How to customize Html.ValidationMessageFor in ASP MVC

The only need for change of the default tag generation was in my case, that spans behavior results in anoying margin setups.

I resolved this by using 'display: block'

Maybe this helps some people..

I am not sure if it's possible to use paragraph instead of default span, as it may make impossible for validation plugin to place error messages. But for div -s, thats easy - you could write custom html helper.

Something along these lines (may need further testing/coding). You will need to include the namespace of this static extension method in your view, or put this into System.Web.Mvc.Html directly.

public static class Validator
{
    public static MvcHtmlString MyValidationMessageFor<TModel, TProperty>(this HtmlHelper<TModel> helper, Expression<Func<TModel, TProperty>> expression)
    {
        TagBuilder containerDivBuilder = new TagBuilder("div");
        containerDivBuilder.AddCssClass("field-error-box");

        TagBuilder topDivBuilder = new TagBuilder("div");
        topDivBuilder.AddCssClass("top");

        TagBuilder midDivBuilder = new TagBuilder("div");
        midDivBuilder.AddCssClass("mid");
        midDivBuilder.InnerHtml = helper.ValidationMessageFor(expression).ToString();

        containerDivBuilder.InnerHtml += topDivBuilder.ToString(TagRenderMode.Normal);
        containerDivBuilder.InnerHtml += midDivBuilder.ToString(TagRenderMode.Normal);

        return MvcHtmlString.Create(containerDivBuilder.ToString(TagRenderMode.Normal));
    }
}

As you see, this uses default ValidationMessageFor method, to not interfere with validation-plugin error message processing.

And you use this simply, as default validation message helper

@Html.MyValidationMessageFor(model => model.SomeRequiredField)

I used another way:

    public static MvcHtmlString DivValidationMessageFor<TModel, TProperty>(this HtmlHelper<TModel> htmlHelper, Expression<Func<TModel, TProperty>> expression)
    {
        return MvcHtmlString.Create(htmlHelper.ValidationMessageFor(expression).ToString().Replace("span", "div"));
    }

This way you can use the built in way, but replace the span with a div.

If you need any other overloads of the function, just duplicate as necessary.

You can implement your own ValidationMessageFor helper to emit your desired output or use some javascript to add/modify the rendered HTML code but the custom ValidationMessageFor implementation is the cleaner approach IMHO.

To implement your own ValidationMessageFor helper take a look at the ValidationExtensions.ValidationMessageFor and ValidationMessageHelper methods in the ASP.NET MVC source code.

Implementation Hints

Since GetFormContextForClientValidation is internal you have to work around that implementation by duplicating the internal functionality in your code:

FormContext formContext = htmlHelper.ViewContext.ClientValidationEnabled ? htmlHelper.ViewContext.FormContext : null;

Some other methods are private in ValidationExtensions like GetUserErrorMessageOrDefault you would need to duplicate that code too. What you can do to avoid duplicating code is to let ValidationExtentensions.ValidationMessageFor render the validation message string that is wrapped in a span and afterwards change the rendered string according to your requirements. Keep in mind that "null" is returned in case no error was found and that you'll need the data- HTML attributes in case you have unobtrusive JavaScript enabled.

You can download the ASP.NET MVC 3 source code from here