Chapter_6_Data_Annotations_and_Validation.md

6 Data Annotations and Validation

Validation restricts user input to be of certain pattern. For example, if user needs to enter his/her age, he might put "Thirdy", instead of 30. Such a logic should be implemented not only in client-side but also from server side as well.

In MVC paradigm, validation pimarily focusses on validating model values. Did the user provide a required value? Is the value in the range? ASP.NET MVC validation features can help us in validating model values. A declarative style of validation using attributes in ASP.NET, is known as Data Annotation.

Data annotations are attributes found in System.ComponentModel.DataAnnotations namespace. Some of them are:

• Required :
  raises validation error if either property value is null or empty thus asserting user to enter the field value.
• StringLength
  sets maximum length for a given property. raises validation error if the value of the property exceeds its limit. You can also optionally specify minimum length using MinimumLength parameter.
• RegularExpression
  Properties like email needs to be in specific format. These can be validated using RegularExpression attribute.
• Range
  This attribute specifies minimum and maximum constraints for a numeric value.
• Compare
  This ensures two properties on a model have the same value. Example: email and confirm email should be same.

You can use ErrorMessage to localize your validation error messages. Example: consider we have Student Class :

public class Student
{
    public string FullName { get; set; }
    public int Age { get; set; }
    public string RollNumber { get; set; }
}

We have a form for this like:

With some data annotations, we can redefine the Student Class like this:

public class Student
{
    [Required(ErrorMessage="Please enter full name")]
    [StringLength(160), MinimumLength=3]
    public string FullName { get; set; }
    
    [Range(18, 60)]
    public int Age { get; set; }
    
    [Required]
    [StringLength(10)]
    public string RollNumber { get; set; }
}

In this way we can validate models using data annotations.