CREATING WEB API IN ASP.NET CORE 2.0

CREATING WEB API IN ASP.NET CORE 2.0

Introduction

Let's create a Web API with the latest version of ASP.NET Core and Entity Framework Core.

In this guide, we'll use WideWorldImporters database to create a Web API.

REST APIs provide at least the following operations:

• GET
• POST
• PUT
• DELETE

There are other operations for REST, but they aren't necessary for this guide.

Those operations allow clients to perform actions through REST API, so our Web API must contain those operations.

WideWorldImporters database contains 4 schemas:

Application Purchasing Sales Warehouse

In this guide, we'll work with Warehouse.StockItems table. We'll add code to work with this entity: allow to retrieve stock items, retrieve stock item by id, create, update and delete stock items from database.

The version for this API is 1.

This is the route table for API:

VERB	URL	DESCRIPTION
GET	api/v1/Warehouse/StockItem	Retrieves stock items
GET	api/v1/Warehouse/StockItem/id	Retrieves a stock item by id
POST	api/v1/Warehouse/StockItem	Creates a new stock item
PUT	api/v1/Warehouse/StockItem/id	Updates an existing stock item
DELETE	api/v1/Warehouse/StockItem/id	Deletes an existing stock item

Keep these routes in mind because API must implement all routes.

Prerequisites

Software

• .NET Core
• NodeJS
• Visual Studio 2017 with last update
• SQL Server
• WideWorldImporters database

Skills

• C#
• ORM (Object Relational Mapping)
• TDD (Test Driven Development)
• RESTful services

Using the Code

For this guide, the working directory for source code is C:\Projects.

Also, This is the GitHub repository for this guide.

Step 01 - Create Project

Open Visual Studio and follow these steps:

Go to File > New > Project
Go to Installed > Visual C# > .NET Core
Set the name for project as WideWorldImporters.API
Click OK

In the next window, select API and the latest version for .ASP.NET Core, in this case is 2.1:

Once Visual Studio has finished with creation for solution, we'll see this window:

Step 02 - Install Nuget Packages

In this step, We need to install the following NuGet packages:

* EntityFrameworkCore.SqlServer
* Swashbuckle.AspNetCore

Now, we'll proceed to install EntityFrameworkCore.SqlServer package from Nuget, right click on WideWorldImporters.API project:

Change to Browse tab and type Microsoft.EntityFrameworkCore.SqlServer:

Next, install Swashbuckle.AspNetCore package:

This is the structure for project.

Now run the project to check if solution is ready, press F5 and Visual Studio will show this browser window:

By default, Visual Studio adds a file with name ValuesController in Controllers directory, remove it from project.

Step 03 - Add Models

Now, create a directory with name Models and add the following files:

Entities.cs
Extensions.cs
Requests.cs
Responses.cs

Entities.cs will contains all code related to Entity Framework Core.

Extensions.cs will contain the extension methods for DbContext.

Requests.cs will contain definitions for request models.

Responses.cs will contain definitions for response models.

Code for Entities.cs file:

using System;
using Microsoft.EntityFrameworkCore;
using Microsoft.EntityFrameworkCore.Metadata.Builders;

namespace WideWorldImporters.API.Models
{
#pragma warning disable CS1591
    public partial class StockItem
    {
        public StockItem()
        {
        }

        public StockItem(int? stockItemID)
        {
            StockItemID = stockItemID;
        }

