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Having the ability to write code to
validate our model is a powerful tool, but

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it can quickly get tedious.

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As an alternative NVC allows us to
express simple validation rules

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using data annotations on our models.

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Let's open our entry data model.

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I'll collapse to definitions
by pressing Ctrl+M, Ctrl+O.

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If we look at our interim model properties
the NPC model binder will automatically

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validate each of the non-nullable value
type properties as required fields.

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So Date, ActivityId,

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Duration, Intensity and

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Exclude are all automatically
required fields.

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The Notes property is of type
string which allows nulls, so

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that property would not automatically
be validated as a required field.

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We can easily add a required
validation for that field

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by adding a required data annotation
attribute to the notes property.

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But that's not all,
what if we want to prevent users

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from providing notes longer
than 200 characters?

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Easy, we just add a max link
data annotation attribute

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right below the required attribute.

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It requires a parameter value
indicating what the max link should be.

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We can also set the error message property
with the message that we want to display

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to users when this validation fails.

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Let's use the notes field cannot
be longer than 200 characters.

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These are just two of the data annotations
that are available for us to use.

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Other data annotations include min length,
range, email address, phone, and more.

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You can even create your own
custom data annotations.

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For more information about
the built in data annotations or

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how to create custom data annotations,
see the teacher's notes.

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Let's test our duration and
notes field validation rules.

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Press F5 to run the app

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Then select an activity value and
save the form.

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That should trigger the duration
greater than zero and

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notes required validation rules.

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In the Auto's window let's
drill down into model state.

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Values, the third item, then errors.

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Here's the duration error message.

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The duration field value
must be greater than zero.

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Then the sixth item.

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Errors, and
here's the notes error message.

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The Notes field is required.

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Now let's test the notes
max length validation.

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Press F5 to continue.

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Enter a value greater than zero for
the duration field, and

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a long string of text for the notes field.

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I'll enter some random text here.

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And then copy and
paste it a number of times.

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That feels more than 200 characters long.

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Okay, save the form.

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And here's the error message for
the max length validation rule,

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go ahead and stop the app.

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Before we wrap up this video let's
undo one of the changes that we made.

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I added the required validation
rule to the notes property

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to demonstrate how easy it is to
make a nullable property required.

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I didn't really intend to make
the notes field required.

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So let's remove the required data
annotation from the notes property.

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That will make it easier to
test our app going forward.

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If you're using GitHub,
let's commit our changes.

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Enter a commit message of,
added validation rules using

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data annotations, and
click the Commit All button.

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Having validation rules is great, but

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unfortunately our users
currently can't see them.

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In the next video, we'll update our Add
Entry view to display validation messages.