📓 Model Validation with Validation Attributes
In the last course section, we created instructions and used simple condition to avoid creating a database error if a user tries to create an Item without there being a Category to associate it with. In the Items/Create.cshtml view we added this message:
...
<h4>Add a new item</h4>
<p><strong>NOTE:</strong> You need to have at least one category before you can add an item!</p>
<p>Go to @Html.ActionLink("this page", "Create", "Categories") to create a category.</p>
...
Then, in the Create() POST action in the ItemsController, we verified whether there was a Category object selected by checking if the categoryId parameter has a value of 0. If so, we simply directed to the Items/Create.cshtml page. Otherwise, we went ahead and saved the new Item to our database.
...
[HttpPost]
public ActionResult Create(Item item)
{
if (item.CategoryId == 0)
{
return RedirectToAction("Create");
}
_db.Items.Add(item);
_db.SaveChanges();
return RedirectToAction("Index");
}
...
In this lesson, we're going to try out a new tool to handle this same issue: model validation with validation attributes. With model validation we configure .NET to validate user input based on rules that we set in our models; we set these rules using validation attributes. As we'll learn a bit later on, "validation attributes" are also commonly called "data annotations". Let's get into this refactor and then revisit these concepts.
Adding Validation Attributes to Item.cs
First, we'll start by adding a [Range] validation attribute to the CategoryId property of our Item model:
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
namespace ToDoList.Models
{
public class Item
{
public int ItemId { get; set; }
public string Description { get; set; }
[Range(1, int.MaxValue)]
public int CategoryId { get; set; }
public Category Category { get; set; }
public List<ItemTag> JoinEntities { get;}
}
}
A validation attribute lets us specify the rules to use when validating a model's property. The validation attribute that we added is [Range(1, int.MaxValue)]. In order to attach a validation attribute to a property, we need to list it directly above the property, which in our case is CategoryId.
In order to use validation attributes at all, we need to include the using directive using System.ComponentModel.DataAnnotations; at the top of the file, which allows us to use the tools within the System.ComponentModel.DataAnnotations namespace. Note that the terms data annotations and validation attributes are used interchangeably.
With [Range(1, int.MaxValue)], we are specifying the rule that the value of CategoryId must be a number between 1 to the int.MaxValue, which is the maximum possible integer value in C#. What this validation rule does for us is ensure that the CategoryId value is never 0. If it is 0, then we can deliver an error message to the user.
Let's update our [Range()] validation attribute to include an error message. To do this, we'll add another argument to [Range()]. Update the Item class as follows:
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
namespace ToDoList.Models
{
public class Item
{
public int ItemId { get; set; }
public string Description { get; set; }
[Range(1, int.MaxValue, ErrorMessage = "You must add your item to a category. Have you created a category yet?")]
public int CategoryId { get; set; }
public Category Category { get; set; }
public List<ItemTag> JoinEntities { get;}
}
}
Notice that we've added a third argument: ErrorMessage = "You must add your item to a category. Have you created a category yet?". The ErrorMessage property can be applied to any validation attribute.
The error message "You must add your item to a category. Have you created a category yet?" will be delivered to the user if they try to create an item without specifying a category (which happens when the CategoryId has a value of 0). However, we have a few more steps to complete before this will actually work! We need to update our Item/Create.cshtml view to display the error message, and we need to update our item's Create() POST controller action to check if our model's state is valid.
Updating Item/Create.cshtml
We'll first update our Item/Create.cshtml view. We'll first remove our hard coded message:
...
<p><strong>NOTE:</strong> You need to have at least one category before you can add an item!</p>
<p>Go to @Html.ActionLink("this page", "Create", "Categories") to create a category.</p>
...
And then we'll add a new line of code in its place (right below <h4>Add a new item</h4>):
...
<p>@Html.ValidationMessageFor(model => model.CategoryId, "", new { @class = "text-danger", @style = "font-weight: 600;" })</p>
...
