how to create an api with microsoft dot net core



This article is part of a series which thus far has been on Angular development.

This is the first article on .Net Core development.  

Since APIs are most often the way data is delivered to an Angular App, and using an Angular based or mock API quickly becomes more trouble than it is worth, we are now at the point where we have to start implementing a proper API. I am going to use Microsoft Asp.Net Core to develop this. Most of the time there would be a database behind the API, however in this case we won't have one.

Create the project using Visual Studio 2019

To create the project follow these steps :

  • Open Visual Studio 2019
  • File - New Project
  • Choose "Asp.Net Core Web Application"
  • Give the project a name
  • Choose "API template". I am using DotNetCore 3.1

The API now exists. Run the project (F5) and it should go to the demo endpoint and you will see this:

After the "https://localhost" in the browser you will see the port number allocated by Visual Studio. In my case the port was 44394. It will be different in your case, and for each project you create. This is the port number you will put in your Angular "environments.ts" file when we hook the API content back to the Angular client.

Create the Data Class

We will now create a class to define the structure of the entities returned by the API. To do this add a file at the project root called "Financials.cs" and paste in the following code, but change the namespace to match your project name:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;


    public class Country
        public int CountryId { get; set; }
        public string CountryName { get; set; }

    public class Stock

        public int StockId { get; set; }
        public string StockName { get; set; }
        public int CountryId { get; set; }

    public class Fund

        public int FundId { get; set; }
        public string FundName { get; set; }
        public int CountryId { get; set; }

    public class CountryList

        public List Countries { get; set; }
        public CountryList()
            Countries = new List();
            Countries.Add(new Country() { CountryId = 1, CountryName = "England" });
            Countries.Add(new Country() { CountryId = 2, CountryName = "France" });
            Countries.Add(new Country() { CountryId = 3, CountryName = "Germany" });
            Countries.Add(new Country() { CountryId = 4, CountryName = "Spain" });
            Countries.Add(new Country() { CountryId = 5, CountryName = "Wales" });

Re-purpose the existing Controller

The API now exists but it has a demo endpoint called WeatherController which we don't want. We will now re-purpose this for our own requirements.

In the Controllers folder right mouse click the "WeatherForecastController.cs" and rename it to "CountryController.cs". This should also rename the class name in "CountryController.cs". Check that it has and if not fix it. 

Delete the "Summaries" object at the top of the file and the code that creates it. Paste the following to replace the existing Get Method in "CountryController.cs":

        public IEnumerable Get()
            var countryList = new CountryList();
            return countryList.Countries;

Run the project and go "https:\\localhost:\country". You should see the following:

Delete the "WeatherForecast.cs" file.

Create the StockController and FundController

We will create a Stock Controller for our API. In the constructor we will use a loop create as many Stock records per Country as we need. To start with 20 records per country is enough. Later on we can change it to see how long it takes using 200, 2000 or 20000 records per country etc. 

Copy "CountryController.cs" and paste into the same folder. Rename the new file "StockController.cs". Now edit the "StockController.cs" and change the name of the class to "StockController". Paste in the following code to replace the constructor and the get method:

        List stockList = new List();
        public StockController()
            var countryList = new CountryList();
            var countries = countryList.Countries;
            stockList = new List();
            for(int i=1; i < 6; i++)
                Country currentCountry = countries.Where(x => x.CountryId == i).First();
                for(int j=1; j<21; j++)
                    stockList.Add(new Stock
                        CountryId = i,
                        StockId = i * j,
                        StockName = "Stock " + i * j + " - " + currentCountry.CountryName


        public IEnumerable Get(int countryId)
            return stockList.Where(x => x.CountryId == countryId);

Run the project, using the url below (with your port) and you should see the following output for the stock endpoint:

Do the same for FundController (and substitute "fund" for "stock" throughout the class). Run the project and test the Fund endpoint.

The Git Repo

We now have a working API. If I have lost you along the way you can access the Github repo here.

Next - add CORS

We can't connect our Angular App to this API yet as modern browsers won't call an API from a different URL, without that API expressly giving permission. CORS is a way of declaring which clients can access which endpoint in an API.

Therefore, the next step is to add CORS to our Dot Net Core Web API.

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