        public int? StockItemID { get; set; }

        public string StockItemName { get; set; }

        public int? SupplierID { get; set; }

        public int? ColorID { get; set; }

        public int? UnitPackageID { get; set; }

        public int? OuterPackageID { get; set; }

        public string Brand { get; set; }

        public string Size { get; set; }

        public int? LeadTimeDays { get; set; }

        public int? QuantityPerOuter { get; set; }

        public bool? IsChillerStock { get; set; }

        public string Barcode { get; set; }

        public decimal? TaxRate { get; set; }

        public decimal? UnitPrice { get; set; }

        public decimal? RecommendedRetailPrice { get; set; }

        public decimal? TypicalWeightPerUnit { get; set; }

        public string MarketingComments { get; set; }

        public string InternalComments { get; set; }

        public string CustomFields { get; set; }

        public string Tags { get; set; }

        public string SearchDetails { get; set; }

        public int? LastEditedBy { get; set; }

        public DateTime? ValidFrom { get; set; }

        public DateTime? ValidTo { get; set; }
    }

    public class StockItemsConfiguration : IEntityTypeConfiguration<StockItem>
    {
        public void Configure(EntityTypeBuilder<StockItem> builder)
        {
            // Set configuration for entity
            builder.ToTable("StockItems", "Warehouse");

            // Set key for entity
            builder.HasKey(p => p.StockItemID);

            // Set configuration for columns

            builder.Property(p => p.StockItemName).HasColumnType("nvarchar(200)").IsRequired();
            builder.Property(p => p.SupplierID).HasColumnType("int").IsRequired();
            builder.Property(p => p.ColorID).HasColumnType("int");
            builder.Property(p => p.UnitPackageID).HasColumnType("int").IsRequired();
            builder.Property(p => p.OuterPackageID).HasColumnType("int").IsRequired();
            builder.Property(p => p.Brand).HasColumnType("nvarchar(100)");
            builder.Property(p => p.Size).HasColumnType("nvarchar(40)");
            builder.Property(p => p.LeadTimeDays).HasColumnType("int").IsRequired();
            builder.Property(p => p.QuantityPerOuter).HasColumnType("int").IsRequired();
            builder.Property(p => p.IsChillerStock).HasColumnType("bit").IsRequired();
            builder.Property(p => p.Barcode).HasColumnType("nvarchar(100)");
            builder.Property(p => p.TaxRate).HasColumnType("decimal(18, 3)").IsRequired();
            builder.Property(p => p.UnitPrice).HasColumnType("decimal(18, 2)").IsRequired();
            builder.Property(p => p.RecommendedRetailPrice).HasColumnType("decimal(18, 2)");
            builder.Property(p => p.TypicalWeightPerUnit).HasColumnType("decimal(18, 3)").IsRequired();
            builder.Property(p => p.MarketingComments).HasColumnType("nvarchar(max)");
            builder.Property(p => p.InternalComments).HasColumnType("nvarchar(max)");
            builder.Property(p => p.CustomFields).HasColumnType("nvarchar(max)");
            builder.Property(p => p.LastEditedBy).HasColumnType("int").IsRequired();

            // Computed columns

            builder
                .Property(p => p.StockItemID)
                .HasColumnType("int")
                .IsRequired()
                .HasComputedColumnSql("NEXT VALUE FOR [Sequences].[StockItemID]");

            builder
                .Property(p => p.Tags)
                .HasColumnType("nvarchar(max)")
                .HasComputedColumnSql("json_query([CustomFields],N'$.Tags')");

            builder
                .Property(p => p.SearchDetails)
                .HasColumnType("nvarchar(max)")
                .IsRequired()
                .HasComputedColumnSql("concat([StockItemName],N' ',[MarketingComments])");

            // Columns with generated value on add or update

            builder
                .Property(p => p.ValidFrom)
                .HasColumnType("datetime2")
                .IsRequired()
                .ValueGeneratedOnAddOrUpdate();

            builder
                .Property(p => p.ValidTo)
                .HasColumnType("datetime2")
                .IsRequired()
                .ValueGeneratedOnAddOrUpdate();
        }
    }

    public class WideWorldImportersDbContext : DbContext
    {
        public WideWorldImportersDbContext(DbContextOptions<WideWorldImportersDbContext> options)
            : base(options)
        {
        }

        protected override void OnModelCreating(ModelBuilder modelBuilder)
        {
            // Apply configurations for entity

            modelBuilder
                .ApplyConfiguration(new StockItemsConfiguration());

            base.OnModelCreating(modelBuilder);
        }

        public DbSet<StockItem> StockItems { get; set; }
    }
#pragma warning restore CS1591
}

Code for Extensions.cs file:

using System.Linq;
using System.Threading.Tasks;
using Microsoft.EntityFrameworkCore;

namespace WideWorldImporters.API.Models
{
#pragma warning disable CS1591
    public static class WideWorldImportersDbContextExtensions
    {
        public static IQueryable<StockItem> GetStockItems(this WideWorldImportersDbContext dbContext, int pageSize = 10, int pageNumber = 1, int? lastEditedBy = null, int? colorID = null, int? outerPackageID = null, int? supplierID = null, int? unitPackageID = null)
        {
            // Get query from DbSet
            var query = dbContext.StockItems.AsQueryable();

            // Filter by: 'LastEditedBy'
            if (lastEditedBy.HasValue)
                query = query.Where(item => item.LastEditedBy == lastEditedBy);

            // Filter by: 'ColorID'
            if (colorID.HasValue)
                query = query.Where(item => item.ColorID == colorID);

            // Filter by: 'OuterPackageID'
            if (outerPackageID.HasValue)
                query = query.Where(item => item.OuterPackageID == outerPackageID);

            // Filter by: 'SupplierID'
            if (supplierID.HasValue)
                query = query.Where(item => item.SupplierID == supplierID);

            // Filter by: 'UnitPackageID'
            if (unitPackageID.HasValue)
                query = query.Where(item => item.UnitPackageID == unitPackageID);

            return query;
        }

        public static async Task<StockItem> GetStockItemsAsync(this WideWorldImportersDbContext dbContext, StockItem entity)
            => await dbContext.StockItems.FirstOrDefaultAsync(item => item.StockItemID == entity.StockItemID);

        public static async Task<StockItem> GetStockItemsByStockItemNameAsync(this WideWorldImportersDbContext dbContext, StockItem entity)
            => await dbContext.StockItems.FirstOrDefaultAsync(item => item.StockItemName == entity.StockItemName);
    }

    public static class IQueryableExtensions
    {
        public static IQueryable<TModel> Paging<TModel>(this IQueryable<TModel> query, int pageSize = 0, int pageNumber = 0) where TModel : class
            => pageSize > 0 && pageNumber > 0 ? query.Skip((pageNumber - 1) * pageSize).Take(pageSize) : query;
    }
#pragma warning restore CS1591
}

Code for Requests.cs file:

using System;
using System.ComponentModel.DataAnnotations;

namespace WideWorldImporters.API.Models
{
#pragma warning disable CS1591
    public class PostStockItemsRequest
    {
        [Key]
        public int? StockItemID { get; set; }

        [Required]
        [StringLength(200)]
        public string StockItemName { get; set; }

        [Required]
        public int? SupplierID { get; set; }

        public int? ColorID { get; set; }

        [Required]
        public int? UnitPackageID { get; set; }

        [Required]
        public int? OuterPackageID { get; set; }

        [StringLength(100)]
        public string Brand { get; set; }

        [StringLength(40)]
        public string Size { get; set; }

        [Required]
        public int? LeadTimeDays { get; set; }

        [Required]
        public int? QuantityPerOuter { get; set; }

        [Required]
        public bool? IsChillerStock { get; set; }

        [StringLength(100)]
        public string Barcode { get; set; }

        [Required]
        public decimal? TaxRate { get; set; }

        [Required]
        public decimal? UnitPrice { get; set; }

        public decimal? RecommendedRetailPrice { get; set; }

        [Required]
        public decimal? TypicalWeightPerUnit { get; set; }

        public string MarketingComments { get; set; }

        public string InternalComments { get; set; }

        public string CustomFields { get; set; }

        public string Tags { get; set; }

        [Required]
        public string SearchDetails { get; set; }

        [Required]
        public int? LastEditedBy { get; set; }

        public DateTime? ValidFrom { get; set; }

        public DateTime? ValidTo { get; set; }
    }

    public class PutStockItemsRequest
    {
        [Required]
        [StringLength(200)]
        public string StockItemName { get; set; }

