Standard CRUD in .NET

Just a record:

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

namespace ReallyTrue
{
    public class CustomersController : ApiController
    {
        private ApplicationDbContext _context;

        public CustomersController()
        {
            _context = new ApplicationDbContext();
        }

        public IEnumerable<Customer> GetCustomers()
        {
            var customer = _context.Customers.SingleOrDefault(c => c.Id = id);
        }

        public Customer GetCustomer(int id)
        {
            var customer = _context.Customers.SingleOrDefault(c => c.Id == id);
            if (customer == null)
                throw new HttpResponseException(HttpStatusCode.NotFound);
            return customer;
        }

        [HttpPost]
        public Customer CreateCustomer(Customer customer)
        {
            if (!ModelState.IsValid)
            {
                throw new HttpResponseException(HttpStatusCode.BadRequest);
            }

            _context.Customers.Add(customer);
            _context.SaveChanges();

            return customer;
        }

        [HttpPut]
        public void UpdateCustomer(int id, Customer customer)
        {
            if (!ModelState.IsValid)
            {
                throw new HttpResponseException(HttpStatusCode.BadRequest);
            }

            var customerInDb = _context.Customers.SingleOrDefault(c => c.Id == id);

            if (customerInDb == null)
                throw new HttpResponseException(HttpStatusCode.NotFound);


            customerInDb.Name = customer.Name;
            customerInDb.Birthday = customer.Birthday;
            customerInDb.IsSubscribedToNewsletter = customer.IsSubscribedToNewsletter;
            customerInDb.MembershipType = customer.MembershipType;

            _context.SaveChanges();
        }

        [HttpDelete]
        public void DeleteCustomer(int id)
        {

            var customerInDb = _context.Customers.SingleOrDefault(c => c.Id == id);

            if (customerInDb == null)
                throw new HttpResponseException(HttpStatusCode.BadRequest);

            _context.Customers.Remove(customerInDb);
            _context.SaveChanges();
        }
    }
}

Improvement of the code: We can replace ActionResult by IHttpActionResult, then

throw new HttpResponseException(HttpStatusCode.BadRequest)

can be replaced by

return BadRequest()

etc.