Asp.Net Web Api Data Annotations And Custom Validation Attributes Complete Guide

Complete Examples, Step by Step for Beginners: ASP.NET Web API Data Annotations and Custom Validation Attributes

Prerequisites:

• Visual Studio 2019 or later
• ASP.NET Web API knowledge basics

Step 1: Create an ASP.NET Core Web API Project

1. Open Visual Studio and select "Create a new project".
2. In the "Create a new project" window, select "ASP.NET Core Web Application".
3. Click "Next".
4. Name your project WebApiValidationExample and choose a location for it.
5. Click "Create".
6. In the "Create a new ASP.NET Core Web Application" dialog, select the "ASP.NET Core Web API" template.
7. Make sure "Authentication" is set to "None".
8. Click "Create".

Step 2: Create a Model

Let's create a model called Employee for your API:

1. Right-click on the Models folder in Solution Explorer and select "Add" > "Class".
2. Name the file Employee.cs and click "Add".
3. Define the Employee class with some properties and use Data Annotations for validation.

using System.ComponentModel.DataAnnotations;

namespace WebApiValidationExample.Models
{
    public class Employee
    {
        [Key]
        public int Id { get; set; }

        [Required(ErrorMessage = "Name is required")]
        [StringLength(50, MinimumLength = 2, ErrorMessage = "Name should be between 2 and 50 characters.")]
        public string Name { get; set; }

        [Required(ErrorMessage = "Email is required")]
        [EmailAddress(ErrorMessage = "Invalid Email Address")]
        [StringLength(100)]
        public string Email { get; set; }

        [Range(18, 60, ErrorMessage = "Age must be between 18 and 60")]
        public int Age { get; set; }

        [Required(ErrorMessage = "Department is required")]
        [StringLength(50)]
        public string Department { get; set; }
    }
}

Step 3: Create a Controller

1. Right-click on the Controllers folder in Solution Explorer and select "Add" > "Controller".
2. Choose "API Controller - Empty", name it EmployeesController, and click "Add".
3. Define basic CRUD operations. Since we're focusing on validation, we'll focus on POST operation for simplicity.

using Microsoft.AspNetCore.Mvc;
using System.Collections.Generic;
using WebApiValidationExample.Models;
using System.Linq;

namespace WebApiValidationExample.Controllers
{
    [Route("api/[controller]")]
    [ApiController]
    public class EmployeesController : ControllerBase
    {
        private static readonly List<Employee> Employees = new List<Employee>();

        // POST: api/Employees
        [HttpPost]
        public IActionResult PostEmployee([FromBody] Employee employee)
        {
            if (!ModelState.IsValid)
            {
                return BadRequest(ModelState);
            }

            Employees.Add(employee);
            
            return CreatedAtAction(nameof(GetEmployeeById), new { id = employee.Id }, employee);
        }

        // GET: api/Employees/{id}
        [HttpGet("{id}")]
        public IActionResult GetEmployeeById(int id)
        {
            var employee = Employees.FirstOrDefault(e => e.Id == id);
            if (employee == null)
            {
                return NotFound();
            }
            return Ok(employee);
        }
    }
}

Step 4: Create a Custom Validation Attribute

Let's create a custom validation attribute named ValidDepartment to check if the department name is one of the valid departments.

1. Right-click on the Models folder in Solution Explorer and select "Add" > "Class".
2. Name the file ValidDepartmentAttribute.cs and click "Add".
3. Implement the ValidDepartmentAttribute class.

using System.ComponentModel.DataAnnotations;

namespace WebApiValidationExample.Models
{
    public class ValidDepartmentAttribute : ValidationAttribute
    {
        private readonly string[] _validDepartments = { "Sales", "Marketing", "IT", "HR" };

        protected override ValidationResult IsValid(object value, ValidationContext validationContext)
        {
            if (value == null)
            {
                return new ValidationResult("Department is required");
            }

            string departmentName = value.ToString();

            if (!_validDepartments.Contains(departmentName))
            {
                return new ValidationResult($"Department should be one of: {string.Join(", ", _validDepartments)}");
            }

            return ValidationResult.Success;
        }
    }
}

4. Update the Employee model to use this custom attribute.

using System.ComponentModel.DataAnnotations;

namespace WebApiValidationExample.Models
{
    public class Employee
    {
        [Key]
        public int Id { get; set; }

        [Required(ErrorMessage = "Name is required")]
        [StringLength(50, MinimumLength = 2, ErrorMessage = "Name should be between 2 and 50 characters.")]
        public string Name { get; set; }

        [Required(ErrorMessage = "Email is required")]
        [EmailAddress(ErrorMessage = "Invalid Email Address")]
        [StringLength(100)]
        public string Email { get; set; }

        [Range(18, 60, ErrorMessage = "Age must be between 18 and 60")]
        public int Age { get; set; }

        [Required(ErrorMessage = "Department is required")]
        [StringLength(50)]
        [ValidDepartment]
        public string Department { get; set; }
    }
}

Step 5: Testing the API

1. Use Postman or Swagger UI to test the API.
2. Incorrect Data:
   • Send an invalid Employee object with incorrect data types or values, e.g., Age as a negative number or non-string departments.
3. Correct Data:
   • Send a valid Employee object ensuring all required fields are provided and values are within the specified ranges.

Example JSON for Post Operation:

Incorrect Example (missing Name and invalid Department):

{
    "email": "employee@example.com",
    "age": 25,
    "department": "RandomDept"
}

Response:

{
    "type": "https://tools.ietf.org/html/rfc7231#section-6.5.1",
    "title": "One or more validation errors occurred.",
    "status": 400,
    "traceId": "|187c0a85-4588a35af05988c0.",
    "errors": {
        "Name": [
            "Name is required"
        ],
        "Department": [
            "Department should be one of: Sales, Marketing, IT, HR"
        ]
    }
}

Correct Example:

{
    "name": "John Doe",
    "email": "john.doe@example.com",
    "age": 30,
    "department": "IT"
}

Response: