1. 创建一个接口：

public interface IUserService {
    User Get(int id);
    IEnumerable GetAll();
}

2. 创建一个实现IUserService接口的类：

public class UserService : IUserService {
    public User Get(int id) {
        // 从数据库中获取用户
        // ...
    }

    public IEnumerable GetAll() {
        // 从数据库中获取所有用户
        // ...
    }
}

3. 在ApiController中注入IUserService：

public class UserController : ApiController {
    private readonly IUserService _userService;
    
    public UserController(IUserService userService) {
        _userService = userService;
    }

    public IHttpActionResult Get(int id) {
        var user = _userService.Get(id);
        if (user == null) {
            return NotFound();
        }
        return Ok(user);
    }

    public IHttpActionResult GetAll() {
        var users = _userService.GetAll();
        if (users.Any()) {
            return Ok(users);
        }
        return NotFound();
    }
}

4. 创建一个MockUserService，实现IUserService接口：

public class MockUserService : IUserService {
    public User Get(int id) {
        // 从内存中获取用户
        // ...
    }

    public IEnumerable GetAll() {
        // 从内存中获取用户
        // ...
    }
}

5. 在Nunit测试中使用MockUserService实例进行测试：

[TestFixture]
public class UserControllerTests {
    [Test]
    public void Get_Should_Return_User() {
        // Arrange
        var userService = new MockUserService();
        userService.Add(new User { Id = 1, Name = "张三" });
        var controller = new UserController(userService);

        // Act
        IHttpActionResult result = controller.Get(1);

        // Assert
        Assert.IsNotNull(result);
        var content = result as OkNegotiatedContentResult;
        Assert.IsNotNull(content);
        Assert.AreEqual("张三", content.Content.Name);
    }

    [Test]
    public void GetAll_Should_Return_All_Users() {
        // Arrange
        var userService = new MockUserService();
        userService.Add(new User { Id = 1, Name = "张三" });