Accessing attributes from Custom Html Helpers in asp.net mvc 4 razor

The [Display] attribute enriches the metadata. So you could fetch the information from the metadata.

For example if you wanted to retrieve the display name inside the helper:

public static class HtmlHelpers
{
    public static IHtmlString Example<TModel, TProperty>(
        this HtmlHelper<TModel> html, 
        Expression<Func<TModel, TProperty>> ex
    )
    {
        var metadata = ModelMetadata.FromLambdaExpression(ex, html.ViewData);
        var displayName = metadata.DisplayName;
        return new HtmlString(html.Encode(displayName));
    }
}

and then if we assume that you have a view model:

public class MyViewModel
{
    [Required]
    [Display(Name = "Your Lastname")]
    public string Lastname { get; set; }
}

you could use the helper in your strongly typed view:

@model MyViewModel

@Html.Example(x => x.Lastname)

Now let's suppose that you wrote a custom metadata attribute:

public class FooBarAttribute : Attribute, IMetadataAware
{
    public FooBarAttribute(string bar)
    {
        Bar = bar;
    }
    public string Bar { get; private set; }

    public void OnMetadataCreated(ModelMetadata metadata)
    {
        metadata.AdditionalValues["foo"] = Bar;
    }
}

that you used to decorate your model with:

public class MyViewModel
{
    [Required]
    [FooBar("This is the bar")]
    public string SomeBar { get; set; }
}

and then inside your helper you could fetch the custom attribute:

public static class HtmlHelpers
{
    public static IHtmlString Example<TModel, TProperty>(
        this HtmlHelper<TModel> html, 
        Expression<Func<TModel, TProperty>> ex
    )
    {
        var metadata = ModelMetadata.FromLambdaExpression(ex, html.ViewData);
        if (metadata.AdditionalValues.ContainsKey("foo"))
        {
            var foo = metadata.AdditionalValues["foo"] as string;
            return new HtmlString(html.Encode(foo));
        }
        return MvcHtmlString.Empty;
    }
}

UPDATE:

It seems that you need to fetch the Required message. No idea why you need to do this in a custom helper but here's an example how you could achieve that:

public static class HtmlHelpers
{
    public static IHtmlString Example<TModel, TProperty>(
        this HtmlHelper<TModel> html, 
        Expression<Func<TModel, TProperty>> ex
    )
    {
        var me = (ex.Body as MemberExpression);
        if (me != null)
        {
            var required = me
                .Member
                .GetCustomAttributes(typeof(RequiredAttribute), false)
                .Cast<RequiredAttribute>()
                .FirstOrDefault();
            if (required != null)
            {
                var msg = required.FormatErrorMessage(me.Member.Name);
                return new HtmlString(html.Encode(msg));
            }
        }
        return MvcHtmlString.Empty;
    }
}