        [Required]
        public int? SupplierID { get; set; }

        public int? ColorID { get; set; }

        [Required]
        public decimal? UnitPrice { get; set; }
    }

    public static class Extensions
    {
        public static StockItem ToEntity(this PostStockItemsRequest request)
            => new StockItem
            {
                StockItemID = request.StockItemID,
                StockItemName = request.StockItemName,
                SupplierID = request.SupplierID,
                ColorID = request.ColorID,
                UnitPackageID = request.UnitPackageID,
                OuterPackageID = request.OuterPackageID,
                Brand = request.Brand,
                Size = request.Size,
                LeadTimeDays = request.LeadTimeDays,
                QuantityPerOuter = request.QuantityPerOuter,
                IsChillerStock = request.IsChillerStock,
                Barcode = request.Barcode,
                TaxRate = request.TaxRate,
                UnitPrice = request.UnitPrice,
                RecommendedRetailPrice = request.RecommendedRetailPrice,
                TypicalWeightPerUnit = request.TypicalWeightPerUnit,
                MarketingComments = request.MarketingComments,
                InternalComments = request.InternalComments,
                CustomFields = request.CustomFields,
                Tags = request.Tags,
                SearchDetails = request.SearchDetails,
                LastEditedBy = request.LastEditedBy,
                ValidFrom = request.ValidFrom,
                ValidTo = request.ValidTo
            };
    }
#pragma warning restore CS1591
}

Code for Responses.cs file:

using System.Collections.Generic;
using System.Net;
using Microsoft.AspNetCore.Mvc;

namespace WideWorldImporters.API.Models
{
#pragma warning disable CS1591
    public interface IResponse
    {
        string Message { get; set; }

        bool DidError { get; set; }

        string ErrorMessage { get; set; }
    }

    public interface ISingleResponse<TModel> : IResponse
    {
        TModel Model { get; set; }
    }

    public interface IListResponse<TModel> : IResponse
    {
        IEnumerable<TModel> Model { get; set; }
    }

    public interface IPagedResponse<TModel> : IListResponse<TModel>
    {
        int ItemsCount { get; set; }

        double PageCount { get; }
    }

    public class Response : IResponse
    {
        public string Message { get; set; }

        public bool DidError { get; set; }

        public string ErrorMessage { get; set; }
    }

    public class SingleResponse<TModel> : ISingleResponse<TModel>
    {
        public string Message { get; set; }

        public bool DidError { get; set; }

        public string ErrorMessage { get; set; }

        public TModel Model { get; set; }
    }

    public class ListResponse<TModel> : IListResponse<TModel>
    {
        public string Message { get; set; }

        public bool DidError { get; set; }

        public string ErrorMessage { get; set; }

        public IEnumerable<TModel> Model { get; set; }
    }

    public class PagedResponse<TModel> : IPagedResponse<TModel>
    {
        public string Message { get; set; }

        public bool DidError { get; set; }

        public string ErrorMessage { get; set; }

        public IEnumerable<TModel> Model { get; set; }

        public int PageSize { get; set; }

        public int PageNumber { get; set; }

        public int ItemsCount { get; set; }

        public double PageCount
            => ItemsCount < PageSize ? 1 : (int)(((double)ItemsCount / PageSize) + 1);
    }

    public static class ResponseExtensions
    {
        public static IActionResult ToHttpResponse(this IResponse response)
        {
            var status = response.DidError ? HttpStatusCode.InternalServerError : HttpStatusCode.OK;

            return new ObjectResult(response)
            {
                StatusCode = (int)status
            };
        }

