Automatic ASP.NET Core Model Validation Using a Custom Action Filter

Create a custom ActionFilterAttribute

[AttributeUsage(AttributeTargets.Class | AttributeTargets.Method)]
public class ValidateModelStateFilterAttribute : ActionFilterAttribute
{
   public override void OnActionExecuting(ActionExecutingContext context)
   {
       if (!context.ModelState.IsValid)
       {
           // For API endpoints, return JSON error response  
           if (IsApiController(context.Controller))
           {
               var errors = context.ModelState
                   .Where(x => x.Value?.Errors.Count > 0)
                   .ToDictionary(
                       kvp => kvp.Key,
                       kvp => kvp.Value?.Errors.Select(e => e.ErrorMessage).ToArray()
                   );
               context.Result = new BadRequestObjectResult(new
               {
                   Message = "Validation failed",
                   Errors = errors
               });
           }
           else
           {
               // For MVC controllers, return view with model errors  
               context.Result = new ViewResult();
           }
       }
       base.OnActionExecuting(context);
   }
   private static bool IsApiController(object controller)
   {
       return controller.GetType()
           .GetCustomAttributes(typeof(ApiControllerAttribute), true).Length != 0;
   }
}

And then decorate the controller or action method you want to validate with the [ValidateModelStateFilter] attribute

If you will like this applied to all your controllers then do this:

services.AddControllersWithViews(options =>
{
   options.Filters.Add(new ValidateModelStateFilterAttribute()); // Custom model validation filter
});

This helps you avoid checking the ModelState in all your controller actions.