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Now that we have our comic books repository, let’s update our controller to use it.
Follow Along
To follow along commiting your changes to this course, you'll need to fork the aspnet-comic-book-gallery repo. Then you can clone, commit, and push your changes to your fork like this:
git clone <your-fork>
cd aspnet-comic-book-gallery
git checkout tags/v4.7 -b updating-the-controller
Keyboard Shortcuts
-
CTRL+COMMA- Opens the “Navigate To” dialog -
CTRL+PERIOD- Opens the quick actions popup
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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
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is what we want to do.
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So we can just press Enter
to perform this action.
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Then, we can continue
with adding our field.
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Let's name the field _comicBookRepository.
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Notice that I'm starting the field
name with an underscore and
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lower case letter, which is a common
naming convention for private fields.
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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.
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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
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with a call to the repository's
get comic book method.
2:36
Be sure to pass the ID
argument into the method call.
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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.
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I promise we'll fix that soon.
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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
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parameter ID of non noble
type system.in32 for
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method system web
NVC Action Results detail in 32.
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In comic book gallery, controllers,
comic books controller.
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An optional parameter must be
a reference type, a null-able type.
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Or be declared as an optional parameter.
3:29
Let's break this down.
3:32
We can see here and here that this
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error message refers to our comic
books controller detail action method.
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Remember how we added an ID parameter
to our detail action method.
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Well, this error message is telling us
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that the request didn't include
a value for the ID parameter.
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So in Mvc was unable to
call our action method.
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Let's take another look
at how Mvc URL routing
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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
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named ComicBooks and
an action method named Detail.
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As it turns out, the actual pattern
that MVC uses is controller/action/id.
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The third part of that pattern,
the ID segment is optional.
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This means that MVC can successfully
match the pattern against a URL,
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even if the path doesn't
include the ID segment.
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Unfortunately, MVC is getting confused
about what to do with our request for
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/ComicBooks/Detail.
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The problem is that even though
the ID parameter should be optional,
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our detail action method only accept
an integer which isn't nullable.
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So, how do we resolve this issue?
5:09
It turns out that C# provides
us with an easy solution so
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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
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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
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by checking to see if
the ID argument is null.
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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
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to return an http not found result object.
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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
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whichever one your development team
has decided is their preference and
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do that consistently
throughout the project.
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Restart the website, so that we can
retest the /comicbooks/detail route.
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Don't forget to manually browse
to the correct route, great.
6:43
I've never been so
happy to see a 404 error.
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Let's change our URL, so that it
includes the optional ID parameter.
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Bomber we just encountered an unhandled
exception in our detailView
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Click OK to dismiss the exception dialog.
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Hm, I wonder what we did wrong.
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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
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