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Validating user input is crucial for maintaining data quality and security in any software application. When working with phone numbers in C#, leveraging data annotations for validation offers a clean and efficient way to ensure that the input conforms to expected formats. This article explores how to implement phone number validation using data annotations in C#, best practices, and tips to handle different phone number formats effectively.

1. What Is Data Annotation Phone Number Validation in C#?
Data annotations are attributes you apply to class business owner database properties in C# to enforce validation rules automatically. The [Phone] attribute, available in the System.ComponentModel.DataAnnotations namespace, is specifically designed to validate phone number formats.

When applied to a property, the [Phone] attribute uses a predefined regular expression to check if the input matches common phone number patterns. This makes validation straightforward without writing custom code or complex regex expressions manually.

Example:

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using System.ComponentModel.DataAnnotations;

public class Contact
{
[Phone(ErrorMessage = "Invalid phone number format.")]
public string PhoneNumber { get; set; }
}
This simple addition validates the PhoneNumber property whenever model validation is triggered.

2. How the [Phone] Data Annotation Works Behind the Scenes
The [Phone] attribute internally uses a regex pattern that validates a broad range of phone number formats, including:

Local phone numbers with or without area codes.

International numbers with + prefix.

Numbers containing spaces, dashes, or parentheses.

However, it is essential to understand that the built-in [Phone] validation is a general-purpose check and may not cover very specific or country-specific formats perfectly.