ASP.NET MVC2 custom validation helper - C#

gistfile1.cs

/// <summary>
/// If the given model field has validation errors, this will emit the given CSS class name
/// </summary>
/// <typeparam name="TModel"></typeparam>
/// <typeparam name="TProperty"></typeparam>
/// <param name="htmlHelper"></param>
/// <param name="expression"></param>
/// <param name="cssClassToEmit"></param>
/// <returns></returns>
public static MvcHtmlString ValidationCSSClassFor<TModel, TProperty>(this HtmlHelper<TModel> htmlHelper, Expression<Func<TModel, TProperty>> expression, string cssClassToEmit)
{
    MvcHtmlString htmlString = null;

    //  Figure out the expression text from the LambdaExpression using our nifty helper
    //  (thank God for RedGate reflector or I would have never figured this one out)
    string expressionText = ExpressionHelper.GetExpressionText((LambdaExpression) expression);

    //  Get the full field name from the expression string:
    string fullHtmlFieldName = htmlHelper.ViewContext.ViewData.TemplateInfo.GetFullHtmlFieldName(expressionText);

    //  Get the model state from the field name:
    ModelState modelState = htmlHelper.ViewData.ModelState[fullHtmlFieldName];

    //  Check to see if we have errors.  If we don't, just get out:
    ModelErrorCollection errors = (modelState == null) ? null : modelState.Errors;
    ModelError error = ((errors == null) || (errors.Count == 0)) ? null : errors[0];
    if((error == null))
    {
        return null;
    }

    //  At this point, we have errors -- but if we don't have a CSS class we're supposed to emit
    //  ... well, we're just SOL
    if(!string.IsNullOrEmpty(cssClassToEmit))
    {
        htmlString = MvcHtmlString.Create(cssClassToEmit);
    }

    return htmlString;
}