Question

I refactored an attribute, which implements Basic Http Authentication in the Web api, to have DI as follows:

 public class BasicHttpAuthAttribute : ActionFilterAttribute
    {
        private readonly ILoginManager _manager;

        public BasicHttpAuthAttribute(ILoginManager manager)
        {
            this._manager = manager;
        }
        public override void OnActionExecuting(HttpActionContext actionContext)
        {
            if (actionContext.Request.Headers.Authorization == null)
            {
                actionContext.Response = actionContext.Request.CreateResponse(HttpStatusCode.Unauthorized);
                actionContext.Response.Content = new StringContent("Missing Auth-Token");
            }
            else
            {

                var authToken = actionContext.Request.Headers.Authorization.Parameter;
                var decodedToken = Encoding.UTF8.GetString(Convert.FromBase64String(authToken));

                string userName = decodedToken.Substring(0, decodedToken.IndexOf(":"));
                string password = decodedToken.Substring(decodedToken.IndexOf(":") + 1);


                UserInfo user;


                if (_manager.LoginPasswordMatch(userName, password, out user))
                {
                    var apiUser = new ApiUser(user.UserID);
                    HttpContext.Current.User = new GenericPrincipal(new ApiIdentity(apiUser), new string[]{});

                    base.OnActionExecuting(actionContext);

                }
                else
                {
                    actionContext.Response = actionContext.Request.CreateResponse(HttpStatusCode.Unauthorized);
                    actionContext.Response.Content = new StringContent("Invalid username or password");
                }



            }
        }
    }

Before the refactoring, I was creating an instance of the LoginManager (which itself did not have DI so I could create an instance using a ctor) inside OnActionExecuting. The problem after refactoring is that the build fails because when I apply the filter to a WebApi method, it is expecting an argument there.

How do I implement DI in this case as the LoginManager itself takes a ILoginRepository in its constructor? Is it even possible?

Answer 1

It’s not easy to do because the way attributes are treated in Web API is a bit different than what you might expect. First of all they are created at various point in time, and secondly they are cached.

With that said, rather than injection through a constructor, the easiest way is to achieve DI is to call the DI framework of your choice at processing time, and retrieving the dependency then, i.e. inside your onActionExecuting:

var s = GlobalConfiguration.Configuration.DependencyResolver.GetService(typeof(IService));