Model Validation in ASP.NET Web API

In my previous articles, I discussed about MVC (Model-View-Controller) architecture as well as its implementation in details. I am expecting that the reader of this article already understand about the role of Model in an ASP.NET MVC application. Model in MVC is basically a representation of our data structure. So, here in this article, we are going to understand and implement the data annotation technique for applying validation on a model class for an ASP.NET Web API application.

In our ASP.NET Web API article series, we have already created an application that is performing all CRUD (Create, Retrieve, Update, Delete) operations using Web API. Here, we will take the same domain model class, i.e., "Student.cs" and apply validation on it using data annotation method. In order to better understand with more validation rules, I modified it a bit. So, here is our Model class:

public class Student
{
        [Range(1, 500)]
        public int StudentID { get; set; }
[Required] [MaxLength(10)] public string 
                FirstName { get; set; } public string LastName 
                { get; set; } }

We have applied some validation to our model class properties in the same manner as a database table's fields have associated validation rules. We have applied the attributes to model properties using System.ComponentModel.DataAnnotations namespace.

Our Model class, i.e., Student has the following validation rules.

• StudentID value must be between 1 and 500.
• FirstName is also required with maximum length of 10.
• LastName has no associated rule.

We will check the validity of Model against the defined validation rules in controller class, i.e., StudentController.

1 public class StudentsController 
                : ApiController
2 {
3 public HttpResponseMessage Post(Student Student)
4 {
5 if (ModelState.IsValid)
6 {
7 // Valid scenario code here...
8 return new HttpResponseMessage(HttpStatusCode.OK);
9 }
. else
. {
. return Request.CreateErrorResponse(HttpStatusCode.BadRequest, ModelState);
. }
. }
. }

Now, when the client will send a POST request in JSON format, it's being converted to Student object instance and that instance will be validated against the rules defined in Model class. In the above code sample, line # 5, Student instance state is validated and accordingly response is generated. Let's suppose if we post the following JSON to our ASP.NET Web API service one by one:

{ "StudentID":942, "FirstName":"Imran", "LastName":"Ghani" }
{ "StudentID":100, "FirstName":"ImraaanAbdul", "LastName":"Ghani" }
{ "StudentID":101, "FirstName":"Imran", "LastName":"Ghani" }

JSON representation 1 & 2 will return with the message "Bad Request....Request is Invalid...." because:

1. StudentID value is not between the defined validation range (1 to 500).
2. FirstName maximum defined length exceeded.

JSON representation 3 is valid and response with status code of 200 will be returned.

Data annotation technique for Model validation is really helpful because it validates data before involving in any further processing. But there are some limitations associated with this approach, i.e., Under-Posting and Over-Posting. Hopefully, I will try to discuss these concepts in more detail in a separate post.

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