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Now, that we have our data
concurrency control implemented,

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we could update our action
method to catch and

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handle the DB update concurrency
Exception that EF is throwing.

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That will allow us to display
a friendly message to the user.

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To do that, we need to wrap these three
lines of code in a try catch statement.

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To quickly add a try catch statement,
you can type try and

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press Tab twice and cut and
paste our code into the try block.

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Then instead of catching any exception,

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let's only catch
the DbUpdateCocurrencyException.

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That way any exception other than
a DBUpdateConcurrencyException

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won't get caught by our cat statement.

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Instead of our cat statement, we can now
add a model error to the model state

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which would display as a validation
message to the end user.

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The comic book being updated has

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already been updated by another user.

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Now, let's test our changes.

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Open the Edit Comment Book page into
two tabs for Bone number three.

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Save the first Tab,
then save the second Tab.

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And we see our error message
instead of a server error.

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That's definitely an improvement
over the previous behavior.

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Our current solution doesn't give the user
any option other than to cancel their edit

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and return to the Comic Book detail page.

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This type of data concurrency
conflict resolution is known as

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server wins because the client's
changes are discarded.

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We can also give the user the option
to resolve the conflict in the client

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wins manner, which will override the
server's values with the client's values.

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Giving the use of the option to force
their changes, may not always make sense.

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So you'll need to consider
your specific situation and

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determine which approach
is the most correct.

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Another improvement that we can make
is to let the user know when the entity

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they're trying to update has
been deleted by another user.

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Let's start with adding the code to detect
if the entity was deleted or just updated.

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To do this we need a reference to
the DbUpdateConcurrencyException.

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We'll be varying the model error
message depending on whether

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the entity was deleted or updated.

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So let's declare a variable for
the message.

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We can use
the DbUpdateConcurrencyException object to

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check if the entity has been deleted, or

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just update it since
it was last retrieved.

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The exception objects entries
property is a collection of DB entity

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entry objects representing the entities
that couldn't be saved to the database.

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Since we're updating a single entity,
we can simply call the single

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method on the entry's property
to get our entity's entry.

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Then we can use the entry's get database
values method to query from the database

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the entity's current values.

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If the entity isn't found in the database

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then the get database values
method will return null.

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So we can use the nullness of
the entity property values variable to

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determine what message
to display to the user.

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Now we can assign a value to the message
variable for each of our use cases.

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First, the deleted use case.

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The comic book being
updated has been deleted by

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another user, and
then the updated use case,

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The comic book being updated has
already been updated by another user.

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And while we're at it,

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let's update the call to the add model
air method to use the message variable.

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Okay, so we've updated our solution to let
the user know when the entity has been

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deleted or updated.

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But we still don't support allowing
the user to choose the option

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to resolve the conflict
in a ClientWins manner.

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Implementing that feature is
really pretty straight forward.

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We just need to update the comic
book's tracking property

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with the current value from the database.

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That way if the user chooses
to submit the form again,

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the tracking property value should
match the record's current road version

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value in the database,
which will allow the update to succeed.

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Remember we already retrieved the entity's
current values from the database by

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calling the get database values method
on the entity's associated entry object.

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The entity property values variable
is a dictionary of property values.

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So we can simply index into
the dictionary using the property name we

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wanna value for.

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Indexing into the dictionary
returns an object, which can't

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be implicitly converted to the row version
properties data type of byte array.

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We can fix this by
adding an explicit cast.

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While this approach will work,
its reliance on a string for

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the property name is
a bit of a code smell.

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Luckily, the entity property values,
DBPropertyValues object,

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gives us a work around We can call
the ToObject method to get an instance of

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the underlying type, populated with
the values from the dictionary.

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The ToObject method returns an object, so

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we need to cast that
to the ComicBook type.

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But then we can just access
the RowVersion property.

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Now that we've given the user the option
to choose the client wins option,

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let's update the error message to
let them know that they can do that.

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If you still want to make your changes,

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then click the Save button again.

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Otherwise click the Cancel button

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to discard your changes.

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Let's make one more change
before we test our changes.

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Currently when the Cancel
button is clicked,

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we send the user to the comic book detail
page, but if the entity has been deleted

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redirecting the user to that page
will result in a 404 not found error.

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To resolve this issue we can set a flag on
the view model to indicate that the entity

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has been deleted.

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We can use that flag in the view to
determine the correct URL with the cancel

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button.

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For the flag,

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let's add a boolean property to view
model named ComicBookHasBeenDeleted.

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Then we can set the property
to true when we detect

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that the entity has been deleted.

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00:07:01,870 --> 00:07:06,333
Now we can update the cancel button to use
our view model flag to conditionally set

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the anchored elements href attribute.

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If the Comic Book has been deleted,
let's set the URL to the list page.

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Otherwise, we'll set it to the detail
page, which was the existing behavior.

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And finally,
let's update the error message so

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that the user will know what will happen
when they click the cancel button.

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Click the Cancel button to
return to the list page.

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00:07:46,884 --> 00:07:49,032
And now, lets test our changes.

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I'll start by adding a new Comic Book,

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so we can test the deleted
concurrency use case.

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I'll select Bone for the series and
set the issue number to 4.

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Jeff Smith for the artist,
and Script for the role.

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And click the Save button.

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Let's test the updated use case.

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Open the Edit Comic Book
page into another tab.

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Save the first tab.

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Then attempt to save the second tab.

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Okay, here's our new error message.

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Hovering over the Cancel button show us
that we'd browse back to the comic book

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detail page if we click the button.

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Let's try clicking the Save button.

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Great, we were able to save by
choosing the client wins option.

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Now let's tested deleted use case.

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To start let's open a comic book
detail page into another tab and

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click the edit button to browse
to Edit Comic Book page.

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Then back in the other tab
let's delete the Comic Book.

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We're back at the list page and
we can see that Bone #4 isn't in the list.

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Let's switch to the other tab and
try to Save.

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And we get the error message letting us
know that the comic book has been deleted.

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And clicking the Cancel button
takes us back to the list page.

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Just like we wanted it to.

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Next step, we'll see how to handle
concurrency when deleting records