Custom validation with DataAnnotations

You can download the source:  https://github.com/stevenh77/CustomValidationWithDataAnnotations

	// because sometimes you have to use MVC :)
	using System;
	using System.Collections.Generic;
	using System.ComponentModel.DataAnnotations;
	namespace CustomValidation
	{
	class Program
	{
	static void Main(string[] args)
	{
	var invalidPerson = new Person()
	{
	FirstName = "Tom",
	Surname = "Tom"
	};
	var validPerson = new Person()
	{
	FirstName = "Steve",
	Surname = "Hollidge"
	};
	Validate(validPerson);
	Validate(invalidPerson);
	}
	private static void Validate(Person person)
	{
	var context = new ValidationContext(person, null, null);
	var results = new List<ValidationResult>();
	var isValid = Validator.TryValidateObject(person, context, results, true);
	if (isValid) return;
	foreach (var validationResult in results)
	{
	Console.WriteLine(validationResult.ErrorMessage);
	}
	}
	}
	class Person
	{
	[ContainsTheLetterTValidationAttribute]
	public string FirstName { get; set; }
	[DifferentValueValidation("FirstName")]
	public string Surname { get; set; }
	}
	public class ContainsTheLetterTValidationAttribute : ValidationAttribute
	{
	protected override ValidationResult IsValid(object value, ValidationContext validationContext)
	{
	// check type
	var stringValue = value as string;
	if (stringValue == null)
	{
	return new ValidationResult(string.Format("Unknown type or null value"));
	}
	// actual validation comparison occurs here
	return stringValue.ToLowerInvariant().Contains("t")
	? null
	: new ValidationResult(FormatErrorMessage(validationContext.DisplayName));
	}
	}
	public class DifferentValueValidationAttribute : ValidationAttribute
	{
	private readonly string other;
	public DifferentValueValidationAttribute(string other)
	{
	this.other = other;
	}
	protected override ValidationResult IsValid(object value, ValidationContext validationContext)
	{
	var property = validationContext.ObjectType.GetProperty(other);
	if (property == null)
	{
	return new ValidationResult(
	string.Format("Unknown property: {0}", other)
	);
	}
	var otherValue = property.GetValue(validationContext.ObjectInstance, null);
	// actual validation comparison occurs here
	return (value ==otherValue)
	? new ValidationResult(FormatErrorMessage(validationContext.DisplayName))
	: null;
	}
	}
	}

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