public void DoWork(Person person)
    {
        ModelValidation<Person>.Validate(person)
            .WhenNullOrEmpty("{0} is required", c => c.FirstName, c => c.LastName)
            .WhenNull("{0} is required", c=>c.BirthDate)
            .WhenTrue("Must be 21 or older", c=> c < 21, c=>c.Age)
            .ThrowIfErrors();

        Console.WriteLine("This person is cool");
    } 

	Example from Visual Studio!