Create a custom MVC Dropdownlist with custom option attributes and retain the validation.

Step 1. Make sure to reference jquery validation in _ValidationScriptsPartial or inside _Layout.cshtml

<script src="~/lib/jquery/dist/jquery.min.js"></script> 
<script src="~/lib/jquery-validation/dist/jquery.validate.min.js"></script>
<script src="~/lib/jquery-validation-unobtrusive/dist/jquery.validate.unobtrusive.min.js"></script>

Step 2. Create a custom SelectListItem and HTML helper

    public class CustomSelectListItem : SelectListItem
    {
        public IDictionary<string, string> HtmlAttributes { get; set; }
    }

    public static class HtmlHelpers
    {
        public static IHtmlContent CustomDropDownListFor<TModel, TProperty>(
            this IHtmlHelper<TModel> htmlHelper,
            Expression<Func<TModel, TProperty>> expression,
            IEnumerable<CustomSelectListItem> customSelectList,
            string optionLabel,
            object htmlAttributes)
        {
            // Create a <select> element
            var selectBuilder = new TagBuilder("select");

            // Add attributes like id and name
            var expressionName = htmlHelper.NameFor(expression).ToString();
            selectBuilder.Attributes.Add("id", expressionName);
            selectBuilder.Attributes.Add("name", expressionName);
            selectBuilder.MergeAttributes(HtmlHelper.AnonymousObjectToHtmlAttributes(htmlAttributes));

            // Add the default optionLabel (placeholder) if provided
            if (!string.IsNullOrEmpty(optionLabel))
            {
                var optionTag = new TagBuilder("option");
                optionTag.InnerHtml.AppendHtml(optionLabel);
                optionTag.Attributes.Add("value", "");
                selectBuilder.InnerHtml.AppendHtml(optionTag);
            }

            // Add <option> elements from customSelectList
            foreach (var item in customSelectList)
            {
                var optionTag = new TagBuilder("option");
                optionTag.Attributes.Add("value", item.Value);
                optionTag.InnerHtml.AppendHtml(item.Text);

                // Add custom attributes
                foreach (var attribute in item.HtmlAttributes)
                    optionTag.Attributes.Add(attribute.Key, attribute.Value ?? string.Empty);

                if (item.Selected)
                    optionTag.Attributes.Add("selected", "selected");

                selectBuilder.InnerHtml.AppendHtml(optionTag);
            }…