What is best way to display error message that all radio buttons must be selected in an ASP.NET Core MVC view?

Question

I have a set of radio buttons in my ASP.NET Core MVC view. If "Submit" (save) is clicked without selecting ALL radio buttons, I want to display a message:

All radio buttons must be selected

in the @HTML.ValidationSummary at the top of my view.

What's the best way to achieve this please?

Answer 1

One of the most simplest method is using ViewBag instead of @HTML.ValidationSummary. @HTML.ValidationSummary will show all list of the error messages instead of custom error message by default, So you can try this simple demo:

<form method="post">        
    @ViewBag.Error
    <input type="radio"..../>
    ..........


</form>

controller

        [HttpPost]
        public IActionResult Index(TestModel model)
        {
            if (!ModelState.IsValid)
            {
                //set error message
                ViewBag.Error = "All radio buttons must be selected.";
                return View(model);
            }
              .......
           
        }

Then, when you don't select all radio buttons and submit the form, The error message will show, You can also add Css style to make it look the way you want.