Specification

To create custom Specification type, just need to implement ISpecification<T> interface from FluentSpecification.Abstractions package.

public class VipCustomerSpecification :
    ISpecification<Customer>
{
    public bool IsSatisfiedBy(Customer candidate)
    {
        return candidate
            .Comments
            .Split(new char[] {';'})
            .Contains("Vip");
    }
}

Type, prepared in this way can be used with other, built-in Specifications.

var customerSpec = Specification
    .NotNull<Customer>()
    // Create 'VipCustomerSpecification' by parameterless constructor
    .And<Customer, VipCustomerSpecification>();