Example controller and its tests

ValuesController.cs

using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Mvc;

namespace WebApp.Controllers
{
    [Route("api/[controller]")]
    public class ValuesController : Controller
    {
        // GET api/values
        [HttpGet]
        public async Task<IEnumerable<string>> Get()
        {
            return await Task
                .FromResult(new string[] { "value1", "value2" }
                .AsEnumerable());
        }

        // GET api/values/5
        [HttpGet("{id}")]
        public async Task<string> Get(int id)
        {
            return await Task.FromResult("value");
        }
    }
}

ValuesController.Spec.cs

using System.Collections.Generic;
using Xunit;

namespace WebApp.Controllers
{
    public class ValuesControllerSpec : IClassFixture<ValuesController>
    {
        private readonly ValuesController controller;

        public ValuesControllerSpec(ValuesController controller)
        {
            this.controller = controller;
        }

        [Fact]
        public async void Get_action_should_return_an_array_of_strings()
        {
            var result = await controller.Get();
            Assert.IsAssignableFrom<IEnumerable<string>>(result);
        }

        [Fact]
        public async void Get_by_id_action_should_return_a_value()
        {
            var result = await controller.Get(123);
            Assert.Equal("value", result);
        }
    }
}