Object Oriented validation rules

gistfile1.cs

// the entity you want to validate
public class Person
{
    public int Age { get; set; }
	public string Name { get; set; }
}

public class ValidationFailure
{
	public ValidationFailure(string description) { Description = description; }
	public string Description { get; set; }
	// perhaps add other properties here if desired
}

public interface IValidationRule<TEntity>
{
	ValidationFailure Test(TEntity entity);
}

public class ValidatesMaxAge : IValidationRule<Person>
{
	public ValidationFailure Test(Person entity)
	{
		if (entity.Age > 100) return new ValidationFailure("Age is too high.");
	}
}

public class ValidatesName : IValidationRule<Person>
{
	public ValidationFailure Test(Person entity)
	{
		if (string.IsNullOrWhiteSpace(entity.Name))
			return new ValidationFailure("Name is required.");
	}
}

// to perform your validation
var rules = new List<IValidationRule> { new ValidatesMaxAge(), new ValidatesName() };
// test each validation rule and collect a list of failures
var failures = rules.Select(rule => rule.Test(person))
	.Where(failure => failure != null);
bool isValid = !failures.Any();