        public static IActionResult ToHttpResponse<TModel>(this ISingleResponse<TModel> response)
        {
            var status = HttpStatusCode.OK;

            if (response.DidError)
                status = HttpStatusCode.InternalServerError;
            else if (response.Model == null)
                status = HttpStatusCode.NotFound;

            return new ObjectResult(response)
            {
                StatusCode = (int)status
            };
        }

        public static IActionResult ToHttpResponse<TModel>(this IListResponse<TModel> response)
        {
            var status = HttpStatusCode.OK;

            if (response.DidError)
                status = HttpStatusCode.InternalServerError;
            else if (response.Model == null)
                status = HttpStatusCode.NoContent;

            return new ObjectResult(response)
            {
                StatusCode = (int)status
            };
        }
    }
#pragma warning restore CS1591
}

Understanding Models

ENTITIES

StockItems class is the representation for Warehouse.StockItems table.

StockItemsConfiguration class contains the mapping for StockItems class.

WideWorldImportersDbContext class is the link between database and C# code, this class handles queries and commits the changes in database and of course, another things.

EXTENSIONS

WideWorldImportersDbContextExtensions contains extension methods to provide linq queries.

IQueryableExtensions contains extension methods to allow paging in IQueryable instances.

REQUESTS

We have the following definitions:

PostStockItemsRequest
PutStockItemsRequest

PostStockItemsRequestModel represents the model to create a new stock item, contains all required properties to save in database.

PutStockItemsRequestModel represents the model to update an existing stock item, in this case contains only 4 properties: StockItemName, SupplierID, ColorID and UnitPrice. This class doesn't contain StockItemID property because id is in route for controller's action.

The models for requests do not require to contain all properties like entities, because we don't need to expose full definition in a request or response, it's a good practice to limit data using models with few properties.

Extensions class contains an extension method for PostStockItemsRequestModel, to return an instance of StockItem class from request model.

RESPONSES

These are the interfaces:

IResponse
ISingleResponse<TModel>
IListResponse<TModel>
IPagedResponse<TModel>

Each one of these interfaces has implementations, why do we need these definitions if it's more simple to return objects without wrapping them in these models? Keep in mind this Web API will provide operations for clients, with UI or without UI and it's more easy to have properties to send message, to have a model or send information if an error occurs, in addition, we set Http status code in response to describe the result from request.

These classes are generic, because in this way, we save time to define responses in future, this Web API only returns a response for a single entity, a list and a paged list.

ISingleResponse represents a response for a single entity.

IListResponse represents a response with a list, for example all shipping to existing order without paging.

IPagedResponse represents a response with pagination, for example all orders in a date range.

ResponseExtensions class contains extension methods to convert a response in a Http response, these methods return InternalServerError (500) status if an error occurs, OK (200) if it's OK and NotFound (404) if an entity does not exist in database or NoContent (204) for list responses without model.

Step 04 - Add Controller

Now, inside of Controllers directory, add a code file with name WarehouseController.cs and add this code:

using System;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Mvc;
using Microsoft.EntityFrameworkCore;
using Microsoft.Extensions.Logging;
using WideWorldImporters.API.Models;

namespace WideWorldImporters.API.Controllers
{
#pragma warning disable CS1591
    [ApiController]
    [Route("api/v1/[controller]")]
    public class WarehouseController : ControllerBase
    {
        protected readonly ILogger Logger;
        protected readonly WideWorldImportersDbContext DbContext;

        public WarehouseController(ILogger<WarehouseController> logger, WideWorldImportersDbContext dbContext)
        {
            Logger = logger;
            DbContext = dbContext;
        }
#pragma warning restore CS1591

