Custom Html helper that can browse DataAnnotations

Question

Say I have a model like this

public class User
{
    [Required]
    [StringLength(14, ErrorMessage = "Can only be 14 characters long")]
    public string UserName;

}

I want to create a Html helper like this:

@Html.ValidatableEditorFor(m => m.UserName)

so that it spits out a text field with the correct format for jQuery Vaidation plugin to be able to validate, like this:

   <input type="text" class="required" maxlength="14" />

From my research, it seems that there is no way to iterate over all the data annotations in a MetaDataModel so that I can check to see which one's are applicable to jQuery Validation.

How I envision it working in pseudo code:

    var tag = new TagBuilder("input");
    tag.mergeAttribute("type", "text");
    foreach(var attribute in metadata.attributes)
    {
       CheckForValidatableAttribute(attribute, tag);
    }

...
    private void CheckForValidatableAttribute(DataAnnotation attribute, TagBuilder tag)
    {
        switch(attribute.type)
       {
          case Required:
             tag.addClass("required");
             break;
          case StringLength
             tag.mergeAttribute("maxlength", attribute.value)
             break;
       }
    }

How could I go about achieving a helper like this? I want it to work on data annotations so that I don't have to duplicate the validation literals.

For instance, the current Html helpers like TextEditorFor do append validatable attributes to their output fields. How does it do this, and how can I make my own implementation?

Cheers

Answer 1

You may use this simple condition:

if(attribute.Type is ValidationAttribute)
{
   string className = attribute.Type.Name.Replace("Attribute", "").ToLower();
}

---

UPDATE

Define an Html helper:

public static MvcHtmlString ValidationEditorFor<TModel, TProperty>(this HtmlHelper<TModel> htmlHelper, 
        Expression<Func<TModel, TProperty>> expression)
{
    ....
}

Create this helper method:

private static string GetPropertyNameFromExpression<TModel, TProperty>(HtmlHelper<TModel> htmlHelper, Expression<Func<TModel, TProperty>> expression)
{
    MemberExpression memberExpression = expression.Body as MemberExpression;
    if (memberExpression == null)
        throw new InvalidOperationException("Not a memberExpression");

    if (!(memberExpression.Member is PropertyInfo))
        throw new InvalidOperationException("Not a property");

    return memberExpression.Member.Name;
}

Now use this in ValidationEditorFor:

var propertyName = GetPropertyNameFromExpression(htmlHelper, expression);
var propertyType = typeof(TModel).GetProperties().Where(x=>x.Name == propertyName).First().PropertyType;
var attributes = propertyType.GetCustomAttributes(true).OfType<ValidationAttribute>();

Now you can check the attributes.... rest is easy.

Answer 2

Slightly altered and extracted into a helper.

using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.Linq;
using System.Linq.Expressions;
using System.Reflection;

namespace Payntbrush.Infrastructure.Web.Mvc
{
    public static class ReflectionHelper
    {
        public static IEnumerable<ValidationAttribute> GetAttributes<TModel, TProperty>(Expression<Func<TModel, TProperty>> expression)
        {
            Type type = typeof(TModel);
            var prop = type.GetProperty(GetPropertyNameFromExpression(expression));
            return prop.GetCustomAttributes(true).OfType<ValidationAttribute>();
        }


        private static string GetPropertyNameFromExpression<TModel, TProperty>(Expression<Func<TModel, TProperty>> expression)
        {
            var memberExpression = expression.Body as MemberExpression;
            if (memberExpression == null)
                throw new InvalidOperationException("Not a memberExpression");

            if (!(memberExpression.Member is PropertyInfo))
                throw new InvalidOperationException("Not a property");

            return memberExpression.Member.Name;
        }
    }
}