ASP.Net

It is straight forward to create a simple API service by creating a class that extends the 'ApiController' class.

using System;

namespace Products.Models
{
    public class Product
    {
        public int     Id       { get; set; }
        public string  Name     { get; set; }
        public string  Category { get; set; }
        public decimal Price    { get; set; }
    }
}

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

namespace Products.Controllers
{
    public class ProductsController : ApiController
    {
        Product[] products = new Product[]
        {
            new Product { Id = 1, Name = "Tomato Soup", Category = "Groceries", Price = 1      },
            new Product { Id = 2, Name = "Yo-yo",       Category = "Toys",      Price = 3.75M  },
            new Product { Id = 3, Name = "Hammer",      Category = "Hardware",  Price = 16.99M }
        };

        public IEnumerable GetAllProducts()
        {
            return products;
        }

        public IHttpActionResult GetProduct(int id)
        {
            var product = products.FirstOrDefault((p) => p.Id == id);
            if (product == null)
            {
                return NotFound();
            }
            return Ok(product);
        }
    }
}

When run, the following is available at the endpoint:

http://localhost/api/products

[{
    "Id": 1,
    "Name": "Tomato Soup",
    "Category": "Groceries",
    "Price": 1.0
}, {
    "Id": 2,
    "Name": "Yo-yo",
    "Category": "Toys",
    "Price": 3.75
}, {
    "Id": 3,
    "Name": "Hammer",
    "Category": "Hardware",
    "Price": 16.99
}]

References

Get Started with ASP.NET Web API 2 (C#)
https://docs.microsoft.com/en-au/aspnet/web-api/overview/getting-started-with-aspnet-web-api/tutorial-your-first-web-api