper method, the label tag is being generated but nothing is shown when my page is rendered.

So my questions are:

• How do I fix this to generate the label to my view?
• What do I have to do so that the generic parameter can be inferred?
• Is there any way to write the Html helper to do the same thing but using an expression? I have no experience using expressions so don't know where to start.

Update

I thought I'd post the final code as I made a few minor changes. First of all I took a look at the helpers in the MVC source code and decided to split the method into three separate methods in line with the provided examples. I also removed all the TagBuilder stuff as all I really needed was to generate the text that was to be injected between the <legend></legend> tags. The final code is below. Once again, thanks to Sylon for helping me out with this.

public static IHtmlString LegendTextFor<TModel, TObject>(this HtmlHelper<TModel> html, Expression<Func<TModel, TObject>> expression)
{
    return LegendTextHelper(html,
        ModelMetadata.FromLambdaExpression(expression, html.ViewData),
        ExpressionHelper.GetExpressionText(expression),
        expression.Compile().Invoke(html.ViewData.Model));
}

private static IHtmlString LegendTextHelper<TModel, TObject>(this HtmlHelper<TModel> html, ModelMetadata metadata, string htmlFieldName, TObject value)
{
    string resolvedLabelText = metadata.DisplayName ?? value.GetDisplayName() ?? metadata.PropertyName ?? htmlFieldName.Split('.').Last();

    if (String.IsNullOrEmpty(resolvedLabelText))
        return MvcHtmlString.Empty;

    return MvcHtmlString.Create(resolvedLabelText);
}

private static string GetDisplayName<T>(this T obj)
{
    if (obj != null)
    {
        var attribute = obj.GetType()
            .GetCustomAttributes(typeof(DisplayNameAttribute), false)
            .Cast<DisplayNameAttribute>()
            .FirstOrDefault();

        return attribute != null ? attribute.DisplayName : null;
    }
    else
    {
        return null;
    }
}

Answer 1

I just created a custom Html helper for label that does what you want:

Html Helper:

public static class HtmlHelperExtensions
{

    public static MvcHtmlString LabelForCustom<TModel, TValue>(this HtmlHelper<TModel> html, Expression<Func<TModel, TValue>> expression)
    {
        ModelMetadata metadata = ModelMetadata.FromLambdaExpression(expression, html.ViewData);

        string customDisplayName = null;

       var value = expression.Compile().Invoke(html.ViewData.Model);

       if (value != null)
       {
          var attribute = value.GetType().GetCustomAttributes(typeof(DisplayNameAttribute), false)
           .Cast<DisplayNameAttribute>()
           .FirstOrDefault();

           customDisplayName = attribute != null ? attribute.DisplayName : null;
       }

         string htmlFieldName = ExpressionHelper.GetExpressionText(expression);
        string labelText = metadata.DisplayName ?? customDisplayName ?? metadata.PropertyName ?? htmlFieldName.Split('.').Last();
        if (String.IsNullOrEmpty(labelText))
        {
            return MvcHtmlString.Empty;
        }

        TagBuilder tag = new TagBuilder("label");
         tag.Attributes.Add("for", html.ViewContext.ViewData.TemplateInfo.GetFullHtmlFieldId(htmlFieldName));
        tag.SetInnerText(labelText);
        return MvcHtmlString.Create(tag.ToString(TagRenderMode.Normal));
    }
}

My example model:

public class Parent
{
    public object Child { get; set; }
}

[DisplayName("yo")]
public class Child 
{
   public int Id { get; set; }
}

View:

@Html.LabelForCustom(m => m.Child)  @*prints yo*@