15.5. Validation and Views

While our application is properly handing errors, we need to display the error messages so that the users know what information they should be adding. Client-side validation like this can prevent faulty data submissions.

Since the filming of this video, ASP.NET requires a different method for changing the color of error messages. Instructions for working with field-validation-valid have been updated below.

15.5.1. Displaying Error Messages in our View - Video


If you want to verify what code this video starts with, check out the handling-errors branch. If you want to verify what code this video ends with, check out the display-error-messages branch.

15.5.2. Displaying Error Messages in our View - Text

Create a separate span element in the form. In the <span> tag, we can add asp-validation-for to specify which property’s validation error messages should be displayed if the conditions are not met.


We don’t have to add anything else to display error messages, because that validation is already built in!

Now when we run our application and enter a bad event name or forget our description, we get the error message displaying what we should be entering!


In the video, we note that the <span> element will only be materialized if the validation fails. This isn’t true. The <span> element to contain the error message is always created, it just doesn’t contain any text if the validation rule it met.

15.5.3. Changing the Color of Error Messages

Adding to the project’s site.css will not change the color of your error messages. We need to add more code to the project.

One way to do this is by using unobtrusive validation methods, such as using a third-party library.

The jQuery Unobtrusive Validation library. contains scripts that will perform validation tasks, such as updating the color of error messages. To utilize this library, we need to point to it in our _Layout.cshtml page and then add more attributes to our Views. _Layout.cshtml Updates

To utilize the library, we need to add the following references to it within the <main> area.

   <script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.6.0/jquery.js"></script>
   <script src="https://cdnjs.cloudflare.com/ajax/libs/jquery-validate/1.19.3/jquery.validate.js"></script>
   <script src="https://cdnjs.cloudflare.com/ajax/libs/jquery-validation-unobtrusive/3.2.12/jquery.validate.unobtrusive.js"></script>

In the Add View, you will need to add more class attributes for the library to target.

 <div asp-controller="Events" asp-action="Add" class="form-group">
     <label asp-for="Name"></label>
     <input asp-for="Name" class="form-control"/>
     <span asp-validation-for="Name" class="text-danger"></span>

Note in line 10 the addition of class="form-control" and in line 11 class="text-danger". These attributes will be noted by the jQuery Unobtrusive Validation library and print our red if the client does not meet the requirements.

You can read more about client-side validation and the jQuery Unobtrusive Validation library here. That is were we found the syntax for this portion of the project.

15.5.4. Check Your Understanding


True or False In order to implement validation, all three elements of MVC applications are involved.