how can i validate model in asp.net web api?

Hello 

I have created this model and it worked for me, let me show you code and class

public class Person
    {
        [DataMember]
        [Required(ErrorMessage ="User name is requried")]
        public string Name { get; set; }

        [DataMember]
        [Required(ErrorMessage = "First name is requried")]
        public string FirstName { get; set; }
    }

and this was the code in web-api controller

[HttpPost]
        public HttpResponseMessage SaveTest(Person p)
        {
            if(ModelState.IsValid)
            {
                //save it in db
                return Request.CreateResponse(HttpStatusCode.OK);
            }
            else{
                var errors = new List<string>();
                foreach (var state in ModelState)
                {
                    foreach (var error in state.Value.Errors)
                    {
                        errors.Add(error.ErrorMessage);
                    }
                }
                return Request.CreateResponse(HttpStatusCode.Forbidden, errors);
            }
          
        }

And I was able to get error in postman when first name was empty

that's it, hope it will resolve your issue