Data Annotations in .NET with Examples

Introduction

Data annotations in .NET offer developers a convenient way to define validation rules and presentation attributes for model classes. By annotating properties with attributes from the System.ComponentModel.DataAnnotations namespace, developers can enforce data integrity and improve user interaction. This article explores 17 commonly used data annotations in .NET, providing detailed examples for each.

Required Attribute

The [Required] attribute ensures that a property must have a non-null value. It’s commonly used to enforce the presence of essential data fields.

Example

public class Person
{
    [Required(ErrorMessage = "First name is required.")]
    public string FirstName { get; set; }

    [Required(ErrorMessage = "Last name is required.")]
    public string LastName { get; set; }
}

StringLength Attribute

The [StringLength] attribute specifies the maximum and minimum lengths of a string property. It’s useful for enforcing length constraints on string values.

Example

public class Product
{
    [StringLength(50, MinimumLength = 3, ErrorMessage = "Product name must be between 3 and 50 characters.")]
    public string Name { get; set; }
}

Range Attribute

The [Range] attribute validates that a numeric property falls within a specified range of values.

Example

public class Product
{
    [Range(0, 100, ErrorMessage = "Price must be between 0 and 100.")]
    public decimal Price { get; set; }
}

RegularExpression Attribute

The [RegularExpression] attribute specifies a regular expression pattern that a string property must match.

Example

public class Customer
{
    [RegularExpression(@"^\d{3}-\d{3}-\d{4}$", ErrorMessage = "Invalid phone number format.")]
    public string PhoneNumber { get; set; }
}

EmailAddress Attribute

The [EmailAddress] attribute validates that a string property contains a valid email address.

Example

public class Contact
{
    [EmailAddress(ErrorMessage = "Invalid email address.")]
    public string Email { get; set; }
}

Phone Attribute

The [Phone] attribute validates that a string property contains a valid phone number.

Example

public class Contact
{
    [Phone(ErrorMessage = "Invalid phone number.")]
    public string PhoneNumber { get; set; }
}

Url Attribute

The [Url] attribute validates that a string property contains a valid URL.

Example

public class Website
{
    [Url(ErrorMessage = "Invalid URL.")]
    public string Url { get; set; }
}

CreditCard Attribute

The [CreditCard] attribute validates that a string property contains a valid credit card number.

Example

public class Payment
{
    [CreditCard(ErrorMessage = "Invalid credit card number.")]
    public string CardNumber { get; set; }
}

DataType Attribute

The [DataType] attribute specifies the data type of a property.

Example

public class Product
{
    [DataType(DataType.Currency)]
    public decimal Price { get; set; }
}

Compare Attribute

The [Compare] attribute compares the value of a property with another property’s value.

Example

public class Account
{
    [Compare("ConfirmPassword", ErrorMessage = "Passwords do not match.")]
    public string Password { get; set; }

    public string ConfirmPassword { get; set; }
}

Display Attribute

The [Display] attribute specifies how a property is displayed in the user interface.

Example

public class Product
{
    [Display(Name = "Product Name")]
    public string Name { get; set; }
}

Editable Attribute

The [Editable] attribute specifies whether a property should be editable in the user interface.

Example

public class User
{
    [Editable(false)]
    public string Username { get; set; }
}

ReadOnly Attribute

The [ReadOnly] attribute specifies that a property should be read-only in the user interface.

Example

public class Customer
{
    [ReadOnly(true)]
    public int Id { get; set; }
}

MaxLength Attribute

The [MaxLength] attribute specifies the maximum length of a string property.

Example

public class Note
{
    [MaxLength(100, ErrorMessage = "Note cannot exceed 100 characters.")]
    public string Content { get; set; }
}

MinLength Attribute

The [MinLength] attribute specifies the minimum length of a string property.

Example

public class User
{
    [MinLength(6, ErrorMessage = "Password must be at least 6 characters long.")]
    public string Password { get; set; }
}

RegularExpression Attribute

The [RegularExpression] attribute can also be used for validating other formats, such as postal codes.

Example

public class Address
{
    [RegularExpression(@"^\d{5}(-\d{4})?$", ErrorMessage = "Invalid ZIP code format.")]
    public string ZipCode { get; set; }
}

Custom ErrorMessage Attribute

All the examples above can include custom error messages by setting the ErrorMessage property.

Conclusion

Data annotations in .NET offer a powerful toolset for defining validation rules and presentation attributes for model classes. By leveraging attributes like [Required], [StringLength], [Range], and others, developers can enhance data integrity and user interaction in their applications. Understanding and effectively utilizing these annotations is essential for building robust and user-friendly .NET applications.