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ASP.NET MVC Forms

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Implementing Server-Side Validation Using Data Annotations

4:36 with James Churchill

Now let’s see how we can use data annotations to write less code when implementing server-side validations.

Follow Along

To follow along commiting your changes to this course, you'll need to fork the aspnet-fitness-frog repo. Then you can clone, commit, and push your changes to your fork like this: 

git clone <your-fork>
cd aspnet-fitness-frog
git checkout tags/v4.3 -b implementing-server-side-validation-using-data-annotations

Additional Learning

For more information about C# Attributes, see this page on MSDN.

For additional information on how ASP.NET MVC validation works, including another look at using data annotations, see this ASP.NET website tutorial.

Here’s a project that extends the built-in data annotations.

Here’s an article that walks through the steps to create a custom data annotation attribute.

Keyboard Shortcuts

  • CTRL-M CTRL-O - Collapse to definitions

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    Having the ability to write code to validate our model is a powerful tool, but 0:00
    it can quickly get tedious. 0:04
    As an alternative NVC allows us to express simple validation rules 0:07
    using data annotations on our models. 0:11
    Let's open our entry data model. 0:14
    I'll collapse to definitions by pressing Ctrl+M, Ctrl+O. 0:19
    If we look at our interim model properties the NPC model binder will automatically 0:23
    validate each of the non-nullable value type properties as required fields. 0:27
    So Date, ActivityId, 0:33
    Duration, Intensity and 0:36
    Exclude are all automatically required fields. 0:42
    The Notes property is of type string which allows nulls, so 0:50
    that property would not automatically be validated as a required field. 0:54
    We can easily add a required validation for that field 0:59
    by adding a required data annotation attribute to the notes property. 1:03
    But that's not all, what if we want to prevent users 1:07
    from providing notes longer than 200 characters? 1:11
    Easy, we just add a max link data annotation attribute 1:14
    right below the required attribute. 1:18
    It requires a parameter value indicating what the max link should be. 1:21
    We can also set the error message property with the message that we want to display 1:25
    to users when this validation fails. 1:29
    Let's use the notes field cannot be longer than 200 characters. 1:32
    These are just two of the data annotations that are available for us to use. 1:41
    Other data annotations include min length, range, email address, phone, and more. 1:47
    You can even create your own custom data annotations. 1:54
    For more information about the built in data annotations or 1:58
    how to create custom data annotations, see the teacher's notes. 2:02
    Let's test our duration and notes field validation rules. 2:06
    Press F5 to run the app 2:09
    Then select an activity value and save the form. 2:15
    That should trigger the duration greater than zero and 2:21
    notes required validation rules. 2:24
    In the Auto's window let's drill down into model state. 2:29
    Values, the third item, then errors. 2:33
    Here's the duration error message. 2:43
    The duration field value must be greater than zero. 2:45
    Then the sixth item. 2:48
    Errors, and here's the notes error message. 2:52
    The Notes field is required. 2:59
    Now let's test the notes max length validation. 3:01
    Press F5 to continue. 3:04
    Enter a value greater than zero for the duration field, and 3:06
    a long string of text for the notes field. 3:10
    I'll enter some random text here. 3:13
    And then copy and paste it a number of times. 3:18
    That feels more than 200 characters long. 3:25
    Okay, save the form. 3:28
    And here's the error message for the max length validation rule, 3:36
    go ahead and stop the app. 3:39
    Before we wrap up this video let's undo one of the changes that we made. 3:44
    I added the required validation rule to the notes property 3:48
    to demonstrate how easy it is to make a nullable property required. 3:52
    I didn't really intend to make the notes field required. 3:56
    So let's remove the required data annotation from the notes property. 3:59
    That will make it easier to test our app going forward. 4:03
    If you're using GitHub, let's commit our changes. 4:06
    Enter a commit message of, added validation rules using 4:11
    data annotations, and click the Commit All button. 4:16
    Having validation rules is great, but 4:23
    unfortunately our users currently can't see them. 4:26
    In the next video, we'll update our Add Entry view to display validation messages. 4:29

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