Data Annotations in ASP.NET Core

Data annotation can be used most commonly in in ASP.NET Core MVC or ASP.Net Data controls to perform model validation. Data annotations (available as part of the System. ComponentModel. DataAnnotations namespace, are attributes that can be applied to classes or class members to specify the relationship between classes, describe how the data is to be displayed in the UI, and specify validation rules. This article briefly tells you about data annotations, why they are useful, and how to use them in our .NET Core applications. 

Types of Data Annotations 

DA attributes are used to specify metadata on a class or a property. The data annotation attributes can broadly be classified into the following: 

1.  Validation attribute — Used to enforce validation rules on the properties of the entities 
2. Display attribute — Used to specify how the data should be displayed in the user interface 
3. Modeling attribute — Used to specify the relationship that exists between classes Attribute classes

 The System.ComponentModel.Annotations namespace comprises several attribute classes that can be used to define metadata for your entity classes or data controls. The most commonly used attributes include the following: 

•  DataType- to specify datatype rules for a specific data type like Date etc
•  Key – specify the property as a Key/Primary Key 
• MaxLength 
• Required
• StringLength
• Timestamp
• IsEmpty Required

 For a complete list of data annotation attibute classes, visit Microsoft Docs at System.ComponentModel.DataAnnotations Namespace | Microsoft Docs (https://docs.microsoft.com/en-us/dotnet/api/system.componentmodel.dataannotations?view=net-5.0) 

public class Customer 

 {

 [Required(ErrorMessage = "{0} is required")]

 [StringLength(50, MinimumLength = 3, ErrorMessage = "First Name should be minimum 3 characters and a maximum of 30 characters")]

 [DataType(DataType.Text)] 

 public string FirstName { get; set; }

 [Required(ErrorMessage = "{0} is required")]

 [StringLength(50, MinimumLength = 4, ErrorMessage = "Last Name should be minimum 3 characters and a maximum of 30 characters")] 

 [DataType(DataType.Text)] 

 public string LastName { get; set; } 

 [DataType(DataType.PhoneNumber)]

 [Phone]

 public string PhoneNumber { get; set; } 

 [DataType(DataType.EmailAddress)] 

 [EmailAddress] 

 public string Email { get; set; } 

 } 

 class Program { static void Main(string[] args) 

 { 

 Customer cust = new Customer();

 cust .FirstName = "Aisha"; 

 cust.LastName = ""; //invalid value entered 

 cust .PhoneNumber = "1234567890"; cust .Email = "abc@gmail.com"; 

 //use this code to validate the customer object for the data 

ValidationContext context = new ValidationContext (author, null, null); 

List validationResults = new List(); 

 bool valid = Validator.TryValidateObject (author, context, validationResults, true); 

 if (!valid) { 

 foreach (ValidationResult validationResult in validationResults)

 { 

Console.WriteLine("{0}", validationResult.ErrorMessage);

 } 

 Console.ReadKey(); 

 }

 }

On execution you get the following error: 

  LastName is required 

For IsEmpty rule validation you can use following data annotation:

[IsEmpty(ErrorMessage = "Should not be null or empty.")] 

public string Name{ get; set; } 

You can take advantage of data annotations to define data validation rules in a single place and thereby avoid having to re-write the same validation code again and again. Data annotation can be used most commonly in in ASP.NET Core MVC or ASP.Net Data controls to perform model validation.