This tutorial shows how to call a web API from a .NET application, using System.Net.Http.HttpClient. The client-program can be found on Github, as well as the server-side code.

using System;
using System.Net;
using System.Net.Http;
using System.Net.Http.Headers;
using System.Threading.Tasks;
using Newtonsoft.Json;

namespace HttpClientSample
{
    /// <summary>
    /// HttpClientExtensions for Dotnet Core
    /// see: https://stackoverflow.com/questions/40027299/where-is-postasjsonasync-method-in-asp-net-core
    /// </summary>
    public static class HttpClientExtensions
    {
        public static Task<HttpResponseMessage> PostAsJsonAsync<T>(this HttpClient httpClient, string url, T data)
        {
            var dataAsString = JsonConvert.SerializeObject(data);
            var content = new StringContent(dataAsString);
            content.Headers.ContentType = new MediaTypeHeaderValue("application/json");

            return httpClient.PostAsync(url, content);
        }

        public static Task<HttpResponseMessage> PutAsJsonAsync<T>(this HttpClient httpClient, string url, T data)
        {
            var dataAsString = JsonConvert.SerializeObject(data);
            var content = new StringContent(dataAsString);
            content.Headers.ContentType = new MediaTypeHeaderValue("application/json");

            return httpClient.PutAsync(url, content);
        }

        public static async Task<T> ReadAsJsonAsync<T>(this HttpContent content)
        {
            var dataAsString = await content.ReadAsStringAsync().ConfigureAwait(false);

            return JsonConvert.DeserializeObject<T>(dataAsString);
        }
    }


    public class Product
    {
        public string Id { get; set; }
        public string Name { get; set; }
        public decimal Price { get; set; }
        public string Category { get; set; }
    }


    internal static class Program
    {
        private static readonly HttpClient _client = new HttpClient();

        private static void ShowProduct(Product product)
        {
            Console.WriteLine($"Name: {product.Name}\tPrice: {product.Price}\tCategory: {product.Category}");
        }

        private static async Task<Uri> CreateProductAsync(Product product)
        {
            var response = await _client.PostAsJsonAsync("api/products", product);
            response.EnsureSuccessStatusCode();

            // return URI of the created resource.
            return response.Headers.Location;
        }

        private static async Task<Product> GetProductAsync(string path)
        {
            Product product = null;
            var response = await _client.GetAsync(path);
            if (response.IsSuccessStatusCode)
            {
                product = await response.Content.ReadAsJsonAsync<Product>();
            }

            return product;
        }

        private static async Task<Product> UpdateProductAsync(Product product)
        {
            var response = await _client.PutAsJsonAsync($"api/products/{product.Id}", product);
            response.EnsureSuccessStatusCode();

            // Deserialize the updated product from the response body.
            product = await response.Content.ReadAsJsonAsync<Product>();

            return product;
        }

        private static async Task<HttpStatusCode> DeleteProductAsync(string id)
        {
            var response = await _client.DeleteAsync($"api/products/{id}");

            return response.StatusCode;
        }

        private static void Main()
        {
            RunAsync().GetAwaiter().GetResult();
        }

        private static async Task RunAsync()
        {
            // Update port # in the following line.
            _client.BaseAddress = new Uri("http://localhost:64195/");
            _client.DefaultRequestHeaders.Accept.Clear();
            _client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));

            try
            {
                // Create a new product
                var product = new Product
                {
                    Name = "Gizmo",
                    Price = 100,
                    Category = "Widgets"
                };

                var url = await CreateProductAsync(product);
                Console.WriteLine($"Created at {url}");

                // Get the product
                product = await GetProductAsync(url.PathAndQuery);
                ShowProduct(product);

                // Update the product
                Console.WriteLine("Updating price...");
                product.Price = 80;
                await UpdateProductAsync(product);

                // Get the updated product
                product = await GetProductAsync(url.PathAndQuery);
                ShowProduct(product);

                // Delete the product
                var statusCode = await DeleteProductAsync(product.Id);
                Console.WriteLine($"Deleted (HTTP Status = {(int) statusCode})");
            }
            catch (Exception e)
            {
                Console.WriteLine(e.Message);
            }

            Console.ReadLine();
        }
    }
}