DataTransferObjectsAndHowToDefineIt.md

DTO and how to define it in ASP.NET Core WebApi

What is a DTO (Data Transfer Object)?

A DTO (Data Transfer Object) is an object that carries data between processes, typically between different layers of a backend application. It is a plain data structure without business logic, used to encapsulate and transport data between the server, database, and client.

Purpose of DTOs in Backend Development

DTOs (Data Transfer Objects) are used to encapsulate, filter, and transform data when transferring it between different layers (e.g., database, business logic, API).

Why Use DTOs?

• Security > Prevent exposing sensitive fields (e.g., passwords).
• Performance > Reduce payload size by sending only necessary data.
• Decoupling > Separate internal models from API contracts for flexibility.
• Validation & Transformation > Format or validate incoming/outgoing data.
• Consistency > Standardize API responses across different endpoints.

Defining DTOs in ASP.NET Core Web API

In ASP.NET Core Web API, DTOs (Data Transfer Objects) are typically defined as plain C# classes without business logic. These DTOs are used to shape the request and response data.

Step-1

Define a DTO class separately in a DTOs directory to keep it organized. An example from our implementation is given below:

namespace backend.DTOs.Todo
{
    public class TodoDto
    {
        public Guid Id { get; set; }
        public string Title { get; set; }
        public bool IsCompleted { get; set; }
        public string Description { get; set; }
        public DateTime CreatedAt { get; set; }
        public Guid? UserId { get; set; }
    }
}

Step-2

To use DTOs in a Controller modify the controller to return DTOs instead of database models. An example from our implementation:

using Microsoft.AspNetCore.Mvc;
using backend.Data;
using backend.Mappers;
namespace backend.Controllers
{
    [ApiController]
    [Route("api/todo")]
    public class TodoController : ControllerBase
    {
        private readonly ApplicationDbContext _context;
        public TodoController(ApplicationDbContext context)
        {
            _context = context;
        }

        [HttpGet]
        public IActionResult Get()
        {
            var todos = _context.Todos.ToList().Select(todo => todo.ToTodoDto());
            return Ok(todos);
        }
    }
}

Here, you can see we use some sort of Mappers to use DTOs. A Mapper is a utility that helps convert one object type to another, typically mapping database models to DTOs (Data Transfer Objects). The most commonly used mapper in ASP.NET Core is AutoMapper. But here we are using custom mappers, because in this process the conversion is totally in our hands.

But why use Mappers?

Without mappers we cloud do it like below which will work perfectly for us.

using Microsoft.AspNetCore.Mvc;
using backend.Data;
using backend.DTOs.Todo;
namespace backend.Controllers
{
    [ApiController]
    [Route("api/todo")]
    public class TodoController : ControllerBase
    {
        private readonly ApplicationDbContext _context;
        public TodoController(ApplicationDbContext context)
        {
            _context = context;
        }

        [HttpGet]
        public IActionResult Get()
        {
            var todos = _context.Todos.ToList().Select(todo => new TodoDto
                                                            {
                                                                Id = todo.Id,
                                                                Title = todo.Title,
                                                                Description = todo.Description,
                                                                IsCompleted = todo.IsCompleted,
                                                                CreatedAt = todo.CreatedAt,
                                                                UserId = todo.UserId
                                                            });
            return Ok(todos);
        }
    }
}

What if this DTO is need in the whole project for 10 times how hectic the process it will be?. For this we use mappers. If you use custom mappers these are more flexible than the AutoMapper.