CustomAttribute reflects html attribute MVC5

If using the standard helpers with the overload to add html attributes is not acceptable, then you can create an attribute implements IMetadataAware that adds properties to metadata.AdditionalValues which can then be used in custom html helpers. A simple example might be

[AttributeUsage(AttributeTargets.Property)]
public class CustomHtmlAttribute : Attribute, IMetadataAware
{
  public static string ValueKey
  {
    get { return "Value"; }
  }
  public string Value { get; set; }
  public void OnMetadataCreated(ModelMetadata metadata)
  {
    if (Value != null)
    {
      metadata.AdditionalValues[ValueKey] = Value;
    }
  }
}

and to create a helper to render a textbox (only one overload shown here)

public static MvcHtmlString CustomHtmlTextBoxFor<TModel, TValue>(this HtmlHelper<TModel> helper, Expression<Func<TModel, TValue>> expression)
{
  ModelMetadata metaData = ModelMetadata.FromLambdaExpression(expression, helper.ViewData);
  object attributes = null;
  if (metaData.AdditionalValues.ContainsKey(ValueKey))
  {
    attributes = new { customhtml = (string)metaData.AdditionalValues[ValueKey] };
  }
  return InputExtensions.TextBoxFor(helper, expression, attributes);
}

and use it as

[CustomHtml(Value = "hello")]
public string CoolValue { get; set; } 

and in the view

@Html.CustomHtmlTextBoxFor(m => m.CoolValue)

to make this a bit more flexible, you could add more properties to the attribute so you could apply it as

[CustomHtml(Value = "hello", Pattern="/d")]
public string CoolValue { get; set; }

and modify the helper to render all the html attributes you define.