We're making use of a new strongly typed HTML helper called ValidationMessageFor() that returns the HTML markup for a validation-error message for the specified model property, in this case CategoryId. The ValidationMessageFor() method takes two arguments, plus a third optional argument:
- The first argument specifies the model property
CategoryIdthat this validation message is for. - The second argument is an empty string acting as a placeholder for the
ErrorMessagewe set in the[Range()]validation attribute. If the model is valid,ValidationMessageFor()will display nothing on the page (per the empty string). If the model is not valid, thenValidationMessageFor()will display our error message replacing the empty string placeholders with"You must add your item to a category. Have you created a category yet?". - The third argument is optional and it contains an object with options that we want to specify for our HTML element. In this case, we're adding a new class set to Bootstrap's
"text-danger"which will make our text red, and a new inline style that will make our text bold.
The last step we have to complete is to actually direct our controller to check the state of our model.
Checking the Model State
In our controllers, we can check the validity of our model's state by accessing the built-in ModelState.IsValid property. ModelState.IsValid is a bool, so we can check if the model state is true/valid or false/invalid. Let's see how this works by updating our ItemsController.
Replace the item's Create() POST action with the following code:
...
[HttpPost]
public ActionResult Create(Item item)
{
if (!ModelState.IsValid)
{
ViewBag.CategoryId = new SelectList(_db.Categories, "CategoryId", "Name");
return View(item);
}
else
{
_db.Items.Add(item);
_db.SaveChanges();
return RedirectToAction("Index");
}
}
...
When we check if (!ModelState.IsValid), we're checking if ModelState.IsValid == false, which means that the model's state is not valid.
Take note that model validation happens automatically, but we still need to determine what to do when the model's state is valid or invalid. In our case, if our item is not valid, then we create a new select list of category objects and return View(item);. This will display Items/Create.cshtml once again, both with a select list of category objects and the invalid item object from which the ValidateMessageFor accesses the validation error message that we set on the Item.CategoryId property.
And with that, we have model validation set up for our Item.CategoryId property. Run your To Do List app now and try adding an item without a category to see the error message displayed.
Adding a Second Validation
Each property of a model can have one or more validation attributes. Let's get more practice by adding the [Required] validation attribute to our Item.Description property. When we require a property, it means that a form input for that property can't be empty. Doing this for our items will ensure that no description input is left empty when a user creates a new item.
First, we'll update the model:
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
namespace ToDoList.Models
{
public class Item
{
public int ItemId { get; set; }
[Required(ErrorMessage = "The item's description can't be empty!")]
public string Description { get; set; }
[Range(1, int.MaxValue, ErrorMessage = "You must add your item to a category. Have you created a category yet?")]
public int CategoryId { get; set; }
public Category Category { get; set; }
public List<ItemTag> JoinEntities { get;}
}
}
Note that we could simply list [Required] without specifying an ErrorMessage.
Next, let's update our view to display the error message if our item's Description is empty. Right below our ValidationMessageFor() for Item.CategoryId, we'll add a new ValidationMessageFor() for Item.Description:
...
<p>@Html.ValidationMessageFor(model => model.CategoryId, "", new { @class = "text-danger", @style = "font-weight: 600;" })</p>
<p>@Html.ValidationMessageFor(model => model.Description, "", new { @class = "text-danger", @style = "font-weight: 600;" })</p>
...
At this point we can run our application and see that we'll get an error message if we leave the description input blank when we create new items. Note that we don't need to make any updates to the item's Create() POST action, because if the model state is invalid we return the entire invalid item object from which the CategoryId and Description properties can be accessed.
More to Explore
There are many more built-in validation attributes to explore, as well as the possibility of creating custom validation attributes. There's also more to learn about model state, including how to manually re-run model validation. To optionally learn more, visit the MS Docs article on Model validation in ASP.NET Core MVC and Razor Pages. For a complete list of available data annotations, visit the System.ComponentModel.DataAnnotations namespace
Up next, we're going to learn about how we can use data annotations to configure our entity (like Item) properties (like Item.Description).