Now that we have our comic books repository, 0:00 we can update our controller to use it. 0:03 Let's open the comic books controller class. 0:05 We could use the solution explore to find the file that we're looking for, but 0:08 let's look at another option. 0:12 Pressing control comma will open Visual Studios, navigate to dialog. 0:14 Now we can start typing the class, method, or property name that we're looking for. 0:19 So in this case, I'll type comic. 0:24 Visual Studio will search across the project and show the matching items. 0:28 I'll use the down arrow key to select the fifth item in the list, 0:32 the comic books controller class, and then press Enter to open the file. 0:36 The navigate to dialogue is a great way to open files 0:41 without having to take your hands off the keyboard. 0:45 Let's add a private field for our repository. 0:48 I'll start by typing private ComicBookRepository. 0:52 Notice that Visual Studio is already letting us know 1:00 that something is wrong with our type name. 1:03 The red squiggle is a dead giveaway. 1:05 If we leave the cursor right at the end of ComicBookRepository, 1:08 we can press control period to open the list of suggested quick actions, 1:12 instead of having to use the mouse to click on the light bulb icon. 1:17 The first item in the list to add the missing using directive 1:21 is what we want to do. 1:25 So we can just press Enter to perform this action. 1:26 Then, we can continue with adding our field. 1:30 Let's name the field _comicBookRepository. 1:33 Notice that I'm starting the field name with an underscore and 1:39 lower case letter, which is a common naming convention for private fields. 1:43 Then set the field value to null. 1:47 I'll collapse the solution explorer panel to give us more room. 1:50 Now, we need an instance of our repository. 1:54 Let's add a constructor and instantiate a repository within it. 1:57 Remember, that constructors are special methods 2:03 that are called when an instance of our class is being instantiated. 2:06 We use constructors to initialize instance members, 2:10 which is exactly what we're doing with our repository field. 2:13 Constructors can be identified by their lack of a return type. 2:17 And their name, which matches the class name. 2:21 Next, let's update the detail method. 2:24 To start, add an ID parameter of type int. 2:28 Then, let's replace the inline instantiation of the comic book model 2:32 with a call to the repository's get comic book method. 2:36 Be sure to pass the ID argument into the method call. 2:41 Ready to preview our changes? 2:44 Save the controller and run the website. 2:46 I don't know about you, but I'm getting tired of seeing our home page 404. 2:51 I promise we'll fix that soon. 2:55 But for now, go ahead and browse. 2:58 The /comic_books/detail and we got an error. 3:00 The parameters dictionary contains a no entry for 3:06 parameter ID of non noble type system.in32 for 3:09 method system web NVC Action Results detail in 32. 3:14 In comic book gallery, controllers, comic books controller. 3:21 An optional parameter must be a reference type, a null-able type. 3:25 Or be declared as an optional parameter. 3:29 Let's break this down. 3:32 We can see here and here that this 3:33 error message refers to our comic books controller detail action method. 3:36 Remember how we added an ID parameter to our detail action method. 3:41 Well, this error message is telling us 3:45 that the request didn't include a value for the ID parameter. 3:48 So in Mvc was unable to call our action method. 3:52 Let's take another look at how Mvc URL routing 3:57 associates URL's with controller action methods. 4:00 In a previous video, we discussed that MVC uses simple 4:04 pattern matching against the URL path to determine the name of the Controller and 4:08 Action should be used in the form of controller/action. 4:13 So, the URL path ComicBooks/Detail would map to a controller 4:18 named ComicBooks and an action method named Detail. 4:24 As it turns out, the actual pattern that MVC uses is controller/action/id. 4:29 The third part of that pattern, the ID segment is optional. 4:37 This means that MVC can successfully match the pattern against a URL, 4:41 even if the path doesn't include the ID segment. 4:47 Unfortunately, MVC is getting confused about what to do with our request for 4:50 /ComicBooks/Detail. 4:55 The problem is that even though the ID parameter should be optional, 4:58 our detail action method only accept an integer which isn't nullable. 5:03 So, how do we resolve this issue? 5:09 It turns out that C# provides us with an easy solution so 5:11 that we can make the necessary changes to our detail action method. 5:15 If we update our parameter type to be nullable by placing a question mark 5:20 after the keyword int, then MVC can successfully pass null for 5:24 the ID parameter if an ID value isn't provided as part of the request. 5:29 Nullable types will be covered in more detail in future courses. 5:34 Now, we need to do a bit of defensive coding 5:39 by checking to see if the ID argument is null. 5:42 And if so, we turn a not found error to the client. 5:45 We can take advantage of the base controllers http not found method 5:50 to return an http not found result object. 5:54 Lastly, notice that visual studio is unhappy about passing 6:00 the ideal argument to the repositories GetComicBook method. 6:04 Argument one cannot convert from question mark to int. 6:08 When using a nullable type, 6:14 you need to use the value property to get at the underlying value. 6:16 You could also use an explicit cast. 6:21 Use whichever one feels the most readable or natural to you or 6:24 whichever one your development team has decided is their preference and 6:29 do that consistently throughout the project. 6:33 Restart the website, so that we can retest the /comicbooks/detail route. 6:36 Don't forget to manually browse to the correct route, great. 6:43 I've never been so happy to see a 404 error. 6:47 Let's change our URL, so that it includes the optional ID parameter. 6:51 Bomber we just encountered an unhandled exception in our detailView 6:56 Click OK to dismiss the exception dialog. 7:02 Hm, I wonder what we did wrong. 7:06 In the next video, we'll walk through how to use the Visual Studio debugger 7:08 to troubleshoot this error. 7:12 See you then. 7:14