Custom validation at controller level in web api

i'm stuck with a problem in a net standard 2.0 library and i really don't know how to approach it.

let's say i have a controller like this:

    [Route("{action}/{Id}")]
    [HttpPut]
    public void UpdateUser([FromBody] UserDTO user, long id )
    {
        _service.UpdateUser(user, id);
    }

easy, it works. now i want to add some custom validation. i know i can use System.ComponentModel.DataAnnotations on the DTO, but the message when there's a bad request is... bad.

i tried to use ValidationAttribute with a isValid,a FormatErrorMessage and add it to my controller like so

    [MyValidation]
    [Route("{action}/{Id}")]
    [HttpPut]
    public void UpdateUser([FromBody] UserDTO user, long id )
    {
        _service.UpdateUser(user, id);
    }

but it never reached the MyValidation code, it just skipped it.

is there a way i can do this so that i can do all the validation i want and check it against my db and do all the stuff i need?

how can i proceed?

all the guides i found are related to .core or .mvc, things i can't use cos i'm on a net standard library even a guide would be wonderful! thanks to anyone who wants to help :D

My Solution!

so, after lots of research i found out that... it's not possibile to do it. at least from what i understood/tried

i did a bit of a workaround because what i used can't be used on a net standard library, whatever, it seems net standard doesn't have this kind of stuff to work with.

i used the IActionFilter.

i created a class and inherited the filter and put in the two methods that it requires, OnActionExecuted and OnActionExecuting.

public class UserFilter : IActionFilter
    {
       public void OnActionExecuted(ActionExecutedContext context)
        {
           
        }

      public void OnActionExecuting(ActionExecutingContext context)
        {
        }
    }

to read what it's inside the payload you can use

    UserDTO model = new UserDTO();
    model = context.ActionArguments["user"] as UserDTO;

where user is the "name" of your payload. to know what it is, you can use the debugger and read what's inside context.actionArguments. remember to use the " " between that name. you have to cast it using the "as" keyword to your model. now you have access to your payload and you can do your own custom validator!!!

in my case i created a list of string that will contain the string errors, validate id and throw an error if the list has at least one element.

example :

     UserDTO model = new UserDTO();
     List<string> CustomError = new List<string>();
     model = context.ActionArguments["user"] as UserDTO;
     if (model.Age == 0) //dumb control, just for the sake of the example
        {
           CustomError.Add("age cannot be 0!")      
        }
     //other controls
     if (CustomError.Count > 0)
        {
            context.Result = new BadRequestObjectResult(CustomError);
        }

the last if is important! if the list you made has at least one element, it will throw a bad request (error 400) and you will get back a json with all the errors.

and on top of the controller you want to custom validate use

     [HttpPost]
     [TypeFilter(typeof(UserFilter))]
     public IValidation<> PostUser(UserDTO user)
       {
        //your controller logic
       }

hope it will help somebody in the future who got stucked like me, trying to custom validate data in a net standard, which it seems is impossible