Usage

using FluentSpecification;
using FluentSpecification.Core;

var customerSpec = Specification
    // Specify not null Customer
    .NotNull<Customer>()

    // with Id between 100 and 200
    .And()
    .InclusiveBetween(c => c.CustomerId, 100, 200)

    // and LastName is "Smith"
    .And()
    .Equal(c => c.LastName, "Smith")

    // and Customer is active
    .And()
    .True(c => c.IsActive)

    // and Customer property "Email" is ...
    .And(Specification.ForProperty<Customer, string>(c => c.Email, Specification
        // ... valid email
        .Email()

        // longer than 15 characters
        .And()
        .MinLength(15)

        // on "gmail.com"
        .And()
        .Match("^.*@gmail.com$")))

    // with not empty Item collection ...
    .And()
    .ForProperty(c => c.Items, Specification
        .NotEmpty<ICollection<Item>>()

        // ... contains Item '1000'
        .And()
        .Contains(new Item { ItemId = 1000 }))

    // and Customer has credit card ...
    .And()
    .NotNull(c => c.CreditCard)

    // ... with valid number
    .And()
    .CreditCard(c => c.CreditCard.CardNumber)

    // and credit card is valid between 2019-03-12 ...
    .And(Specification
        .GreaterThanOrEqual<Customer, DateTime>(c => c.CreditCard.ValidityDate, 
            new DateTime(2019, 3, 12))
        
        // ... and 2019-05-31
        .Or()
        .LessThan(c => c.CreditCard.ValidityDate, new DateTime(2019, 6, 1)));

Is satisfied by

customerSpec.IsSatisfiedBy(new Customer
{
    CustomerId = 125,
    LastName = "Smith",
    IsActive = true,
    Email = "asmith@gmail.com",
    Items = new List()
    {
        new Item { ItemId = 1000 }
    },
    CreditCard = new CreditCard
    {
        CardNumber = "5500 0000 0000 0004",
        ValidityDate = DateTime.Parse("2019-03-12")
    }
}); // return true

Validation

customerSpec.IsSatisfiedBy(new Customer
{
    CustomerId = 90,
    LastName = "Jones",
    IsActive = false,
    Email = "mjones@hotmail.com",
    Items = null,
    CreditCard = new CreditCard
    {
        CardNumber = "5500 0000 1",
        ValidityDate = DateTime.Parse("2019-03-01")
    }
}, out var specResult);    // return false

Console.WriteLine(specResult.ToString());
// Field 'CustomerId' value is not valid
// Field 'CustomerId': [Value is not between [100] and [200]]
// Field 'LastName' value is not valid
// Field 'LastName': [Object is not equal to [Smith]]
// Field 'IsActive' value is not valid
// Field 'IsActive': [Value is False]
// Field 'Email' value is not valid
// Field 'Email': [String not match pattern [^.*@gmail.com$]]
// Field 'Items' value is not valid
// Field 'Items': [Object is empty]
// Field 'Items': [Collection not contains [FluentSpecification.Integration.Tests.Data.Item]]
// Field 'CreditCard.CardNumber' value is not valid
// Field 'CreditCard.CardNumber': [Value is not correct credit card number]

Querying

var customers = new List<Customer>()
{
    // fill customers
};
var result = customers
    .Where(customerSpec.AsPredicate()).ToList();

var dbResult = Context.Customers
    .Where(customerSpec.GetExpression()).ToList();    // Or customerSpec.AsExpression()

Translations / Custom messages

// Single error message for whole specifications chain
customerSpec
    .WithMessage(c => $"Validation failed: Incorrect Customer - ID '{c.CustomerId}'")
    .IsSatisfiedBy(new Customer
    {
        CustomerId = 90,
        LastName = "Jones",
        IsActive = false,
        Email = "mjones@hotmail.com",
        Items = null,
        CreditCard = new CreditCard
        {
            CardNumber = "5500 0000 1",
            ValidityDate = DateTime.Parse("2019-03-01")
        }
    }, out var specResult);    // return false

Console.WriteLine(specResult.ToString());
// Validation failed: Incorrect Customer - ID '90'


// Custom messages for each specification
var idSpec = Specification
    .NotEmpty<Customer, int>(c => c.CustomerId)
    .WithMessage(c => $"Unknown Customer ID: '{c.CustomerId}'");

var activeSpec = Specification
    .True<Customer>(c => c.IsActive)
    .WithMessage("Customer is archived");

idSpec.And(activeSpec)
    .IsSatisfiedBy(new Customer(), out var specResult);    // return false

Console.WriteLine(specResult.ToString());
// Unknown Customer ID: '0'
// Customer is archived