        // GET
        // api/v1/Warehouse/StockItem

        /// <summary>
        /// Retrieves stock items
        /// </summary>
        /// <param name="pageSize">Page size</param>
        /// <param name="pageNumber">Page number</param>
        /// <param name="lastEditedBy">Last edit by (user id)</param>
        /// <param name="colorID">Color id</param>
        /// <param name="outerPackageID">Outer package id</param>
        /// <param name="supplierID">Supplier id</param>
        /// <param name="unitPackageID">Unit package id</param>
        /// <returns>A response with stock items list</returns>
        /// <response code="200">Returns the stock items list</response>
        /// <response code="500">If there was an internal server error</response>
        [HttpGet("StockItem")]
        [ProducesResponseType(200)]
        [ProducesResponseType(500)]
        public async Task<IActionResult> GetStockItemsAsync(int pageSize = 10, int pageNumber = 1, int? lastEditedBy = null, int? colorID = null, int? outerPackageID = null, int? supplierID = null, int? unitPackageID = null)
        {
            Logger?.LogDebug("'{0}' has been invoked", nameof(GetStockItemsAsync));

            var response = new PagedResponse<StockItem>();

            try
            {
                // Get the "proposed" query from repository
                var query = DbContext.GetStockItems();

                // Set paging values
                response.PageSize = pageSize;
                response.PageNumber = pageNumber;

                // Get the total rows
                response.ItemsCount = await query.CountAsync();

                // Get the specific page from database
                response.Model = await query.Paging(pageSize, pageNumber).ToListAsync();

                response.Message = string.Format("Page {0} of {1}, Total of products: {2}.", pageNumber, response.PageCount, response.ItemsCount);

                Logger?.LogInformation("The stock items have been retrieved successfully.");
            }
            catch (Exception ex)
            {
                response.DidError = true;
                response.ErrorMessage = "There was an internal error, please contact to technical support.";

                Logger?.LogCritical("There was an error on '{0}' invocation: {1}", nameof(GetStockItemsAsync), ex);
            }

            return response.ToHttpResponse();
        }

        // GET
        // api/v1/Warehouse/StockItem/5

        /// <summary>
        /// Retrieves a stock item by ID
        /// </summary>
        /// <param name="id">Stock item id</param>
        /// <returns>A response with stock item</returns>
        /// <response code="200">Returns the stock items list</response>
        /// <response code="404">If stock item is not exists</response>
        /// <response code="500">If there was an internal server error</response>
        [HttpGet("StockItem/{id}")]
        [ProducesResponseType(200)]
        [ProducesResponseType(404)]
        [ProducesResponseType(500)]
        public async Task<IActionResult> GetStockItemAsync(int id)
        {
            Logger?.LogDebug("'{0}' has been invoked", nameof(GetStockItemAsync));

            var response = new SingleResponse<StockItem>();

            try
            {
                // Get the stock item by id
                response.Model = await DbContext.GetStockItemsAsync(new StockItem(id));
            }
            catch (Exception ex)
            {
                response.DidError = true;
                response.ErrorMessage = "There was an internal error, please contact to technical support.";

                Logger?.LogCritical("There was an error on '{0}' invocation: {1}", nameof(GetStockItemAsync), ex);
            }

            return response.ToHttpResponse();
        }

        // POST
        // api/v1/Warehouse/StockItem/

        /// <summary>
        /// Creates a new stock item
        /// </summary>
        /// <param name="request">Request model</param>
        /// <returns>A response with new stock item</returns>
        /// <response code="200">Returns the stock items list</response>
        /// <response code="201">A response as creation of stock item</response>
        /// <response code="400">For bad request</response>
        /// <response code="500">If there was an internal server error</response>
        [HttpPost("StockItem")]
        [ProducesResponseType(200)]
        [ProducesResponseType(201)]
        [ProducesResponseType(400)]
        [ProducesResponseType(500)]
        public async Task<IActionResult> PostStockItemAsync([FromBody]PostStockItemsRequest request)
        {
            Logger?.LogDebug("'{0}' has been invoked", nameof(PostStockItemAsync));

            var response = new SingleResponse<StockItem>();

            try
            {
                var existingEntity = await DbContext
                    .GetStockItemsByStockItemNameAsync(new StockItem { StockItemName = request.StockItemName });

                if (existingEntity != null)
                    ModelState.AddModelError("StockItemName", "Stock item name already exists");

