Validation is the act of checking the information the user enters into the form and ensuring that it is correct, proper, and meets the business logic of the application. Data annotations are the most used method for validation in MVC applications, and validation is conducted in the model. These annotations declare validation constraints, With submittals validation rules are also executed. Proper validation not only includes quality in the data set but also helps the user if there is an error or mistake he/she made because there is feedback.

Types of Validation in MVC

1. Client-Side Validation: View level validation is the most basic level of validation where the check is performed on the client side of the browser. This is faster than the other because you will not be frequently interacting with the server. But with the inclusion of a more responsive interface, one can also solve this problem with ready-made libraries like jQuery Validation. But client-side validation should not, however, be used alone because it can and should be ignored.

2. Server-Side Validation: Server-side validation takes place when the information entered is sent to a server. Such validation is more secure as it cannot be easily circumvented by users who would disable JavaScript in the browsers that they use. This is why, in case client-side validation is utilized, it is impossible to ignore server-side validation – it is one of the primary requirements for data validation.

Both of these types can be employed on the same page, and frameworks like ASP.NET MVC guarantee that MVC validation indeed is constructive and viable in multiple-user situations.

Applying Validation along with Data Annotation

The most used approach when it comes to validation in MVC is Data Annotation. These are attributes you apply to the model properties and they state the validation rules directly. Here are some of the common data annotation attributes used in ASP.NET MVC:

• [Required]: Makes sure that any given field is completed at all times.
• [StringLength]: Limits the amount of characters that can be typed in one string input, also may set the minimum number of characters allowed.
• [Range]: Filters input by a range of values in the form of an integer.
• [RegularExpression]: Converts a string using a predefined regular expression pattern which will be used to test strings or the email address or phone number.

For example, if you want to make sure a username is mandatory and its length falls between 5 and 50 characters, you would define it as:

• >[Required(ErrorMessage = “Username is required.”)]
• @[RegularExpression(@"^[a-zA-Z0-9]{5,50}$”), ErrorMessage = “Username must be between 5 and 50 characters with no special characters allowed.”)]
• public string Username { public get; private set; }

Validating Data in the Controller

After data entry, the controller determines if the model is valid or not according to the ModelState.IsValid. If the data does not pass the validations then error messages generated are sent back to the view. This also helps users in modifying their inputs without having to complete the entire form all over the place.

Here’s a conceptual process:

• The controller action first checks the ModelState.
• If the state is invalid then the form is submitted back with error messages.
• The user is then informed that he/she has entered the data incorrectly and the form should be completed afresh.

Displaying Validation Messages in Views

In the view, validation errors are output with the help of helpers such as @Html.ValidationSummary() and @Html.ValidationMessageFor(). These helpers produce HTML that puts plain text right in the form to tell the user what went wrong.

For instance, you can display validation messages for a form field as follows:

• concerning the @Html.LabelFor(model => model.Username)Below is text box HTML to let user enter application user name @Html.TextBoxFor(model => model.Username)The purpose of using @Html.ValidationMessageFor(model => model.Username)

Thus, the rationale for this setup is to guarantee the users get quick feedback about any validation errors in a tidy manner.

Remote Validation in MVC

Another enhancement of MVC is Remote Validation, which allows you to verify the value’s admissibility on the server without submitting the form. For instance when entering a username in a form and you want to know first if the username is already taken, with remote validation, you can kick off an AJAX request to the server to let you determine the validity of the specified username without having to submit the entire form.

This is done by applying the [Remote] attribute to your model property and specifying the action method that should handle the validation:

• [Remote("IsUsernameAvailable", "User")]
• public string Username { get; set; }

From the server’s perspective, the corresponding action method executes the check and returns to the client a JSON value of true if the username is available.

Custom Validation

MVC also provides flexibility in cases when the built-in validation attributes are not enough as it enables the creation of the so-called custom validation attributes inheriting the Validation Attribute class. This is helpful especially when validation is required that cannot be solved through the use of simple annotations.

For instance, if your business policy dictates that you have to validate whether a username already exists in a database of your business then you could make a new attribute that validates this and return an error message when true.

Conclusion

Data validation in MVC applications is particularly important when it comes to form validation as it checks if data conforms to application requirements. Therefore, to make server-side validation and enable client-side validation both meet the needed efficiency as well as make sure that the application is secure, data annotations can be applied. Remote validation and custom attributes added another level of customization and durability to your validation logic.

Thus, by mastering these techniques, you will enhance and make secure, efficient, and easily usable MVC applications.