FluentValidation

public static partial class Config
{   
    public static void Register(HttpConfiguration config)
    {
        FluentValidationModelValidatorProvider.Configure(config);     
    } 
}

[Validator(typeof(TestModelValidator))]
public class Test
{
    public Test()
    {
        this.address = new Address();
    }
    [Key]
    public int Id { get; set; }
    public string email { get; set; }
    public bool needValidAddress { get; set; }        
    public Address address { get; set; }
}
public class TestModelValidator : AbstractValidator<Test>
{
    public TestModelValidator()
    {
        RuleFor(x => x.address.text)
            .NotEmpty()
            .SetValidator(new CustomValidator<int>())
            .When(o => o.needValidAddress)
            .WithMessage("address text required");

        When(o => !string.IsNullOrWhiteSpace(o.email), () =>
        {
            RuleFor(c => c.email).Matches(@"^([\w-\.]+)@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.)|(([\w-]+\.)+))([a-zA-Z]{2,4}|[0-9]{1,3})(\]?)$");
        });

        Custom(o => {
            return o.Id >= 10
                ? new ValidationFailure("Id","more than 10")
                : null;
        });
    }
}
public class CustomValidator<T> : PropertyValidator
{

    public CustomValidator()
        : base("Property {PropertyName} contains more than 10 items!")
    {

    }

    protected override bool IsValid(PropertyValidatorContext context)
    {
        var list = context.PropertyValue as IList<T>;

        if (list != null && list.Count >= 10)
        {
            return false;
        }

        return true;
    }
}
[ComplexType]
public class Address
{
    public string text { get; set; }
}