String property allowed values with dropdown box and MVC model validation

AllowedValuesAttribute.cs

// Usage:
// [AttributeUsage("Option1", "Option2", "Option3")]
// public string MyTestAttribute { get; set; }
// 
// In view:
// @Html.DropDownListFor(x => x.MyTestAttribute)
//
// Result:
// Dropdown list with 3 options.

namespace Extensions
{
	using System;
	using System.ComponentModel.DataAnnotations;
	using System.Linq;

	/// <summary>
	/// AllowedValues attribute for string types
	/// </summary>
	[AttributeUsage(AttributeTargets.Property, Inherited = false)]
	public sealed class AllowedValuesAttribute : ValidationAttribute
	{
		public AllowedValuesAttribute(params string[] args)
		{
			Values = args;
		}

		public string[] Values { get; set; }

		public override bool IsValid(object value)
		{
			if (Values.Contains(value))
				return true;

			return false;
		}
	}
}

HtmlExtensions.cs

		/// <summary>
		/// DropDownListFor model property which has AllowedValuesAttribute
		/// </summary>
		/// <typeparam name="TModel">Model to create a dropdown for</typeparam>
		/// <param name="helper">HTML Helper</param>
		/// <param name="expression">Expression to use</param>
		/// <param name="htmlAttributes">Attributes to use</param>
		/// <returns>MVC Html String of Dropdownlist</returns>
		public static MvcHtmlString DropDownListFor<TModel>(this HtmlHelper<TModel> helper, Expression<Func<TModel, object>> expression, object htmlAttributes)
		{
			ModelMetadata metadata = ModelMetadata.FromLambdaExpression(expression, helper.ViewData);
			var info = GetPropertyInformation(expression.Body);
			if (info == null) return helper.DropDownListFor(expression, htmlAttributes);

			var attr = info.GetAttribute<AllowedValuesAttribute>(true);
			if (attr == null) return helper.DropDownListFor(expression, htmlAttributes);

			IEnumerable<SelectListItem> items =
				attr.Values.Select(
					value => new SelectListItem() { Text = value, Value = value, Selected = value.Equals(metadata.Model) });

			return helper.DropDownListFor(expression, items, htmlAttributes);
		}

		/// <summary>
		/// Gets property information
		/// </summary>
		/// <param name="expression">Expression to use</param>
		/// <returns>Member info of property</returns>
		public static MemberInfo GetPropertyInformation(Expression expression)
		{
			var member = expression as MemberExpression;
			if (member == null)
			{
				var unary = expression as UnaryExpression;
				if (unary != null && unary.NodeType == ExpressionType.Convert)
				{
					member = unary.Operand as MemberExpression;
				}
			}

			if (member != null && member.Member.MemberType == MemberTypes.Property)
			{
				return member.Member;
			}

			return null;
		}

		/// <summary>
		/// Attempts to get attribute from an object
		/// </summary>
		/// <typeparam name="T">Attribute to get</typeparam>
		/// <param name="member">Member to get attribute from</param>
		/// <param name="isRequired">Is the attribute required</param>
		/// <returns>Required attribute</returns>
		public static T GetAttribute<T>(this MemberInfo member, bool isRequired) where T : Attribute
		{
			var attribute = member.GetCustomAttributes(typeof(T), false).SingleOrDefault();
			if (attribute == null && isRequired) return null;
			return (T)attribute;
		}