UserServiceTest.cs

using System;
using Moq;
using Xunit;

namespace UnitTests
{
	public class UserServiceTest
	{
		[Fact]
		public void Should_GivenValidCreateUserRequest_CallUserServiceWithMappedData()
		{
			var addUserRequest = new AddUserRequest{ Username = "foo", Email = "foo@bar.com"};
			var userService = new Mock<UserService>();  

			var sut = new UserController(userService.Object);
			sut.AddUser(addUserRequest);
			userService.Verify(mock => mock.Create("foo", "foo@bar.com"), Times.Once);
		}
	}

	public class UserController
	{
		private UserService _userService;

		public UserController(UserService userService)
		{
			_userService = userService;
		}
		
		public AddUserResponse AddUser(AddUserRequest addUserRequest){
			var created = _userService.Create(addUserRequest.Username, addUserRequest.Email);

			if(created){
				return AddUserResponse.OK();
			}else{
				return AddUserResponse.Fail();
			}
		}
	}
}