.Net Core display model validation with validation filter

I created a basic .Net Core Mvc application and I want to automatically set ModelState.IsValid for all my action here is the code of Validation Filter

public class ValidationFilter : IAsyncActionFilter
{
    public async Task OnActionExecutionAsync(ActionExecutingContext context, ActionExecutionDelegate next)
    {
        if (!context.ModelState.IsValid)
        {
        }

        await next();
    }
}

and here is my action code

[HttpPost]
    public IActionResult AddUser(LoginRequestViewModel loginRequestViewModel)
    {
        return View(loginRequestViewModel);
    }

What should I return to display the errors in the asp-validation-for?
Without using any javascript code..

Answers:

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Method 1

According to the test of your code, global filtering can be achieved. Need to make ValidationFilter a global filter in startup.cs.

services.AddControllersWithViews(option=>
            {
                option.Filters.Add(new ValidationFilter());
            });

Then, create a model.

 public class LoginRequestViewModel
    {
        [MinLength(3)]
        public string username { get; set; }
        public string email { get; set; }
}

Define error messages in ValidationFilter.

public class ValidationFilter : IAsyncActionFilter
{
    public async Task OnActionExecutionAsync(ActionExecutingContext context, ActionExecutionDelegate next)
    {
        if (!context.ModelState.IsValid)
        {
            //Customize your error message
            string messages = string.Join("; ",context.ModelState.Values
                     .SelectMany(x => x.Errors)
                     .Select(x => !string.IsNullOrWhiteSpace(x.ErrorMessage) ? x.ErrorMessage : x.Exception.Message.ToString()));
            context.RouteData.Values.Add("mes", messages);
        }
        
        await next();
    }
}

In action, get RouteData.

[HttpPost]
        public IActionResult AddUser(LoginRequestViewModel loginRequestViewModel)
        {
            ViewData["error"]= RouteData.Values["mes"];
            return View(loginRequestViewModel);
        }

Create AddUser.cshtml and use form.

 @model solution921.Controllers.LoginRequestViewModel
<form action="/home/AddUser" method="post">
@ViewData["error"]
    <input type="text" name="username" value="" />
    <span asp-validation-for="username"></span>
    <input type="text" name="email" value="" />
    <input type="submit" name="" value="sub" />
</form>

It can return correct validation.

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