Sample API controller. C# ASP.NET

APIController.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Net.Http;
using System.Web.Http;

namespace Controllers.Api
{
    [RoutePrefix("api/posts")]
    public class PostsApiController : BaseApiController
    {
        // Dependency Injection
        private IPostsService _postsService; //_postsService is a field a.k.a a var

        public PostsApiController (IPostsService postsService)
        {
            _postsService = postsService;
        }
        //public interface IPostsService
        [Route(""), HttpPost]
        public HttpResponseMessage Create(PostCreateRequest model)
        {
            if (model == null)
            {
                return Request.CreateErrorResponse(HttpStatusCode.BadRequest, "Payload cannot be empty");
            }
            if (model.GameId != null && model.ArtworkId == null || model.GameId == null && model.ArtworkId != null)
            {
                return Request.CreateErrorResponse(HttpStatusCode.BadRequest, "GameId and ArtworkId combination is not valid. Both must be null or not null");
            }
            if (!ModelState.IsValid)
            {
                return Request.CreateErrorResponse(HttpStatusCode.BadRequest, ModelState);
            }
            else
            {
                ItemResponse<int> response = new ItemResponse<int>();
                response.Item = _postsService.Create(model);
                return Request.CreateResponse(HttpStatusCode.OK, response);
            }
        }

        [Route("{id:int}"), HttpPut]
        public HttpResponseMessage Update(PostUpdateRequest model, int id)
        {
            if (model == null)
            {
                return Request.CreateErrorResponse(HttpStatusCode.BadRequest, "Payload cannot be empty");
            }
            if (model.Id != id)
            {
                ModelState.AddModelError("id", "Id is not valid");
            }
            if ((model.GameId != null && model.ArtworkId == null) || (model.GameId == null && model.ArtworkId != null))
            {
                return Request.CreateErrorResponse(HttpStatusCode.BadRequest, "GameId and ArtworkId combination is not valid. Both must be null or not null");
            }

            if (!ModelState.IsValid)
            {
                return Request.CreateResponse(HttpStatusCode.BadRequest, ModelState);
            }

            _postsService.Update(model);
            SuccessResponse response = new SuccessResponse();
            return Request.CreateResponse(HttpStatusCode.OK, response);

        }

        [Route(""), HttpGet]
        // Declaring another method GetAll
        public HttpResponseMessage GetAll()
        {
            ItemsResponse<Post> response = new ItemsResponse<Post>(); 
            response.Items = _postsService.GetAll();
            return Request.CreateResponse(HttpStatusCode.OK, response);
        }

        // Delete
        [Route("{id:int}"), HttpDelete]
        public HttpResponseMessage Delete(int id)
        {
            _postsService.Delete(id);
            SuccessResponse response = new SuccessResponse();
            return Request.CreateResponse(HttpStatusCode.OK, response);
        }

        // Edit
        [Route("{id:int}"), HttpGet]
        public HttpResponseMessage GetById(int id)
        {
            Post post = _postsService.GetById(id);
            ItemResponse<Post> response = new ItemResponse<Post>(); 
            response.Item = post;
            return Request.CreateResponse(HttpStatusCode.OK, response);
        }

        [Route(""), HttpGet]
        public HttpResponseMessage GetRecentByUserId(int userId, string order, int limit)
        {

            if (order != "dateCreated")
            {
                return Request.CreateResponse(HttpStatusCode.BadRequest, "Order is not valid");
            }

            ItemsResponse<Post> response = new ItemsResponse<Post>();
            response.Items = _postsService.GetRecentByUserId(userId, order, limit);
            return Request.CreateResponse(HttpStatusCode.OK, response);
        }

        [Route(""), HttpGet]
        public HttpResponseMessage GetRecent(string order, int limit)
        {
            if (order != "dateCreated")
            {
                return Request.CreateResponse(HttpStatusCode.BadRequest, "Order is not valid");
            }

            ItemsResponse<Post> response = new ItemsResponse<Post>();
            response.Items = _postsService.GetRecent(order, limit);
            return Request.CreateResponse(HttpStatusCode.OK, response);
        }

    }
}