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

                // Create entity from request model
                var entity = request.ToEntity();

                // Add entity to repository
                DbContext.Add(entity);

                // Save entity in database
                await DbContext.SaveChangesAsync();

                // Set the entity to response model
                response.Model = entity;
            }
            catch (Exception ex)
            {
                response.DidError = true;
                response.ErrorMessage = "There was an internal error, please contact to technical support.";

                Logger?.LogCritical("There was an error on '{0}' invocation: {1}", nameof(PostStockItemAsync), ex);
            }

            return response.ToHttpResponse();
        }

        // PUT
        // api/v1/Warehouse/StockItem/5

        /// <summary>
        /// Updates an existing stock item
        /// </summary>
        /// <param name="id">Stock item ID</param>
        /// <param name="request">Request model</param>
        /// <returns>A response as update stock item result</returns>
        /// <response code="200">If stock item was updated successfully</response>
        /// <response code="400">For bad request</response>
        /// <response code="500">If there was an internal server error</response>
        [HttpPut("StockItem/{id}")]
        [ProducesResponseType(200)]
        [ProducesResponseType(400)]
        [ProducesResponseType(500)]
        public async Task<IActionResult> PutStockItemAsync(int id, [FromBody]PutStockItemsRequest request)
        {
            Logger?.LogDebug("'{0}' has been invoked", nameof(PutStockItemAsync));

            var response = new Response();

            try
            {
                // Get stock item by id
                var entity = await DbContext.GetStockItemsAsync(new StockItem(id));

                // Validate if entity exists
                if (entity == null)
                    return NotFound();

                // Set changes to entity
                entity.StockItemName = request.StockItemName;
                entity.SupplierID = request.SupplierID;
                entity.ColorID = request.ColorID;
                entity.UnitPrice = request.UnitPrice;

                // Update entity in repository
                DbContext.Update(entity);

                // Save entity in database
                await DbContext.SaveChangesAsync();
            }
            catch (Exception ex)
            {
                response.DidError = true;
                response.ErrorMessage = "There was an internal error, please contact to technical support.";

                Logger?.LogCritical("There was an error on '{0}' invocation: {1}", nameof(PutStockItemAsync), ex);
            }

            return response.ToHttpResponse();
        }

        // DELETE
        // api/v1/Warehouse/StockItem/5

        /// <summary>
        /// Deletes an existing stock item
        /// </summary>
        /// <param name="id">Stock item ID</param>
        /// <returns>A response as delete stock item result</returns>
        /// <response code="200">If stock item was deleted successfully</response>
        /// <response code="500">If there was an internal server error</response>
        [HttpDelete("StockItem/{id}")]
        [ProducesResponseType(200)]
        [ProducesResponseType(500)]
        public async Task<IActionResult> DeleteStockItemAsync(int id)
        {
            Logger?.LogDebug("'{0}' has been invoked", nameof(DeleteStockItemAsync));

            var response = new Response();

            try
            {
                // Get stock item by id
                var entity = await DbContext.GetStockItemsAsync(new StockItem(id));

                // Validate if entity exists
                if (entity == null)
                    return NotFound();

                // Remove entity from repository
                DbContext.Remove(entity);

                // Delete entity in database
                await DbContext.SaveChangesAsync();
            }
            catch (Exception ex)
            {
                response.DidError = true;
                response.ErrorMessage = "There was an internal error, please contact to technical support.";

                Logger?.LogCritical("There was an error on '{0}' invocation: {1}", nameof(DeleteStockItemAsync), ex);
            }

            return response.ToHttpResponse();
        }
    }
}

The process for all controller's actions is:

Log the invocation for method.
Create the instance for response according to action (Paged, list or single).
Perform access to database through DbContext instance.
If invocation for DbContext extension method fails, set DidError property as true and set ErrorMessage property with: There was an internal error, please contact to technical support., because it isn't recommended to expose error details in response, it's better to save all exception details in log file.
Return result as Http response.

