aspnet-basics/5-model-validation-and-designing-apis.md

marp	paginate	math	theme	title
true	true	mathjax	buutti	5. Model Validation & API Design

Model Validation & API Design

Model validation in ASP.NET

• ASP.NET has a built-in system for validating whether the data sent in requests fits the model set in the code
• Additional requirements can be set with attributes

  public class Contact
  {
    public int Id { get; set; }
  
    [Required]
    [MaxLength(50)]
    public string Name { get; set; }
  
    [MaxLength(100)]
    public string Email { get; set; }
  }
  

TryValidateModel

• An object can be validated at any time with TryValidateModel method:

  [HttpPost]
  public IActionResult Post([FromBody] Contact contact)
  {
    if (!TryValidateModel(contact))
    {
      return BadRequest(ModelState);
    }
    Contacts.Add(contact);
    return Created(Request.Path, contact);
  }
  

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• The attributes set a corresponding error message and information to the ModelState which is sent with the bad request result

---

• Sometimes you might have custom requirements and the action should, according to the standard, return a bad request if the action does not fit those requirements
• For example, the simplest possible way to validate the format of an email is to check whether it contains the @ symbol
  • Add an error with AddModelError if it does not contain the symbol:

    if (!contact.Email.Contains('@'))
    {
      ModelState.AddModelError("Description", "The email is not in valid form.");
    }
    

  • Then check the model state:

    if (!ModelState.IsValid)
    {
      return BadRequest(ModelState);
    }
    

Exercise 1: Add validation

Continue working on CourseAPI.

1. Mark all the properties of the Course class with the [Required] attribute
2. Create an endpoint for POST requests with the URI api/courses
3. All the contents of the new course should come from the request body and the new course should be added to the Courses list
4. The maximum number of credits should be 20 and minimum 1 (Tip: Use Range)
5. Return an appropriate response

Limiting PATCH with validation

• Suppose we only want to allow the PATCH operation to affect Name and Email properties of the Contact class
• Begin by declaring a class with only those fields:

  // Models/ContactPatch.cs
  public class ContactPatch
  {
  public string Name { get; set; }
  public string Email { get; set; }
  }
  

---

• Use the ContactPatch class instead of the actual class for patching:

  // Controllers/ContactsController.cs
  [HttpPatch("{id}")]
  public IActionResult Patch(int id, [FromBody] JsonPatchDocument<ContactPatch> patchDocument)
  {
    Contact initialContact = _contactRepository.GetContact(id);
    ContactPatch patchContact = new ContactPatch
      { Name = initialContact.Name, Email = initialContact.Email };
  
    patchDocument.ApplyTo(patchContact, ModelState);
  
    if (!ModelState.IsValid)
    {
      return BadRequest();
    }
  
    initialContact.Name = patchContact.Name;
    initialContact.Email = patchContact.Email;
  
    _contactRepository.UpdateContact(id, initialContact);
    return Ok(initialContact);
  }
  

Designing APIs

• It can be useful to sketch an overview of the API before starting to implement it.
• Write a table with all the endpoints, writing out the
  • HTTP method
  • Description of the method
  • HTTP code on success
  • HTTP codes on different failure states
• See the following example!

---

Method	Endpoint	Description	Success	Failure
GET	/api/contacts	Return all contacts	200 OK	400 Bad Request, 404 Not Found
POST	/api/contacts	Add a new contact	201 Created	400 Bad Request
PUT	/api/contacts/{id}	Update a contact	204 No Content	400 Bad Request, 404 Not Found
PATCH	/api/contacts/{id}	Partially update a contact	204 No Content	400 Bad Request, 404 Not Found
DELETE	/api/contacts/{id}	Remove a contact	200 OK	404 Not Found