How to do Validation with Data Annotations?

The idea to use data annotation is to add constraint meta data to properties of the view model. The meta data then picked up by default binder when executes the model binding process and used to include additional validation to the data in the form field.

By adding constraints meta data to view model and allowing default binder to do validation as part of model binding process, we get certain benefits that is given below:

• The validation rule is enforced across whole application and not just single action method.

• We do not have to call method, instantiate class or do anything for the validation rules that need to be enforced, default binder will do this automatically.

• The amount of codes that required to implement validation rules for annotations is

considerably very less than when do it manually.

Now modify the code to see data annotations in action. 1st, remove manually added validations and make action method then we will accomplish following tasks:

• Make Form's Email and Message fields required.

• Set maximum length of the From field to 100 characters which is the length defined in the database field. Same constraint will be applied to Email and Subject fields.

• Set maximum length of the Message field to 2000 characters.

• Ensure that the email address is formatted properly.

• Specify the custom error message for all the annotations for further helping user to fix errors.

The result is view model which shown in Figure 1. Note that all data annotations allow us to specify custom error message with ErrorMessage property.

Figure 1. Adding Data Annotations to the View Model

There are few other annotations that we can use to enforce validation rule in view model. Some will work at client browser and give the user immediate feedback and opportunity to fix entries before sending them to server.

Data Annotations List:

[CreditCard] => It defines the value as a credit card number which can have 15 or 16 digits without spaces or dashes.

[EmailAddress] => It indicates that the value must be well formatted email address.

[EnumDataType(typeof(name of enum))] => It enables .NET Framework enumeration that can be mapped to a data column. Its value must be one which is defined in enumeration.

[FileExtension(Extensions="")] => It validates the file extension of uploaded file which should be in the acceptable defined extensions. For example [FileExtensions(Extensions=".jpg,.png,.gif")].

[Range(min, max)] => It indicates that the value must be between specified minimum and

maximum range.

[RegularExpression(pattern)] => It indicates that the value must match specified pattern (regular expression).

[Required] => It indicates that the field must contain a value. The validation exception will be raised if the field is null or contains empty string (blank) or contains only white space characters. If we want to treat white pace characters as a valid character then we need to use [Required(AllowEmptyStrings=true)].

[StringLength(max)] => It indicates the maximum character length of the value can have and optionally, the minimum character length. Type of this properties should  be string.

[URL] => It indicates that its value is well formatted URL.

Now if we execute the application, we can see different type of errors in the browser

(client side code) as shown in Figure 2.

Figure 2. Validation errors of the form fields based on view model annotations