Keep in mind all names for methods that end with Async sufix because all operations are async but in Http attributes, we don't use this suffix.

Step 05 - Setting Up Dependency Injection

ASP.NET Core enables dependency injection in native way, this means we don't need any 3rd party framework to inject dependencies in controllers.

This is a great challenge because we need to change our mind from Web Forms and ASP.NET MVC, for those technologies use a framework to inject dependencies it was a luxury, now in ASP.NET Core dependency injection is a basic aspect.

Project template for ASP.NET Core has a class with name Startup, in this class we must to add the configuration to inject instances for DbContext, Repositories, Loggers, etc.

Modify the code of Startup.cs file to look like this:

using System;
using System.IO;
using System.Reflection;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Mvc;
using Microsoft.EntityFrameworkCore;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Logging;
using Swashbuckle.AspNetCore.Swagger;
using WideWorldImporters.API.Controllers;
using WideWorldImporters.API.Models;

namespace WideWorldImporters.API
{
#pragma warning disable CS1591
    public class Startup
    {
        public Startup(IConfiguration configuration)
        {
            Configuration = configuration;
        }

        public IConfiguration Configuration { get; }

        // This method gets called by the runtime. Use this method to add services to the container.
        public void ConfigureServices(IServiceCollection services)
        {
            services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_1);

            // Add configuration for DbContext
            // Use connection string from appsettings.json file
            services.AddDbContext<WideWorldImportersDbContext>(options =>
            {
                options.UseSqlServer(Configuration["AppSettings:ConnectionString"]);
            });

            // Set up dependency injection for controller's logger
            services.AddScoped<ILogger, Logger<WarehouseController>>();

            // Register the Swagger generator, defining 1 or more Swagger documents
            services.AddSwaggerGen(options =>
            {
                options.SwaggerDoc("v1", new Info { Title = "WideWorldImporters API", Version = "v1" });

                // Get xml comments path
                var xmlFile = $"{Assembly.GetExecutingAssembly().GetName().Name}.xml";
                var xmlPath = Path.Combine(AppContext.BaseDirectory, xmlFile);

                // Set xml path
                options.IncludeXmlComments(xmlPath);
            });
        }

        // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
        public void Configure(IApplicationBuilder app, IHostingEnvironment env)
        {
            if (env.IsDevelopment())
                app.UseDeveloperExceptionPage();

            // Enable middleware to serve generated Swagger as a JSON endpoint.
            app.UseSwagger();

            // Enable middleware to serve swagger-ui (HTML, JS, CSS, etc.), specifying the Swagger JSON endpoint.
            app.UseSwaggerUI(c =>
            {
                c.SwaggerEndpoint("/swagger/v1/swagger.json", "WideWorldImporters API V1");
            });

            app.UseMvc();
        }
    }
#pragma warning restore CS1591
}

The ConfigureServices method specifies how dependencies will be resolved, in this method. We need to set up DbContext, Repositories and Logging.

The Configure method adds the configuration for Http request runtime.

Step 06 - Running Web API

Before you run Web API project, add the connection string in appsettings.json file:

{
  "Logging": {
    "LogLevel": {
      "Default": "Warning"
    }
  },
  "AllowedHosts": "*",
  "AppSettings": {
    "ConnectionString": "server=(local);database=WideWorldImporters;integrated security=yes;"
  }
}

In order to show descriptions in help page, enable XML documentation for your Web API project:

1. Right click on Project > Properties
2. Go to Build > Output
3. Enable XML documentation file
4. Save changes

Now, press F5 to start debugging for Web API project, if everything it's OK, we'll get the following output in the browser: